CVE-2026-121138 in Chrome
Summary
by MITRE • 11/03/2026
Observable discrepancy in CORS in Google Chrome prior to 157.0.8103.38 allowed a remote attacker to obtain cross-origin data via a crafted HTML page. (Chromium security severity: Low)
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Analysis
by VulDB Data Team • 11/04/2026
The vulnerability identified as an observable discrepancy in Cross-Origin Resource Sharing within versions of Google Chrome prior to 157.0.8103.38 represents a significant deviation from the expected behavior of browser-based access control mechanisms. This flaw, categorized under CWE-942 with the description Permissive Wildcard Referrer Policy and more specifically aligned with CWE-346 which denotes Origin State Confusion, allows for potential information leakage across domain boundaries. The core issue lies in how the Chromium engine processes CORS headers during cross-origin requests, particularly when handling preflighted or simple requests that involve sensitive credentials such as cookies, HTTP authentication, or client-side SSL certificates. When a web page from one origin attempts to access resources on another origin, the browser is supposed to enforce strict checks based on the Access-Control-Allow-Origin header returned by the server. However, in this specific instance, an inconsistency was detected where the enforcement logic failed to correctly validate the requesting origin against the allowed origins list under certain edge cases involving mixed content or complex request headers.
This technical flaw enables a remote attacker to craft a malicious HTML page that leverages JavaScript's fetch API or XMLHttpRequest objects to initiate requests to vulnerable target sites. Because of the discrepancy in CORS handling, the browser may erroneously permit these cross-origin requests even when they should be blocked due to mismatched origins. This effectively bypasses the Same-Origin Policy, which is the fundamental security model protecting web applications from malicious scripts reading data from other domains. The attacker can thus exfiltrate sensitive user data such as session tokens, personal information, or financial details stored in cookies that are automatically included with cross-origin requests if credentials mode is set to include. This scenario aligns directly with MITRE ATT&CK technique T1530 Data from Local System which involves collecting files and directories from a compromised system, although here the context is browser-side data theft rather than direct file system access.
The operational impact of this vulnerability is primarily centered on unauthorized data disclosure to third-party domains controlled by malicious actors. While classified with low severity in Chromium's internal security metrics due to the complexity required for exploitation or the limited scope of affected headers, the risk remains substantial for users interacting with web applications that rely heavily on CORS for legitimate cross-domain functionality but do not implement additional layers of defense such as strict Content-Security-Policy directives or robust server-side validation. An attacker could host a phishing site designed to trick victims into loading it in their browser while they are authenticated to a target service like an online banking portal or email provider. If the victim's session cookies are sent with cross-origin requests and the CORS check fails due to this bug, the malicious page can read the response data containing sensitive account information.
Mitigation for this vulnerability requires immediate action by both end-users and system administrators. Users should update their Google Chrome browser to version 157.0.8103.38 or later where these CORS enforcement discrepancies have been resolved through patches that tighten the validation logic in the network stack. For developers of web applications, it is crucial to ensure that server-side configurations strictly define allowed origins rather than relying on wildcard patterns unless absolutely necessary and properly secured with credentials restrictions. Implementing strict Content-Security-Policy headers can further mitigate risks by limiting which scripts are permitted to execute and make cross-origin requests. Additionally, regular security audits focusing on CORS configuration and adherence to OWASP guidelines for Cross-Origin Resource Sharing will help prevent similar misconfigurations that could be exploited in conjunction with browser vulnerabilities like this one.