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CVE-2026-8959

CRITICAL 9.6

Published 2026-05-19 · Last modified 2026-05-19

Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. This vulnerability was fixed in Firefox 151, Firefox ESR 140.11, Thunderbird 151, and Thunderbird 140.11.

ELEVATED IMPACT

Severe if exploited (CVSS 9.6), but no known exploitation and low modeled probability. Patch on a normal cadence.

Exploitation likelihood

0.4%chance of exploitation in 30 days · 33rd percentile

○ In CISA KEV ○ Public exploit / PoC

Impact if exploited

9.6CVSS 3.1 · CRITICAL

  • ConfidentialityHigh
  • IntegrityHigh
  • AvailabilityHigh

What an attacker needs

  • ✓Access: Reachable over the network — no local access needed
  • ✓Privileges: No account or privileges required
  • ⚠User interaction: A user must take an action (click / open a file)
  • ✓Complexity: No special conditions — reliably repeatable

✓ lowers the bar for an attacker · ⚠ raises it

Affected

Vendors Mozilla

Products Firefox Thunderbird

Weakness (CWE)

  • CWE-119: Memory buffer overflow
  • CWE-20: Improper input validation
  • CWE-693

CVSS vector

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

References

Technical & other

Sources: NVD · CVE.org · EPSS