CVE-2026-78903 in Chrome
Summary
by MITRE • 08/26/2026
Incomplete cleanup in SiteIsolation in Google Chrome prior to 152.0.7977.65 allowed a remote attacker who had compromised the renderer process to bypass site isolation via a crafted HTML page. (Chromium security severity: Medium)
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Analysis
by VulDB Data Team • 08/26/2026
The vulnerability identified as an incomplete cleanup issue within the SiteIsolation mechanism of Google Chrome prior to version 152.0.7977.65 represents a significant breach in browser-based isolation policies. Site Isolation is a critical security feature designed to prevent cross-site scripting attacks and data theft by ensuring that each site runs in its own separate process, thereby preventing malicious scripts from accessing sensitive information belonging to other sites. The flaw arises when the cleanup procedures associated with this isolation mechanism fail to properly release or reset specific resources after a page navigation or context switch occurs. This oversight creates a window of opportunity where stateful data intended for one origin may inadvertently persist in memory accessible by another, effectively weakening the boundaries that Site Isolation is meant to enforce.
From a technical perspective, the core issue lies in the lifecycle management of renderer processes and their associated security contexts. When a user navigates between different origins or when tabs are closed and reopened under certain conditions, the browser fails to fully sanitize the internal state related to site isolation checks. This allows an attacker who has already achieved code execution within a compromised renderer process through other means, such as exploiting a separate vulnerability in web content rendering, to leverage this residual state. By crafting specific HTML pages that trigger these incomplete cleanup routines, the attacker can manipulate the browser's perception of which origin is currently active or authorized for certain operations. This manipulation enables the bypassing of site isolation boundaries, allowing data from one site to be read by another, violating the principle of same-origin policy enforcement at a process level.
The operational impact of this vulnerability is severe in scenarios where an attacker has already gained foothold within a renderer process. While the Chromium security severity is rated as Medium, indicating that it requires specific conditions or additional steps to exploit fully, the consequence of success is high. An attacker can potentially exfiltrate sensitive user data such as session cookies, authentication tokens, personal identifiable information, and other confidential content from sites visited by the same browser instance. This undermines the fundamental trust model of web browsing, where users expect that visiting a malicious site will not compromise their accounts on banking or email services running in separate tabs. The ability to bypass site isolation effectively neutralizes one of Chrome's most robust defenses against cross-site data theft and privilege escalation attacks within the browser environment.
This vulnerability aligns with CWE-250, which describes execution with unnecessary privileges, as it allows a process that should be restricted by its origin context to perform actions or access resources beyond its intended scope due to improper state management. Furthermore, in terms of offensive security frameworks like MITRE ATT&CK for Enterprise, this behavior relates to techniques involving browser-based attacks and privilege escalation within the client software environment. The exploitation vector involves crafting malicious web content that triggers the flawed cleanup logic, which falls under categories such as Client-side Execution or potentially Data Exfiltration from Local Systems if successful data access is achieved through the isolation bypass.
Mitigation for this vulnerability primarily relies on updating Google Chrome to version 152.0.7977.65 or later, where these cleanup routines have been corrected to ensure that all site-specific state is properly cleared upon navigation or context changes. Administrators and end-users should enable automatic updates to ensure timely patching of such browser vulnerabilities. Additionally, defense-in-depth strategies remain crucial; users should employ extensions that enforce strict content security policies, avoid visiting untrusted websites whenever possible, and utilize multi-process isolation features where available. Security researchers and developers reviewing similar implementations in other browsers or web engines should audit their own cleanup procedures for cross-origin data handling to prevent analogous state leakage issues that could lead to site isolation bypasses.