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Description
The software 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.
Modes of Introduction:
– Architecture and Design
Likelihood of Exploit: High
Related Weaknesses
CWE-74
CWE-74
CWE-494
CWE-352
Consequences
Access Control, Confidentiality: Bypass Protection Mechanism, Read Application Data
The most common attack performed with cross-site scripting involves the disclosure of information stored in user cookies. 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). 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.
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.
Potential Mitigations
Phase: Architecture and Design
Effectiveness:
Description:
Phase: Implementation, Architecture and Design
Effectiveness:
Description:
Phase: Architecture and Design, Implementation
Effectiveness: Limited
Description:
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.
This technique has limited effectiveness, but can be helpful when it is possible to store client state and sensitive information on the server side instead of in cookies, headers, hidden form fields, etc.
Phase: Architecture and Design
Effectiveness:
Description:
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.
Phase: Architecture and Design
Effectiveness:
Description:
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.
Phase: Implementation
Effectiveness:
Description:
Phase: Implementation
Effectiveness:
Description:
With Struts, write all data from form beans with the bean’s filter attribute set to true.
Phase: Implementation
Effectiveness: Defense in Depth
Description:
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.
Phase: Implementation
Effectiveness:
Description:
Phase: Architecture and Design
Effectiveness:
Description:
When the set of acceptable objects, such as filenames or URLs, is limited or known, create a mapping from a set of fixed input values (such as numeric IDs) to the actual filenames or URLs, and reject all other inputs.
Phase: Operation
Effectiveness: Moderate
Description:
Use an application firewall that can detect attacks against this weakness. It can be beneficial in cases in which the code cannot be fixed (because it is controlled by a third party), as an emergency prevention measure while more comprehensive software assurance measures are applied, or to provide defense in depth.
An application firewall might not cover all possible input vectors. In addition, attack techniques might be available to bypass the protection mechanism, such as using malformed inputs that can still be processed by the component that receives those inputs. Depending on functionality, an application firewall might inadvertently reject or modify legitimate requests. Finally, some manual effort may be required for customization.
Phase: Operation, Implementation
Effectiveness:
Description:
When using PHP, configure the application so that it does not use register_globals. During implementation, develop the application so that it does not rely on this feature, but be wary of implementing a register_globals emulation that is subject to weaknesses such as CWE-95, CWE-621, and similar issues.
CVE References
- CVE-2017-9764
- Web stats program allows XSS through crafted HTTP header.
- CVE-2014-5198
- Web log analysis product allows XSS through crafted HTTP Referer header.
- CVE-2006-4308
- Chain: incomplete denylist (CWE-184) only checks “javascript:” tag, allowing XSS (CWE-79) using other tags
- CVE-2007-5727
- Chain: incomplete denylist (CWE-184) only removes SCRIPT tags, enabling XSS (CWE-79)
- CVE-2008-4730
- Reflected XSS not properly handled when generating an error message
- CVE-2006-3211
- Stored XSS in a guestbook application using a javascript: URI in a bbcode img tag.
- CVE-2006-3295
- Chain: library file is not protected against a direct request (CWE-425), leading to reflected XSS (CWE-79).
