CVE-2026-2790 – This CVE describes a same-origin policy bypass ... (How to Fix)

Q: How do I fix CVE-2026-2790?

1. Open Firefox menu > Help > About Firefox. 2. Allow browser to check for updates. 3. If update is available, click 'Restart to update Firefox'. 4. For enterprise deployments, use your standard patch management system to deploy Firefox 148+ or Firefox ESR 140.8+.

Q: What is the severity of CVE-2026-2790?

CVE-2026-2790 has a CVSS score of 9.8 (CRITICAL). This CVE describes a same-origin policy bypass vulnerability in Firefox's JAR (Java Archive) networking component. It allows malicious websites to access data from other origins, potentially leading to information disclosure. Affected users are those running Firefox versions below 148 or Firefox ESR versions below 140.8.

📋 TL;DR

This CVE describes a same-origin policy bypass vulnerability in Firefox's JAR (Java Archive) networking component. It allows malicious websites to access data from other origins, potentially leading to information disclosure. Affected users are those running Firefox versions below 148 or Firefox ESR versions below 140.8.

💻 Affected Systems

Products:

Mozilla Firefox
Mozilla Firefox ESR

Versions: Firefox < 148, Firefox ESR < 140.8

Operating Systems: All platforms supported by Firefox

Default Config Vulnerable: ⚠️ Yes

Notes: All default configurations of affected versions are vulnerable. No special settings required for exploitation.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Complete compromise of user session data, credentials, and sensitive information from other websites the user has open.

🟠

Likely Case

Targeted data theft from specific websites where the user is authenticated.

🟢

If Mitigated

Limited impact if user has no sensitive sessions open or uses strict browser isolation.

🌐 Internet-Facing: HIGH - Web browsers are inherently internet-facing and this vulnerability can be exploited by visiting malicious websites.

🏢 Internal Only: LOW - This is primarily an external threat vector through web browsing.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

Exploitation requires user to visit a malicious website but no authentication is needed. The vulnerability is in the browser's core networking component.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Firefox 148, Firefox ESR 140.8

Vendor Advisory: https://www.mozilla.org/security/advisories/mfsa2026-13/

Restart Required: Yes

Instructions:

1. Open Firefox menu > Help > About Firefox. 2. Allow browser to check for updates. 3. If update is available, click 'Restart to update Firefox'. 4. For enterprise deployments, use your standard patch management system to deploy Firefox 148+ or Firefox ESR 140.8+.

🔧 Temporary Workarounds

Disable JAR file handling

all

Prevents JAR files from being processed by the browser

about:config
Set network.jar.open-unsafe-types to false

Use browser isolation

all

Run Firefox in a sandboxed or isolated environment

🧯 If You Can't Patch

Implement network segmentation to restrict browser access to sensitive internal resources
Use application whitelisting to prevent execution of unauthorized browser instances

🔍 How to Verify

Check if Vulnerable:

Check Firefox version via menu > Help > About Firefox. If version is below 148 (or below 140.8 for ESR), the system is vulnerable.

Check Version:

firefox --version

Verify Fix Applied:

After update, verify version is Firefox 148+ or Firefox ESR 140.8+ in About Firefox dialog.

📡 Detection & Monitoring

Log Indicators:

Unusual cross-origin resource requests in web server logs
Multiple failed same-origin policy violations

Network Indicators:

Unexpected JAR file downloads from untrusted sources
Cross-origin requests that should be blocked

SIEM Query:

source="firefox.log" AND ("same-origin violation" OR "cross-origin" OR "JAR")

📊 Metadata

CVE ID: CVE-2026-2790

CVSS v3 Score: 9.8 (CRITICAL)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CWE: CWE-346

Published: February 24, 2026

Last Updated: February 26, 2026

🔗 References

https://bugzilla.mozilla.org/show_bug.cgi?id=2008426 Issue Tracking,Permissions Required
https://www.mozilla.org/security/advisories/mfsa2026-13/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-15/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-16/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-17/ Vendor Advisory

📤 Share & Export

📄 Export Markdown 📋 Export JSON

🔗 Related Vulnerabilities

If you're affected by CVE-2026-2790, you might also want to check these: