Mozilla Foundation Security Advisory 2026-87

Security Vulnerabilities fixed in Thunderbird 140.15

Announced
September 1, 2026
Impact
high
Products
Thunderbird
Fixed in
  • Thunderbird 140.15

Many of these flaws cannot be exploited through email in the Thunderbird product because scripting is disabled when reading mail, however they may pose risks in browser or browser-like contexts.

#CVE-2026-84639: Uninitialized memory in MIME parsing

Reporter
Ramesh Adhikari, Dr. Faruk Kazi (CoE-CNDS Lab, VJTI, Mumbai, India)
Impact
high
Description

Triggering an error condition in certain MIME bodies would cause uninitialized memory to be used.

References

#CVE-2026-84640: One byte overflow read in mail parser

Reporter
Ramesh Adhikari, Dr. Faruk Kazi (CoE-CNDS Lab, VJTI, Mumbai, India)
Impact
medium
Description

A maliciously constructed mail header could lead to a one byte read past the end of a buffer.

References

#CVE-2026-84641: Information disclosure due to malicious IMAP server response

Reporter
ABDULAZIZ ALASAIQAH
Impact
low
Description

A malicious IMAP server can trigger use-after-free and heap-memory disclosure by sending a crafted ID response. Heap contents can ultimately be persisted to prefs.js.

References

#CVE-2026-75874: Sandbox escape in the Remote Settings Client component

Reporter
crixer
Impact
high
References

#CVE-2026-16365: Privilege escalation in the DOM: Workers component

Reporter
Khanh Nguyen
Impact
high
References

#CVE-2026-84119: Sandbox escape due to use-after-free in the DOM: Navigation component

Reporter
Yaqoub Aldurayhim
Impact
high
References

#CVE-2026-84120: Use-after-free in the Audio/Video component

Reporter
Ukyo Akai
Impact
high
References

#CVE-2026-84121: Sandbox escape due to use-after-free in the DOM: Security component

Reporter
Yaqoub Aldurayhim
Impact
high
References

#CVE-2026-84122: Use-after-free in the Audio/Video component

Reporter
Hyeonjun Ahn
Impact
high
References

#CVE-2026-84124: Use-after-free in the DOM: Core & HTML component

Reporter
Hyeonjun Ahn
Impact
high
References

#CVE-2026-16371: Privilege escalation in the DOM: Navigation component

Reporter
Steven Julian
Impact
moderate
References

#CVE-2026-84131: Privilege escalation due to invalid pointer in the Graphics component

Reporter
navapon
Impact
moderate
References

#CVE-2026-84143: Internally found bugs fixed in Thunderbird 155, Thunderbird ESR 153.2 and Thunderbird ESR 140.15

Reporter
Jan de Mooij, Tom Ritter and the Mozilla Fuzzing Team
Impact
high
Description

Internally found bugs present in Thunderbird 154, Thunderbird ESR 153.1 and Thunderbird ESR 140.14. Some of these bugs showed evidence of memory corruption or another security-relevant defect and we presume that with enough effort some of these could have been exploited.

References

#CVE-2026-84145: Internally found bugs fixed in Thunderbird 155, Thunderbird ESR 153.2 and Thunderbird ESR 140.15

Reporter
Leo Tenenbaum, Tom Ritter and the Mozilla Fuzzing Team
Impact
high
Description

Internally found bugs present in Thunderbird 154, Thunderbird ESR 153.1 and Thunderbird ESR 140.14. Some of these bugs showed evidence of memory corruption or another security-relevant defect and we presume that with enough effort some of these could have been exploited.

References