Cyber Resilience

CVE-2026-6758

High

Published: 21 April 2026

Published
21 April 2026
Modified
22 April 2026
KEV Added
—
Patch
—
CVSS Score v3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
EPSS Score 0.0043 34.5th percentile
Risk Priority 55 floored blend · peak EPSS

Summary

CVE-2026-6758 is a high-severity Use After Free (CWE-416) vulnerability in Mozilla Firefox. Its CVSS base score is 7.5 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Application or System Exploitation (T1499.004); ranked at the 34.5th 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-16 (Memory Protection).

Deeper analysis

CVE-2026-6758 is a use-after-free vulnerability (CWE-416) in the JavaScript WebAssembly component of Mozilla Firefox and Thunderbird. Published on 2026-04-21, it carries a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H), indicating high severity with primary impact on availability. The issue was addressed in Firefox version 150 and Thunderbird version 150.

Remote attackers can exploit this vulnerability over the network with low attack complexity, requiring no privileges, user interaction, or special scope changes. Successful exploitation results in denial of service, such as browser or application crashes, without affecting confidentiality or integrity.

Mozilla's security advisories MFSA2026-30 and MFSA2026-33 document the patch, recommending upgrade to Firefox 150 or Thunderbird 150 for mitigation. Further technical details are available in Bugzilla entry 2013619.

EU & UK References

Vulnerability details

Use-after-free in the JavaScript: WebAssembly component. This vulnerability was fixed in Firefox 150 and Thunderbird 150.

CWE(s)
CWE-416

Related Threats

MITRE ATT&CK Enterprise TechniquesAI

T1499.004 Application or System Exploitation Impact
Adversaries may exploit software vulnerabilities that can cause an application or system to crash and deny availability to users.
attack.mitre.org →
Why these techniques?

Use-after-free in browser WebAssembly component directly enables remote exploitation causing application crashes, mapping to Endpoint Denial of Service via Application or System Exploitation.

Confidence: HIGH · MITRE ATT&CK Enterprise v19.0

CVEs Like This One

CVE-2026-6759Same product: Mozilla Firefox
CVE-2025-1931Same product: Mozilla Firefox
CVE-2026-6754Same product: Mozilla Firefox
CVE-2026-6747Same product: Mozilla Firefox
CVE-2026-6746Same product: Mozilla Firefox
CVE-2025-1012Same product: Mozilla Firefox
CVE-2026-2758Same product: Mozilla Firefox
CVE-2026-2763Same product: Mozilla Firefox
CVE-2025-1010Same product: Mozilla Firefox
CVE-2024-2612Same product: Mozilla Firefox

Affected Assets

mozilla
firefox
≤ 150.0
mozilla
thunderbird
≤ 150.0

Mitigating Controls

Mitigating Controls (NIST 800-53 r5) AI

SI-2 Flaw Remediation good match
prevent

Directly mandates timely patching and flaw remediation for the specific use-after-free vulnerability fixed in Firefox and Thunderbird version 150.

SI-16 Memory Protection good match
prevent

Provides memory protection mechanisms that prevent unauthorized access to freed memory, comprehensively mitigating use-after-free exploits like CVE-2026-6758.

RA-5 Vulnerability Monitoring and Scanning good match
preventdetect

Requires vulnerability scanning to identify and prioritize remediation of systems with vulnerable Firefox or Thunderbird versions affected by this CVE.

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 (1 rule)
  • V-248592 OL 8 must clear memory when it is freed to prevent use-after-free attacks. via CWE-416
RHEL 8 (1 rule)
  • V-230279 RHEL 8 must clear memory when it is freed to prevent use-after-free attacks. via CWE-416
RHEL 9 (1 rule)
  • V-257794 RHEL 9 must clear memory when it is freed to prevent use-after-free attacks. via CWE-416

References