CVE-2026-78941

Executive Summary

CVE-2026-78941 is a medium-severity information leak vulnerability in Google Chrome's Core component, affecting all versions prior to 152.0.7977.65. The flaw allows a remote attacker who has already compromised the renderer process to bypass Chrome's Site Isolation security mechanism by serving a crafted HTML page to the victim; the attack succeeds when the victim navigates to that page. Successful exploitation results in unauthorized disclosure of data that should be isolated by origin or site boundaries, potentially exposing cross-site information to the already-compromised renderer. The vulnerability is addressed in Chrome 152.0.7977.65, and although no public exploit code or PoC has been observed, the attack surface is significant given Chrome's ubiquitous deployment. Patch confidence is rated 2/4 (CLAIMED), meaning the fix is asserted by a single source (NVD) and is not yet independently confirmed by a second vendor advisory or reproducible test. The CVE is not listed in CISA's Known Exploited Vulnerabilities catalog, and no EPSS score has been assigned to date.

Technical Analysis

The NVD description defines this vulnerability as an information leak in the "Core" module of Google Chrome. The Core component encompasses the browser's foundational subsystems — including the browser process, feature registration, and the primary process boundary management logic. The vulnerability specifically permits a remote attacker who has already compromised the renderer process to bypass Site Isolation protections by delivering a crafted HTML page.

Vulnerability Mechanism. Site Isolation in Chrome is a security architecture that places pages from different sites (eTLD+1) into separate renderer processes, such that a compromised renderer — which may contain a memory corruption bug or a logic error — cannot read or exfiltrate data belonging to a different origin. The vulnerability described here defeats that boundary at the information-disclosure level. The attack flow is as follows:

1. Precondition — Renderer Compromise: The attacker first exploits a separate, likely memory-corruption vulnerability in the renderer process. This gives the attacker the ability to execute code within a sandboxed renderer context and intercept or forge messages that the renderer sends to the browser process.
2. Crafted HTML Page: The attacker crafts an HTML page designed to trigger the information leak. The exact mechanism — whether it relies on a specific DOM API, a race condition, a service worker interaction, or a navigation timing side channel — is not publicly disclosed in the NVD description.
3. Bypass of Site Isolation: Because the attacker controls renderer-side execution, the crafted page manipulates the browser's internal state such that data from a cross-site page is readable within the compromised renderer's context, thereby bypassing the Site Isolation boundary. The result is that data that should be compartmentalized and protected from a compromised renderer becomes accessible.

Attack Surface. The attack surface is constrained by two factors. First, the attacker must already have achieved code execution in the renderer process — this is typically achieved through a separate vulnerability (e.g., a use-after-free in a JavaScript engine or a bug in a Chromium component). Second, the victim must be induced to load the crafted HTML page, either by direct navigation or via a link-click. The information leak is confined to the browser's in-memory data accessible to the renderer; the NVD description does not indicate that filesystem access, credential theft, or network-level data exfiltration occurs. Rather, the impact is the compromise of cross-origin data confidentiality within the browser session.

Code-Level Details. The NVD description does not enumerate specific function names, classes, or API endpoints for this vulnerability. The absence of internal details in public advisories is consistent with Chromium's routine practice of withholding technical specifics until a majority of users have updated. For a vulnerability class of this nature, the underlying defect likely resides in the browser's process-isolation enforcement logic — most plausibly in how navigation requests are validated or how cross-origin responses are admitted to a renderer's memory space. Without access to the patch diff, no further code-level assertions can be made with confidence.

CWE Classification. The vulnerability maps to CWE-200: Exposure of Sensitive Information to an Unauthorized Actor. In this context, the "unauthorized actor" is the compromised renderer process, and the "sensitive information" is data belonging to a different origin or site that would normally be protected by Site Isolation.

Exploitation Scenarios

The following attack chain is inferred from the vulnerability class and the NVD description; no public PoC is available, and no specific implementation details are disclosed. Consequently, technical assertions are made at the class level.

Step 1 — Reconnaissance / Target Identification. The attacker identifies a target who uses a vulnerable version of Google Chrome (i.e., prior to 152.0.7977.65). Target selection may be driven by high-value objectives such as corporate credentials, personal financial data, or session tokens. The attacker may profile browser versions through fingerprinting services (e.g., User-Agent strings, client-hints) or by enticing the target to visit a benign-looking page that reports its own browser version back to the attacker.

Step 2 — Triggering the Flaw. The attacker must first achieve code execution inside the renderer process. This is accomplished by exploiting a separate renderer-side vulnerability (e.g., a memory-corruption bug in a JavaScript engine or a DOM implementation). Once renderer code execution is attained, the attacker's payload crafts an HTML page — or modifies the response of a page the target is viewing — to invoke the specific browser behavior that leaks cross-site data. Because the NVD description identifies "crafted HTML page" as the trigger, the leak is most plausibly triggered by the manipulation of DOM properties, navigation states, or resource loading directives that confuse the Site Isolation enforcement logic.

Step 3 — Payload / Execution Method. At the vulnerability-class level, the payload is an HTML document that contains one or more elements designed to force the browser to expose in-memory data from another origin — for instance, by requesting a cross-site resource and observing metadata (headers, timing, rendering behavior) or by abusing a service worker or a shared worker to read cross-origin data. The attacker's renderer-side payload then reads the leaked data and exfiltrates it to an attacker-controlled server via an HTTP request or WebSocket connection.

Step 4 — Post-Exploitation Impact. The leaked data is a subset of what is held in memory at the time of exploitation — typically rendering state, response bodies, or tokens associated with other open tabs. The confidentiality of currently-open sites is the primary loss. After exfiltration, the attacker may use the harvested tokens or session identifiers to impersonate the user against other applications, pivot to lateral movement if the data includes internal network details, or update the attack against other targets. Because the leak occurs in the browser session, the impact scope is limited to the user's active sessions and is non-persistent across browser restarts.

Impact Assessment

CVSS Score. No Common Vulnerability Scoring System (CVSS) vector or score is assigned for this CVE. The Chromium security team rates this bug as "Medium" severity (as stated in the NVD description), reflecting a meaningful but bounded security impact. The severity is moderated by the prerequisite that the attacker must first compromise the renderer process — a separate, typically high-severity vulnerability. The Medium rating indicates that the leak is not remotely exploitable against an unmodified browser, and that the confidentiality impact is limited to data already present in browser memory. The absence of assigned CVSS subscores precludes granular scoring of attack complexity, user interaction, or impact metrics; analysts should treat the Medium severity as the authoritative qualitative risk rating.

Affected Systems. All deployments of Google Chrome prior to version 152.0.7977.65 are affected. Given Chrome's status as the dominant web browser across Windows, macOS, Linux, Android, and ChromeOS, the vulnerability has a broad potential footprint. Chromium-based browsers that embed the same Core components — however, their inclusion is inferred and not explicitly confirmed by the NVD description — may also be affected if they fail to track upstream patches. Enterprise environments that manage Chrome through group policies or third-party patch management software should verify that the version pushed to endpoints is at least 152.0.7977.65.

Blast Radius. The blast radius is bounded by the following: (1) an attacker must already possess a renderer-code-execution primitive, (2) the leak is confined to in-browser session data, and (3) the impact is limited to confidentiality — not integrity or availability. In practice, the most significant business risk is the exposure of enterprise session tokens, internal web application data, or credentials stored in browser memory. A successful exploitation could lead to account takeovers or data breaches in downstream applications. However, the lack of observed in-the-wild exploitation, absence from CISA KEV, and unavailability of a PoC lower the immediate operational risk.

Detection Guidance

The following detection rules are designed to identify attempts to exploit CVE-2026-78941. Given the vulnerability's nature (in-browser information leak), network-level detection is challenging because the exploit trigger is a benign-looking HTML page, and the exfiltration occurs over ordinary HTTPS connections. Detection should therefore focus on anomalous renderer behavior, unusual navigation patterns, and the presence of known-vulnerable browser versions on the network. All rules below are generic and should be tuned to the environment's baseline behavior.

Sigma Rule

title: Chrome Renderer Compromise and Site Isolation Bypass Attempt
id: 27bbdf24-9a2b-4f6f-8e3d-1a0a4f6c1b2f
status: experimental
description: Detects browser telemetry indicating a renderer process accessing cross-origin data, consistent with a Site Isolation bypass (CVE-2026-78941).
author: Valtersi Threat Intelligence
date: 2026/01/01
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-78941
logsource:
    category: process_creation
    product: windows
    definition: Requires Chrome telemetry or process events exported to Sysmon or EDR.
detection:
    selection_weakened_isolation:
        Image|endswith: '\\chrome.exe'
        CommandLine|contains:
            - '--disable-site-isolation-trials'
            - '--no-site-isolation'
            - '--site-per-process'
    selection_compromised_renderer:
        Image|endswith: '\\chrome.exe'
        ParentImage|endswith: '\\chrome.exe'
        CommandLine|contains|all:
            - '--renderer'
            - '--enable-crash-reporter'
        CommandLine|contains:
            - '--allow-file-access-from-files'
            - '--disable-web-security'
    condition: selection_weakened_isolation or selection_compromised_renderer
falsepositives:
    - Advanced Chrome flags used by developers or QA engineers for specific functional testing.
level: high

YARA Rule

rule CHROME_SITE_ISOLATION_BYPASS_HTML
{
    meta:
        author = "Valtersi Threat Intelligence"
        description = "Detects HTML pages crafted to trigger a Site Isolation bypass in Chrome (CVE-2026-78941)."
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-78941"
        date = "2026-01-01"
        severity = "medium"
    strings:
        $xhr_cross = /new XMLHttpRequest\(\)\s*;\s*(?:xhr\.)?(?:open|send)/ nocase
        $fetch_promise = /fetch\([^)]*\)\s*\.then\s*\(/ nocase
        $iframe_dynamic = /<iframe[^>]+(?:srcdoc|src)\s*=/ nocase
        $message_posting = /postMessage\s*\(/ nocase
        $blob_url = /URL\.createObjectURL\s*\(/ nocase
        $xs_test = /XSLTProcessor|DOMParser|XMLSerializer/ nocase
        $leak_x = /\.responseText\s*[=!]=/ nocase
    condition:
        (uint16(0) == 0x3C3C or uint16(0) == 0x0D0A or uint16(0) == 0x203C)  // HTML file signature
        and 2 of ($xhr_cross, $fetch_promise, $iframe_dynamic, $message_posting)
        and (1 of ($blob_url, $xs_test, $leak_x))
}

Suricata IDS Rule

alert http any any -> any any (msg:"Chrome Site Isolation Bypass - Suspicious HTML Content (CVE-2026-78941)"; flow:to_client,established; content:"<html"; http_client_body; pcre:"/(?i)(fetch\(|XMLHttpRequest|postMessage|createObjectURL)/"; classtype:attempted-information-leak; reference:url,https://nvd.nist.gov/vuln/detail/CVE-2026-78941; sid:202608941; rev:1;)

Elastic Detection

// EQL rule to detect Chrome renderer processes with flags that weaken Site Isolation,
// suggesting an active attempt to exploit CVE-2026-78941.
// Place this in the EQL search bar with an appropriate index pattern.
any where process.name == "chrome.exe" and
  (
    process.args in ("--disable-site-isolation-trials", "--no-site-isolation",
                     "--disable-web-security", "--allow-file-access-from-files")
    and process.args == "--renderer"
  )

Splunk SPL Query

index=endpoint earliest=-7d latest=now
| search (Image="*chrome.exe" AND CommandLine="*--renderer*")
    OR (Image="*chrome.exe" AND (CommandLine="*--disable-site-isolation-trials*"
        OR CommandLine="*--no-site-isolation*"
        OR CommandLine="*--disable-web-security*"))
| stats count by _time, host, User, Image, CommandLine, ParentImage
| where count > 5
| table _time, host, User, Image, CommandLine, ParentImage, count
| sort - _time
| rename _time as timestamp

Wazuh OSSEC Rule

<group name="google_chrome,">
  <rule id="100941" level="12">
    <if_sid>80700</if_sid>
    <field name="win.eventdata.image" type="pcre2">(?i)chrome\.exe</field>
    <field name="win.eventdata.commandLine" type="pcre2">(?i)--(disable-site-isolation-trials|no-site-isolation|disable-web-security|allow-file-access-from-files).*--renderer</field>
    <description>Chrome renderer launched with flags disabling Site Isolation - potential CVE-2026-78941 exploitation.</description>
    <group>vulnerability,information_leak,chrome,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence Assessment. The patch is rated 2/4 — CLAIMED. A single authoritative source (NVD) reports that the fix is in version 152.0.7977.65. However, this has not yet been corroborated by a second independent source (e.g., an official Chrome Releases blog post, an advisory from Google, or a mirrored advisory from another trusted entity). Chromium's security fix process typically withholds exact patch details until a majority of users update. As of this report, no Chrome Releases advisory specific to CVE-2026-78941 has been observed, and no second source has confirmed the version number. Analysts should treat the patch as likely-correct but should independently verify the availability of update 152.0.7977.65 in their environment before declaring remediation complete.

1) Immediate Mitigations (prior to patching).

• Update Chrome immediately: The only complete mitigation is to upgrade to Chrome 152.0.7977.65 or later. Test in a pilot group and then push via your patch management tool (SCCM, Intune, Google Admin Console, etc.).
• Disable or restrict auto-fill of credentials: Reducing the sensitive data in browser memory limits the impact if a renderer is compromised.
• Limit extension use: Disable unused extensions; the renderer code-execution prerequisite for this vulnerability is often introduced through malicious or vulnerable extensions.
• Enable network segmentation for high-value users: Place users with access to critical internal applications into a separate VLAN with stricter egress filtering to reduce exfiltration bandwidth.

2) Patch / Upgrade Steps.

• Google Admin Console (managed Chrome): Set the Chrome Update policy to force-update all managed browsers to version 152.0.7977.65 or later. Verify rollout with the "Google Update" status report.
• End-User Manual Update: Direct users to chrome://settings/help. Chrome will download and stage the update; a browser restart is required to apply it.
• Unmanaged/BYO devices: Publish awareness to end users and enforce a minimum-browser-version policy via conditional access on corporate web applications.
• Verification: After rollout, use device management or telemetry to confirm 100% of Chrome instances are running ≥ 152.0.7977.65. Pay particular attention to Windows and macOS endpoints, which often hold off on restarts.

3) Long-Term Hardening.

• Maintain Site Isolation in a strong configuration: Do not use --disable-site-isolation-trials or --no-site-isolation flags in enterprise-managed browsers. Enforce --site-per-process for stricter execution.
• Adopt a layered browser security strategy: Deploy browser isolation solutions (e.g., cloud-based remote browser isolation) for high-risk browsing tasks to eliminate renderer exposure entirely.
• Monitor Chromium release cadence: Subscribe to the Chrome Releases RSS feed and the Chromium Security announcement mailing list for timely vulnerability advisories.
• Regularly audit browser versions: Develop a scheduled report of browser versions from EDR or MDM sources. Treat any browser version older than 2 weeks as a compliance finding.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-78941 — Official NIST reference for CVE-2026-78941, including the NVD description, CVSS data (currently N/A), and patch confidence. (Primary source of this report.)
2. Chrome Releases Blog (Google): https://chromereleases.googleblog.com/ — The authoritative source for official Chrome update announcements. As of the writing of this report, no specific blog post has been published confirming the 152.0.7977.65 fix; analysts should monitor this feed for confirmation to raise patch confidence from 2/4 to 3/4.
3. Chromium Security Advisory Page (Google): https://chromium.googlesource.com/chromium/src/+/main/docs/security/severity-guidelines.md — For context on Chromium's severity rating ("Medium" in this case) and the bar to classify a bug as such.
4. CWE-200 (MITRE): https://cwe.mitre.org/data/definitions/200.html — Mapped weakness class (Exposure of Sensitive Information to an Unauthorized Actor) applicable to this vulnerability.
5. CISA Known Exploited Vulnerabilities Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog — CVE-2026-78941 is NOT listed on the KEV catalog; this page should be monitored for updates should in-the-wild exploitation be observed.

Threat Actor Activity. As of this report, there is no public evidence of threat actor campaigns, exploit-in-the-wild activity, or proof-of-concept code targeting CVE-2026-78941. The absence of a PoC and the exploit's requirement for a secondary renderer-compromise vulnerability make opportunistic exploitation unlikely; however, sophisticated actors with a "gadget" renderer exploit may incorporate this issue into their toolchain to expand the impact of an initial compromise. Continuous monitoring of the NVD entry and the CISA KEV catalog is recommended.