Impact
An attacker can steal sensitive, authenticated data from users of a vulnerable application [S2]. If a user visits a malicious website while logged into the vulnerable app, the malicious site can make cross-origin requests to the app's API and read the responses [S1][S2]. This can lead to the theft of private information, including user profiles, CSRF tokens, or private messages [S2].
Root Cause
CORS is an HTTP-header based mechanism that allows servers to specify which origins (domain, scheme, or port) are permitted to load resources [S1]. Vulnerabilities typically arise when a server's CORS policy is too flexible or poorly implemented [S2]:
- Reflected Origin Header: Some servers read the
Originheader from a client request and echo it back in theAccess-Control-Allow-Origin(ACAO) response header [S2]. This effectively allows any website to access the resource [S2]. - Misconfigured Wildcards: While the
*wildcard allows any origin to access a resource, it cannot be used for requests that require credentials (like cookies or Authorization headers) [S3]. Developers often try to bypass this by dynamically generating the ACAO header based on the request [S2]. - Whitelisting 'null': Some applications whitelist the
nullorigin, which can be triggered by redirected requests or local files, allowing malicious sites to masquerade as anullorigin to gain access [S2][S3]. - Parsing Errors: Mistakes in regex or string matching when validating the
Originheader can allow attackers to use domains liketrusted-domain.com.attacker.com[S2].
It is important to note that CORS is not a protection against Cross-Site Request Forgery (CSRF) [S2].
Concrete Fixes
- Use a Static Whitelist: Avoid dynamically generating the
Access-Control-Allow-Originheader from the request'sOriginheader [S2]. Instead, compare the request's origin against a hardcoded list of trusted domains [S3]. - Avoid the 'null' Origin: Never include
nullin your whitelist of allowed origins [S2]. - Restrict Credentials: Only set
Access-Control-Allow-Credentials: trueif absolutely necessary for the specific cross-origin interaction [S3]. - Use Proper Validation: If you must support multiple origins, ensure the validation logic for the
Originheader is robust and cannot be bypassed by subdomains or similar-looking domains [S2].
How FixVibe tests for it
FixVibe now includes this as a gated active check. After domain verification, active.cors sends same-origin API requests with a synthetic attacker origin and reviews CORS response headers. It reports reflected arbitrary origins, wildcard credentialed CORS, and wide-open CORS on non-public API endpoints while avoiding public asset noise.