79 (Weakness Base)

"CSS" was once used as the acronym for this problem, but this can cause confusion with the "Cascading Style Sheets," so this acronym has declined significantly. Its use is discouraged by CWE.

High to Very High

Resultant (Weakness is typically related to the presence of some other weaknesses)

Explicit (This is an explicit weakness resulting from behavior of the developer)

Confidentiality: The most common attack performed with cross-site scripting involves the disclosure of information stored in user cookies.

Access control: In some circumstances it may be possible to run arbitrary code on a victim's computer when cross-site scripting is combined with other flaws.

If successful, cross-site scripting vulnerabilities can be exploited to manipulate or steal cookies, create requests that can be mistaken for those of a valid user, compromise confidential information, or execute malicious code on the end user systems for a variety of nefarious purposes.

Carefully check each input parameter against a rigorous positive specification (white list) defining the specific characters and format allowed. All input should be sanitized, not just parameters that the user is supposed to specify, but all data in the request, including hidden fields, cookies, headers, the URL itself, and so forth. A common mistake that leads to continuing XSS vulnerabilities is to validate only fields that are expected to be redisplayed by the site. We often encounter data from the request that is reflected by the application server or the application that the development team did not anticipate. Also, a field that is not currently reflected may be used by a future developer. Therefore, validating ALL parts of the HTTP request is recommended.

This involves "HTML Entity Encoding" all non-alphanumeric characters from data that was received from the user and is now being written to the request.

With Struts, you should write all data from form beans with the bean's filter attribute set to true.

To help mitigate XSS attacks against the user's session cookie, set the session cookie to be HttpOnly. In browsers that support the HttpOnly feature (such as Internet Explorer), this attribute prevents the user's session cookie from being accessed by client-side scripts, including scripts inserted due to a XSS attack.

The following JSP code segment reads an employee ID, eid, from an HTTP request and displays it to the user.

JSP Example:

The following ASP.NET code segment reads an employee ID number from an HTTP request and displays it to the user.

ASP.NET Example:

The code in this example operates correctly if eid contains only standard alphanumeric text. If eid has a value that includes meta-characters or source code, then the code will be executed by the web browser as it displays the HTTP response. Initially this might not appear to be much of a vulnerability. After all, why would someone enter a URL that causes malicious code to run on their own computer? The real danger is that an attacker will create the malicious URL, then use e-mail or social engineering tricks to lure victims into visiting a link to the URL. When victims click the link, they unwittingly reflect the malicious content through the vulnerable web application back to their own computers. This mechanism of exploiting vulnerable web applications is known as Reflected XSS.

The following JSP code segment queries a database for an employee with a given ID and prints the corresponding employee's name.

JSP Example:

The following ASP.NET code segment queries a database for an employee with a given employee ID and prints the name corresponding with the ID.

ASP.NET Example:

This code functions correctly when the values of name are well-behaved, but it does nothing to prevent exploits if they are not. Again, this code can appear less dangerous because the value of name is read from a database, whose contents are apparently managed by the application. However, if the value of name originates from user-supplied data, then the database can be a conduit for malicious content. Without proper input validation on all data stored in the database, an attacker can execute malicious commands in the user's web browser. This type of exploit, known as Stored XSS, is particularly insidious because the indirection caused by the data store makes it more difficult to identify the threat and increases the possibility that the attack will affect multiple users. XSS got its start in this form with web sites that offered a "guestbook" to visitors. Attackers would include JavaScript in their guestbook entries, and all subsequent visitors to the guestbook page would execute the malicious code. As the examples demonstrate, XSS vulnerabilities are caused by code that includes unvalidated data in an HTTP response. There are three vectors by which an XSS attack can reach a victim: * As in the previous example, data is read directly from the HTTP request and reflected back in the HTTP response. Reflected XSS exploits occur when an attacker causes a user to supply dangerous content to a vulnerable web application, which is then reflected back to the user and executed by the web browser. The most common mechanism for delivering malicious content is to include it as a parameter in a URL that is posted publicly or e-mailed directly to victims. URLs constructed in this manner constitute the core of many phishing schemes, whereby an attacker convinces victims to visit a URL that refers to a vulnerable site. After the site reflects the attacker's content back to the user, the content is executed and proceeds to transfer private information, such as cookies that may include session information, from the user's machine to the attacker or perform other nefarious activities. * As in this example, the application stores dangerous data in a database or other trusted data store. The dangerous data is subsequently read back into the application and included in dynamic content. Stored XSS exploits occur when an attacker injects dangerous content into a data store that is later read and included in dynamic content. From an attacker's perspective, the optimal place to inject malicious content is in an area that is displayed to either many users or particularly interesting users. Interesting users typically have elevated privileges in the application or interact with sensitive data that is valuable to the attacker. If one of these users executes malicious content, the attacker may be able to perform privileged operations on behalf of the user or gain access to sensitive data belonging to the user. * A source outside the application stores dangerous data in a database or other data store, and the dangerous data is subsequently read back into the application as trusted data and included in dynamic content.

Cross-site scripting weakness occurs when dynamically generated web pages display input, such as login information, that is not properly validated, allowing an attacker to embed malicious scripts into the generated page and then execute the script on the machine of any user that views the site. If successful, Cross-site scripting vulnerabilities can be exploited to manipulate or steal cookies, create requests that can be mistaken for those of a valid user, compromise confidential information, or execute malicious code on the end user systems for a variety of nefarious purposes.

Cross-site scripting (XSS) vulnerabilities occur when an attacker uses a web application to send malicious code, generally JavaScript, to a different end user. When a web application uses input from a user in the output it generates without filtering it, an attacker can insert an attack in that input and the web application sends the attack to other users. The end user trusts the web application, and the attacks exploit that trust to do things that would not normally be allowed. Attackers frequently use a variety of methods to encode the malicious portion of the tag, such as using Unicode, so the request looks less suspicious to the user. XSS attacks can generally be categorized into two categories: stored and reflected. Stored attacks are those where the injected code is permanently stored on the target servers in a database, message forum, visitor log, and so forth. Reflected attacks are those where the injected code takes another route to the victim, such as in an email message, or on some other server. When a user is tricked into clicking a link or submitting a form, the injected code travels to the vulnerable web server, which reflects the attack back to the user's browser. The browser then executes the code because it came from a 'trusted' server. For a reflected XSS attack to work, the victim must submit the attack to the server. This is still a very dangerous attack given the number of possible ways to trick a victim into submitting such a malicious request, including clicking a link on a malicious Web site, in an email, or in an inner-office posting. XSS flaws are very likely in web applications, as they require a great deal of developer discipline to avoid them in most applications. It is relatively easy for an attacker to find XSS vulnerabilities. Some of these vulnerabilities can be found using scanners, and some exist in older web application servers. The consequence of an XSS attack is the same regardless of whether it is stored or reflected. The difference is in how the payload arrives at the server. XSS can cause a variety of problems for the end user that range in severity from an annoyance to complete account compromise. The most severe XSS attacks involve disclosure of the user's session cookie, which allows an attacker to hijack the user's session and take over their account. Other damaging attacks include the disclosure of end user files, installation of Trojan horse programs, redirecting the user to some other page or site, and modifying presentation of content. Cross-site scripting (XSS) vulnerabilities occur when: 1. Data enters a Web application through an untrusted source, most frequently a web request. 2. The data is included in dynamic content that is sent to a web user without being validated for malicious code. The malicious content sent to the web browser often takes the form of a segment of JavaScript, but may also include HTML, Flash or any other type of code that the browser may execute. The variety of attacks based on XSS is almost limitless, but they commonly include transmitting private data like cookies or other session information to the attacker, redirecting the victim to web content controlled by the attacker, or performing other malicious operations on the user's machine under the guise of the vulnerable site.

Cross-site scripting attacks can occur wherever an untrusted user has the ability to publish content to a trusted web site. Typically, a malicious user will craft a client-side script, which -- when parsed by a web browser -- performs some activity (such as sending all site cookies to a given E-mail address). If the input is unchecked, this script will be loaded and run by each user visiting the web site. Since the site requesting to run the script has access to the cookies in question, the malicious script does also. There are several other possible attacks, such as running "Active X" controls (under Microsoft Internet Explorer) from sites that a user perceives as trustworthy; cookie theft is however by far the most common. All of these attacks are easily prevented by ensuring that no script tags -- or for good measure, HTML tags at all -- are allowed in data to be posted publicly.

Cross-site scripting attacks may occur anywhere that possibly malicious users are allowed to post unregulated material to a trusted web site for the consumption of other valid users. The most common example can be found in bulletin-board web sites which provide web based mailing list-style functionality.

PLOVER - Cross-site scripting (XSS)

7 Pernicious Kingdoms - Cross-site Scripting

CLASP - Cross-site scripting

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