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CVE-2026-2776: Sandbox escape due to incorrect boundary conditions in the Telemetry component in External Software

• Severity: critical (CVSS 10.0 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-02-24 · updated 2026-07-15
• Weakness: CWE-119

Description

Sandbox escape due to incorrect boundary conditions in the Telemetry component in External Software. This vulnerability was fixed in Firefox 148, Firefox ESR 115.33, Firefox ESR 140.8, Thunderbird 148, and Thunderbird 140.8.

Affected products

• Mozilla Firefox: before 115.33.0; before 148.0; from 128.0 before 140.8.0 — fixed in 115.33.0, 140.8.0, 148.0
• Mozilla Thunderbird: before 140.8.0; before 148.0 — fixed in 140.8.0, 148.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 115.33.0, 140.8.0, 148.0; Mozilla Thunderbird 140.8.0, 148.0. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=2015266
• https://www.mozilla.org/security/advisories/mfsa2026-13/
• https://www.mozilla.org/security/advisories/mfsa2026-14/
• https://www.mozilla.org/security/advisories/mfsa2026-15/
• https://www.mozilla.org/security/advisories/mfsa2026-16/
• https://www.mozilla.org/security/advisories/mfsa2026-17/
• https://access.redhat.com/errata/RHSA-2026:3338
• https://access.redhat.com/errata/RHSA-2026:3339
• https://access.redhat.com/errata/RHSA-2026:3361
• https://access.redhat.com/errata/RHSA-2026:3491
• https://access.redhat.com/errata/RHSA-2026:3492
• https://access.redhat.com/errata/RHSA-2026:3493

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