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 13.7th 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 RA-5 (Vulnerability Monitoring and Scanning) and SI-2 (Flaw Remediation).
Threat & Defense at a Glance
Threat & Defense Details
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.
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.
NVD Description
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.
Deeper analysisAI
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).
Details
- CWE(s)