CVE-2026-121134 in Chrome
Summary
by MITRE • 11/03/2026
Information loss in BackgroundFetch in Google Chrome prior to 157.0.8103.38 allowed a remote attacker to bypass web origin policy 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 information loss flaw within the Background Fetch API of Google Chrome, specifically in versions prior to 157.0.8103.38, represents a significant deviation from expected web isolation principles. The Background Fetch API is designed to allow websites to initiate long-running downloads that continue even if the user navigates away from or closes the page hosting the script. This functionality is critical for applications requiring reliable data synchronization in low-connectivity environments or when users are not actively interacting with the browser interface. However, the security severity of this issue was classified as Low by the Chromium team because it primarily affects origin isolation rather than leading to direct code execution or full system compromise. The core technical flaw lies in how the background fetch tasks handle and retain references to resources associated with their initiating context. When a crafted HTML page initiates a background fetch, the browser fails to properly segregate the resulting data from the originating web origin's scope under certain conditions. This misconfiguration allows the fetched content to be accessible or interpretable by contexts that should not have access to it based on standard Same-Origin Policy rules.
From a technical perspective, this vulnerability exploits the asynchronous nature of background fetch operations and the way they interact with storage mechanisms such as IndexedDB or service worker caches. In normal operation, data retrieved via Background Fetch is scoped strictly to the origin that initiated the request. This scoping ensures that one website cannot read sensitive data fetched by another site running in a different tab or window. The flaw arises when specific edge cases involving cross-origin requests or complex navigation sequences occur during the fetch lifecycle. A remote attacker can craft an HTML page containing malicious JavaScript that leverages these edge cases to trigger a background fetch operation. By carefully manipulating the request headers, timing, and subsequent interactions with the browser's storage APIs, the attacker creates a scenario where the fetched data leaks out of its intended origin boundary. This leakage does not necessarily expose raw binary data in an easily readable format but rather allows for the exfiltration or unauthorized access to information that was meant to be isolated within the victim site's sandboxed environment.
The operational impact of this vulnerability centers on the potential breach of confidentiality and integrity for web applications relying heavily on background synchronization features. While a remote attacker cannot directly execute arbitrary code on the user's machine, they can potentially read sensitive data such as authentication tokens, personal identifiers, or proprietary content that was fetched by a victim site in the background. This undermines the trust model of the modern web, where users expect strict isolation between different websites to prevent cross-site scripting and data theft attacks. For enterprise environments using Chrome for business applications, this could lead to unauthorized access to internal resources if those resources are accessed via Background Fetch APIs without sufficient additional validation layers. The low severity rating reflects the difficulty in exploiting this flaw reliably across all browser versions and configurations, as well as the limited scope of damage compared to high-severity vulnerabilities like remote code execution or privilege escalation.
Mitigation strategies for this vulnerability primarily involve updating Google Chrome to version 157.0.8103.38 or later, where the Chromium team has implemented fixes to ensure proper origin scoping during background fetch operations. Developers utilizing the Background Fetch API should also review their implementations to ensure they are not relying solely on browser-level isolation for sensitive data protection. Implementing additional server-side validation and token-based authentication checks can provide defense in depth against potential information leakage. Furthermore, organizations should enforce strict Content Security Policy directives that limit which origins can initiate background fetch operations and restrict the types of resources that can be fetched. Regular security audits focusing on service worker implementations and storage API usage are recommended to identify similar isolation weaknesses before they can be exploited by malicious actors aiming to bypass web origin policies through crafted HTML pages or automated scripts targeting known browser vulnerabilities.
This incident aligns with Common Weakness Enumeration CWE-200, which covers Information Exposure, specifically where sensitive information is disclosed due to improper access control mechanisms within the application logic or underlying framework. In terms of offensive security frameworks such as MITRE ATT&CK, this vulnerability facilitates techniques related to Collection and Exfiltration over Alternative Protocol, allowing attackers to gather data from web applications in a manner that may evade traditional network monitoring tools designed for standard HTTP traffic patterns. The ability to bypass origin policies through background processes highlights the ongoing challenge of maintaining strict security boundaries in complex asynchronous browser environments where multiple contexts interact simultaneously.