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CVE-2026-74986

CRITICAL 9.1

Published 2026-08-18 · Last modified 2026-08-20

Site isolation issue in the CSS Parsing and Computation component. This vulnerability was fixed in Firefox 154, Firefox ESR 153.1, Thunderbird 154, and Thunderbird 153.1.

ELEVATED IMPACT

Severe if exploited (CVSS 9.1), but no known exploitation and low modeled probability. Patch on a normal cadence.

Exploitation likelihood

0.3%chance of exploitation in 30 days · 28th percentile

○ In CISA KEV ○ Public exploit / PoC

Impact if exploited

9.1CVSS 3.1 · CRITICAL

  • ConfidentialityHigh
  • IntegrityNone
  • AvailabilityHigh

What an attacker needs

  • ✓Access: Reachable over the network — no local access needed
  • ✓Privileges: No account or privileges required
  • ✓User interaction: No user interaction needed
  • ✓Complexity: No special conditions — reliably repeatable

✓ lowers the bar for an attacker · ⚠ raises it

Proof of concept & exploit code

Test against your own equipment

curl -s https://vulnpedia.com/cve/CVE-2026-74986/poc.jsonMachine-readable PoC index for this CVE (for automation).

Listed for defensive triage, patch verification, and authorized testing on systems you own. Machine-readable: /cve/CVE-2026-74986/poc.json

Affected

Vendors Mozilla

Products Firefox Thunderbird

Weakness (CWE)

  • CWE-200: Exposure of Sensitive Information to an Unauthorized Actor

CVSS vector

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

All CVSS metrics

  • CRITICAL 9.1 v3.1 · ADP Primary
    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
  • CRITICAL 9.1 v3.1 · NVD Secondary
    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H

References

Technical & other

Sources: NVD · CVE.org · EPSS