Thunderbird MIME header OBO read before 153
CVE-2026-14899 Published on July 22, 2026
Off-by-one out of bounds read in MIME header parser for forwarding
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.
Vulnerability Analysis
CVE-2026-14899 can be exploited with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have a high impact on confidentiality, with no impact on integrity and availability.
Weakness Type
What is an off-by-five Vulnerability?
A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value.
CVE-2026-14899 has been classified to as an off-by-five vulnerability or weakness.
Products Associated with CVE-2026-14899
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Affected Versions
Mozilla Thunderbird:- Version 140.13, <= 140.* is unaffected.
- Version 153, <= * is unaffected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.