Vulnerability · Published Jul 22, 2026 · Assigned by mozilla
CVE-2026-14899Off-by-one out of bounds read in MIME header parser for forwarding
High 7.5
In plain English
A high-severity security flaw in Thunderbird from Mozilla. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 41% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
The code to parse MIME headers for display when forwarding a message (if the setting to view all headers was enabled) had an off-by-one error, allowing a single byte to be read from the memory after the buffer for the headers, and potentially crashing Thunderbird. This vulnerability was fixed in Thunderbird 153 and Thunderbird 140.13.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| AlmaLinux:10 | thunderbird | 140.13.0-1.el10_2.alma.1 | ALSA-2026:49621 |
| AlmaLinux:8 | thunderbird | 140.13.0-1.el8_10.alma.1 | ALSA-2026:49922 |
| AlmaLinux:9 | thunderbird | 140.13.0-1.el9_8.alma.1 | ALSA-2026:49921 |
| Debian:12 | thunderbird | 1:140.13.0esr-2~deb12u1 | DEBIAN-CVE-2026-14899 |
| Debian:13 | thunderbird | 1:140.13.0esr-2~deb13u1 | DEBIAN-CVE-2026-14899 |
| Debian:14 | thunderbird | 1:140.13.0esr-1 | DEBIAN-CVE-2026-14899 |
| Rocky Linux:10 | thunderbird | 0:140.13.0-1.el10_2 | RLSA-2026:49621 |
| Rocky Linux:8 | thunderbird | 0:140.13.0-1.el8_10 | RLSA-2026:49922 |
| Rocky Linux:9 | thunderbird | 0:140.13.0-1.el9_8 | RLSA-2026:49921 |
| Ubuntu:22.04:LTS | thunderbird | no fix listed | UBUNTU-CVE-2026-14899 |
| Ubuntu:24.04:LTS | thunderbird | no fix listed | UBUNTU-CVE-2026-14899 |
| Ubuntu:26.04:LTS | thunderbird | no fix listed | UBUNTU-CVE-2026-14899 |
| Ubuntu:26.10 | thunderbird | no fix listed | UBUNTU-CVE-2026-14899 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Mozilla | Thunderbird | see references |
Weakness: CWE-193 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N