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CWE-79: Improper Neutralization of Input During Web Page… | Glexia

CWE-79 (Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')) weakness overview with consequences, detection methods, mitigations,…

Release 4.20weaknessStable

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Glexia's Take · Automated analysis

CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Executive Impact

Access Control Confidentiality: Bypass Protection Mechanism Read Application Data: The most common attack performed with cross-site scripting involves the disclosure of private information stored in user cookies, such as session information. Typically, a malicious user will craft a client-side script, which -- when parsed by a web browser -- performs some activity on behalf of the victim to an attacker-controlled system (such as sending all site cookies to a given E-mail address). This could be especially dangerous to the site if the victim has administrator privileges to manage that site. 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.
Integrity Confidentiality Availability: Execute Unauthorized Code or Commands: 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, for example, "drive-by hacking."
Confidentiality Integrity Availability Access Control: Execute Unauthorized Code or Commands Bypass Protection Mechanism Read Application Data: 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. Some 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. Other damaging attacks include the disclosure of end user files, installation of Trojan horse programs, redirecting the user to some other page or site, running "Active X" controls (under Microsoft Internet Explorer) from sites that a user perceives as trustworthy, and modifying presentation of content.

Developer Pattern

CWE-79 is the kind of defect developers can usually prevent with explicit validation, safer framework defaults, and tests that exercise hostile input or unsafe state transitions.

Automation confidence

high confidence from CWE-79, 4.20.

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Official CWE Definition

CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

There are many variants of cross-site scripting, characterized by a variety of terms or involving different attack topologies. However, they all indicate the same fundamental weakness: improper neutralization of dangerous input between the adversary and a victim.

Type: weakness
Abstraction: Base
Status: Stable
Source: MITRE CWE definition

Developer And Remediation Guidance

How teams prevent and detect this weakness

Causes

The following code displays a welcome message on a web page based on the HTTP GET username parameter (covers a Reflected XSS (Type 1) scenario). Because the parameter can be arbitrary, the url of the page could be modified so $username contains scripting syntax, such as This results in a harmless alert dialog popping up. 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. More realistically, the attacker can embed a fake login box on the page, tricking the user into sending the user's password to the attacker: If a user clicks on this link then Welcome.php will generate the following HTML and send it to the user's browser: The trustworthy domain of the URL may falsely assure the user that it is OK to follow the link. However, an astute user may notice the suspicious text appended to the URL. An attacker may further obfuscate the URL (the following example links are broken into multiple lines for readability): The same attack string could also be obfuscated as: Both of these attack links will result in the fake login box appearing on the page, and users are more likely to ignore indecipherable text at the end of URLs.
The following code displays a Reflected XSS (Type 1) scenario. The following JSP code segment reads an employee ID, eid, from an HTTP request and displays it to the user. The following ASP.NET code segment reads an employee ID number from an HTTP request and displays it to the user. The code in this example operates correctly if the Employee ID variable contains only standard alphanumeric text. If it 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.
The following code displays a Stored XSS (Type 2) scenario. The following JSP code segment queries a database for an employee with a given ID and prints the corresponding employee's name. 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. 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.
The following code consists of two separate pages in a web application, one devoted to creating user accounts and another devoted to listing active users currently logged in. It also displays a Stored XSS (Type 2) scenario. CreateUser.php The code is careful to avoid a SQL injection attack (CWE-89) but does not stop valid HTML from being stored in the database. This can be exploited later when ListUsers.php retrieves the information: ListUsers.php The attacker can set their name to be arbitrary HTML, which will then be displayed to all visitors of the Active Users page. This HTML can, for example, be a password stealing Login message.
The following code is a simplistic message board that saves messages in HTML format and appends them to a file. When a new user arrives in the room, it makes an announcement: An attacker may be able to perform an HTML injection (Type 2 XSS) attack by setting a cookie to a value like: The raw contents of the message file would look like: For each person who visits the message page, their browser would execute the script, generating a pop-up window that says "Hacked". More malicious attacks are possible; see the rest of this entry.
The following code attempts to stop XSS attacks by removing all occurences of "script" in an input string. Because the code only checks for the lower-case "script" string, it can be easily defeated with upper-case script tags.

Remediation

Architecture and Design: Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid [REF-1482]. Examples of libraries and frameworks that make it easier to generate properly encoded output include Microsoft's Anti-XSS library, the OWASP ESAPI Encoding module, and Apache Wicket.
Implementation Architecture and Design: Understand the context in which your data will be used and the encoding that will be expected. This is especially important when transmitting data between different components, or when generating outputs that can contain multiple encodings at the same time, such as web pages or multi-part mail messages. Study all expected communication protocols and data representations to determine the required encoding strategies. For any data that will be output to another web page, especially any data that was received from external inputs, use the appropriate encoding on all non-alphanumeric characters. Parts of the same output document may require different encodings, which will vary depending on whether the output is in the: HTML body Element attributes (such as src="XYZ") URIs JavaScript sections Cascading Style Sheets and style property etc. Note that HTML Entity Encoding is only appropriate for the HTML body. Consult the XSS Prevention Cheat Sheet [REF-724] for more details on the types of encoding and escaping that are needed.
Architecture and Design Implementation: Understand all the potential areas where untrusted inputs can enter your software: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide data to the application. Remember that such inputs may be obtained indirectly through API calls.
Architecture and Design: For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
Architecture and Design: If available, use structured mechanisms that automatically enforce the separation between data and code. These mechanisms may be able to provide the relevant quoting, encoding, and validation automatically, instead of relying on the developer to provide this capability at every point where output is generated.
Implementation: Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component. The problem of inconsistent output encodings often arises in web pages. If an encoding is not specified in an HTTP header, web browsers often guess about which encoding is being used. This can open up the browser to subtle XSS attacks.
Implementation: With Struts, write all data from form beans with the bean's filter attribute set to true.
Implementation: 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 more recent versions of Internet Explorer and Firefox), this attribute can prevent the user's session cookie from being accessible to malicious client-side scripts that use document.cookie. This is not a complete solution, since HttpOnly is not supported by all browsers. More importantly, XmlHttpRequest and other powerful browser technologies provide read access to HTTP headers, including the Set-Cookie header in which the HttpOnly flag is set.

Detection

Automated Static Analysis: Use automated static analysis tools that target this type of weakness. Many modern techniques use data flow analysis to minimize the number of false positives. This is not a perfect solution, since 100% accuracy and coverage are not feasible, especially when multiple components are involved.
Black Box: Use the XSS Cheat Sheet [REF-714] or automated test-generation tools to help launch a wide variety of attacks against your web application. The Cheat Sheet contains many subtle XSS variations that are specifically targeted against weak XSS defenses.

Mappings

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ATT&CK Relevance

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