Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
CWE-942 Variant Incomplete
Permissive Cross-domain Security Policy with Untrusted Domains
This vulnerability occurs when a web application's cross-domain security policy, like a Content Security Policy (CSP), explicitly allows communication with untrusted or overly permissive external…
Definition
What is CWE-942?
This vulnerability occurs when a web application's cross-domain security policy, like a Content Security Policy (CSP), explicitly allows communication with untrusted or overly permissive external domains.
A permissive cross-domain policy undermines a key web security control. By listing untrusted domains or using overly broad wildcards (e.g., *.example.com), you grant those external sites the ability to interact with your application's data and user session, effectively inviting potential attackers into a trusted context.
Attackers hosted on these permitted domains can often launch exploits, such as data theft or session hijacking, without any visible warning to the end user. This makes the vulnerability particularly dangerous, as a compromise can occur silently during normal browsing, bypassing the intended protections of the security policy.
Real-world impact
Real-world CVEs caused by CWE-942
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Product has a Silverlight cross-domain policy that does not restrict access to another application, which allows remote attackers to bypass the Same Origin Policy.
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The default Flash Cross Domain policies in a product allows remote attackers to access user files.
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Chain: Adobe Flash Player does not sufficiently restrict the interpretation and usage of cross-domain policy files, which makes it easier for remote attackers to conduct cross-domain and cross-site scripting (XSS) attacks.
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Chain: Adobe Flash Player and earlier does not properly interpret policy files, which allows remote attackers to bypass a non-root domain policy.
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Chain: Adobe Flash Player does not properly handle unspecified encodings during the parsing of a cross-domain policy file, which allows remote web servers to bypass intended access restrictions via unknown vectors.
How attackers exploit it
Step-by-step attacker path
- 1
These cross-domain policy files mean to allow Flash and Silverlight applications hosted on other domains to access its data:
- 2
Flash crossdomain.xml :
- 3
Silverlight clientaccesspolicy.xml :
- 4
These entries are far too permissive, allowing any Flash or Silverlight application to send requests. A malicious application hosted on any other web site will be able to send requests on behalf of any user tricked into executing it.
Vulnerable code example
Vulnerable XML
Flash crossdomain.xml :
<cross-domain-policy xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="http://www.adobe.com/xml/schemas/PolicyFile.xsd">
<allow-access-from domain="*.example.com"/>
<allow-access-from domain="*"/>
</cross-domain-policy> Secure code example
Secure pseudo
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
const safe = validateAndEscape(input);
return executeWithGuards(safe);
} What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist
How to prevent CWE-942
- Architecture and Design / Operation Define a restrictive Content Security Policy [REF-1486] or cross-domain policy file.
- Architecture and Design / Operation Avoid using wildcards in the CSP / cross-domain policy file. Any domain matching the wildcard expression will be implicitly trusted, and can perform two-way interaction with the target server.
- Architecture and Design / Operation For Flash, modify crossdomain.xml to use meta-policy options such as 'master-only' or 'none' to reduce the possibility of an attacker planting extraneous cross-domain policy files on a server.
Detection signals
How to detect CWE-942
CWE-942
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Frequently asked questions What is CWE-942?
How serious is CWE-942?
What languages or platforms are affected by CWE-942?
How can I prevent CWE-942?
How does Plexicus detect and fix CWE-942?
Where can I learn more about CWE-942?
Frequently asked questions
What is CWE-942?
This vulnerability occurs when a web application's cross-domain security policy, like a Content Security Policy (CSP), explicitly allows communication with untrusted or overly permissive external domains.
How serious is CWE-942?
MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.
What languages or platforms are affected by CWE-942?
MITRE lists the following affected platforms: Web Based.
How can I prevent CWE-942?
Define a restrictive Content Security Policy [REF-1486] or cross-domain policy file. Avoid using wildcards in the CSP / cross-domain policy file. Any domain matching the wildcard expression will be implicitly trusted, and can perform two-way interaction with the target server.
How does Plexicus detect and fix CWE-942?
Plexicus's SAST engine matches the data-flow signature for CWE-942 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.
Where can I learn more about CWE-942?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/942.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-942
CWE-863 Parent
Incorrect Authorization
This vulnerability occurs when an application checks if a user is allowed to perform an action or access data, but the check is flawed or…
CWE-1244 Sibling
Internal Asset Exposed to Unsafe Debug Access Level or State
This vulnerability occurs when a system's debug or test interface supports multiple access levels, but an internal asset is incorrectly…
CWE-551 Sibling
Incorrect Behavior Order: Authorization Before Parsing and Canonicalization
This vulnerability occurs when a web server checks access permissions before fully processing and normalizing a URL, potentially allowing…
CWE-639 Sibling
Authorization Bypass Through User-Controlled Key
This vulnerability occurs when an application's authorization system fails to verify that a user is allowed to access specific data before…
CWE-647 Sibling
Use of Non-Canonical URL Paths for Authorization Decisions
This vulnerability occurs when an application's authorization logic relies on specific URL paths but fails to enforce a single,…
CWE-804 Sibling
Guessable CAPTCHA
This vulnerability occurs when a CAPTCHA challenge is too easy for automated bots to solve, either by guessing or using pattern…
CWE-668 Can precede
Exposure of Resource to Wrong Sphere
This vulnerability occurs when an application unintentionally makes a resource accessible to users or systems that should not have…
Resources
Further reading
- MITRE — official CWE-942 https://cwe.mitre.org/data/definitions/942.html
- Content Security Policy Cheat Sheet https://cheatsheetseries.owasp.org/cheatsheets/Content_Security_Policy_Cheat_Sheet.html
- Setting a crossdomain.xml file for HTTP streaming https://web.archive.org/web/20121124184922/http://www.adobe.com/devnet/adobe-media-server/articles/cross-domain-xml-for-streaming.html
- Cross-domain policy for Flash movies http://kb2.adobe.com/cps/142/tn_14213.html
- HTTP Communication and Security with Silverlight https://learn.microsoft.com/en-us/previous-versions/windows/silverlight/dotnet-windows-silverlight/cc838250(v=vs.95)?redirectedfrom=MSDN
- Network Security Access Restrictions in Silverlight https://learn.microsoft.com/en-us/previous-versions/windows/silverlight/dotnet-windows-silverlight/cc645032(v=vs.95)
- Analyzing the Crossdomain Policies of Flash Applications http://cseweb.ucsd.edu/~hovav/dist/crossdomain.pdf
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