CVE-2026-2779
Critical boundary-condition flaw in Networking: JAR affects Firefox <148 and Thunderbird <148 (ESR <140.8).
Is CVE-2026-2779 being exploited?
Not confirmed. CVE-2026-2779 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.60% probability of exploitation in the next 30 days.
How severe is CVE-2026-2779?
CVE-2026-2779 is rated Critical with a CVSS score of 9.8. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-2779?
Yes. A fix has been recorded for CVE-2026-2779. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-2779 affect?
CVE-2026-2779 affects Firefox < 148, Firefox ESR < 140.8, Thunderbird < 148, Thunderbird < 140.8. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-2779?
Upgrade to latest Firefox/Thunderbird versions (>=148 / ESR >=140.8).
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
RCE or crash via crafted network data in Networking: JAR.
Patch when possible