Mozilla Foundation Security Advisory 2026-86

Security Vulnerabilities fixed in Thunderbird 155

Announced
September 1, 2026
Impact
high
Products
Thunderbird
Fixed in
  • Thunderbird 155

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-2026-84642: Allowed UNC hostnames for attachments interpreted as a regular expression

Reporter
ChinhNguyen, Lowk3yz
Impact
low
Description

The values of the mail.allowed_attachment_hostnames advanced config setting were used in a regular expression without escaping. For some possible valid hostnames, this could allow certain unintended hostnames to also match and serve remote attachments.

References

#CVE-2026-84118: Use-after-free in the JavaScript: GC component

Reporter
x0e
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-2026-84123: Privilege escalation due to use-after-free in the Graphics: WebGPU component

Reporter
Yaqoub Aldurayhim
Impact
high
References

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

Reporter
Hyeonjun Ahn
Impact
high
References

#CVE-2026-84125: Use-after-free in the DOM: Core & HTML component

Reporter
Yaqoub Aldurayhim
Impact
high
References

#CVE-2026-84126: Incorrect boundary conditions in the Layout: Grid component

Reporter
Irvan Kurniawan
Impact
high
References

#CVE-2026-84128: Privilege escalation in the WebDriver BiDi component

Reporter
Tomoya Nakanishi
Impact
moderate
References

#CVE-2026-84129: Site isolation issue in the DOM: Navigation component

Reporter
Yaqoub Aldurayhim
Impact
moderate
References

#CVE-2026-84130: Information disclosure in the Graphics: WebGPU component

Reporter
5up3rh3i
Impact
moderate
References

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

Reporter
navapon
Impact
moderate
References

#CVE-2026-84132: Information disclosure in the Networking: HTTP component

Reporter
Shu Takahashi
Impact
moderate
References

#CVE-2026-84133: Site isolation issue in the DOM: Push Subscriptions component

Reporter
pakhunov.anton.n
Impact
low
References

#CVE-2026-84134: Other issue in the Profile Backup component

Reporter
5up3rh3i
Impact
low
References

#CVE-2026-84136: Other issue in the DOM: Navigation component

Reporter
Apentota
Impact
low
References

#CVE-2026-84137: Spoofing issue in the DOM: Core & HTML component

Reporter
Riski Muhammad Ivan
Impact
low
References

#CVE-2026-84138: Denial-of-service in the PDF Viewer component

Reporter
Uwez Khan
Impact
low
References

#CVE-2026-84139: Clickjacking issue in the DOM: Events component

Reporter
Long Nguyen
Impact
low
References

#CVE-2026-84140: Site isolation issue in the DOM: Navigation component

Reporter
Mohamed Mbarek
Impact
low
References

#CVE-2026-84141: Integer overflow in the Graphics: ImageLib component

Reporter
nguyentuanhung1149
Impact
low
References

#CVE-2026-84142: Internally found bugs fixed in Thunderbird 155

Reporter
Alexandre Poirot, Christian Holler, Sebastian Hengst and the Mozilla Fuzzing Team
Impact
moderate
Description

Internally found bugs present in Thunderbird 154. 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-2026-84143: 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-2026-84144: Internally found bugs fixed in Thunderbird 155 and Thunderbird ESR 153.2

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

Internally found bugs present in Thunderbird 154 and Thunderbird ESR 153.1. 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