CVE-2026-6766
Published: 21 April 2026
Summary
CVE-2026-6766 is a high-severity Improper Check for Unusual or Exceptional Conditions (CWE-754) vulnerability in Mozilla Firefox. Its CVSS base score is 7.5 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Data from Local System (T1005); ranked at the 17.0th 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 RA-5 (Vulnerability Monitoring and Scanning) and SI-2 (Flaw Remediation).
Deeper analysis
CVE-2026-6766 is a vulnerability involving incorrect boundary conditions in the Libraries component of NSS (Network Security Services). NSS is a set of libraries used for cryptographic and security functions in Mozilla products, specifically affecting Firefox, Firefox ESR, and Thunderbird prior to their respective patched versions.
The vulnerability has a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N), indicating it can be exploited remotely by unauthenticated attackers with low complexity and no user interaction required. Successful exploitation allows attackers to achieve high-impact confidentiality violations, such as disclosing sensitive information processed or stored by affected applications.
Mozilla security advisories (MFSA 2026-30 through 2026-34) and the associated Bugzilla entry detail the fix applied in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10. Security practitioners should ensure these updates are applied to mitigate the issue, as classified under CWE-754 (Improper Check for Unusual or Exceptional Conditions).
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-24107
Vulnerability details
Incorrect boundary conditions in the Libraries component in NSS. 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?
The vulnerability enables remote unauthenticated disclosure of sensitive information from affected applications via an out-of-bounds condition in NSS, directly facilitating collection of data from the local system.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Directly mandates timely remediation of identified flaws, such as applying patches for the NSS boundary condition vulnerability in CVE-2026-6766 affecting Firefox and Thunderbird.
Requires vulnerability scanning to identify systems running unpatched versions of NSS in Mozilla products exploitable by CVE-2026-6766.
Ensures organizations receive and act on security advisories like Mozilla MFSA 2026-30 detailing the NSS vulnerability and required updates.
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 (3 rules)
- V-248574 YUM must be configured to prevent the installation of patches, service packs, device drivers, or OL 8 system components that have not been digitally signed using a certificate that is recognized and approved by the organization. via CWE-754
- V-248573 The OL 8 file integrity tool must notify the system administrator (SA) when changes to the baseline configuration or anomalies in the operation of any security functions are discovered within an organizationally defined frequency. via CWE-754
- V-248575 OL 8 must prevent the installation of software, patches, service packs, device drivers, or operating system components of local packages without verification they have been digitally signed using a certificate that is issued by a Certificate Authority (CA) that is recognized and approved by the organization. via CWE-754
Oracle Linux 9 (1 rule)
- V-271452 OL 9 must use a Linux Security Module configured to enforce limits on system services. via CWE-754