CVE-2026-6771
Published: 21 April 2026
Summary
CVE-2026-6771 is a critical-severity Authentication Bypass Using an Alternate Path or Channel (CWE-288) vulnerability in Mozilla Firefox. Its CVSS base score is 9.8 (Critical).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Client Execution (T1203); ranked at the 22.7th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 SI-2 (Flaw Remediation) and RA-5 (Vulnerability Monitoring and Scanning).
Deeper analysis
CVE-2026-6771 is a mitigation bypass vulnerability in the DOM Security component affecting Mozilla Firefox versions prior to 150, Firefox ESR prior to 140.10, Thunderbird prior to 150, and Thunderbird prior to 140.10. Assigned CWE-288, it carries a CVSS v3.1 base score of 9.8, indicating critical severity due to its network accessibility, low attack complexity, lack of required privileges or user interaction, and high impacts on confidentiality, integrity, and availability.
Remote attackers require no privileges or user interaction to exploit this vulnerability over the network with low complexity. Successful exploitation enables high-impact compromise of confidentiality, integrity, and availability, potentially allowing attackers to bypass security mitigations in the browser's DOM handling.
Mozilla security advisories (MFSA2026-30 through MFSA2026-34) and the associated Bugzilla entry detail the fix applied in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10, recommending immediate upgrades to patched versions for mitigation.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-24112
Vulnerability details
Mitigation bypass in the DOM: Security component. This vulnerability was fixed in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
This is a remote client-side mitigation bypass vulnerability in browser DOM security (Firefox/Thunderbird) with no privileges or user interaction required and critical CIA impact, directly enabling exploitation of client application vulnerabilities for code execution or compromise.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Directly mandates timely patching of known flaws like CVE-2026-6771 in Firefox and Thunderbird to prevent exploitation of the DOM security mitigation bypass.
Requires receiving and acting on vendor security advisories such as Mozilla's MFSA2026-30 series that detail the vulnerability and patched versions.
Facilitates vulnerability scanning to identify systems running vulnerable Firefox or Thunderbird versions affected by the DOM security bypass.
Hardening callouts derived
Configuration rules from DISA STIG baselines that reduce the attack surface for weaknesses of the type cited by this CVE. Derived transitively via CVE→CWE→STIG over `controls_xwalks` (authoritative rows only).
Oracle Linux 8 (2 rules)
- V-248598 There must be no ".shosts" files on the OL 8 operating system. via CWE-288
- V-248827 OL 8 must not have the rsh-server package installed. via CWE-288
Oracle Linux 9 (2 rules)
- V-271757 OL 9 file systems must not contain shosts.equiv files. via CWE-288
- V-271758 OL 9 file systems must not contain .shosts files. via CWE-288
RHEL 7 (3 rules)
- V-204424 The Red Hat Enterprise Linux operating system must not allow accounts configured with blank or null passwords. via CWE-288
- V-204440 Red Hat Enterprise Linux operating systems version 7.2 or newer using Unified Extensible Firmware Interface (UEFI) must require authentication upon booting into single-user and maintenance modes. via CWE-288
- V-204442 The Red Hat Enterprise Linux operating system must not have the rsh-server package installed. via CWE-288