CVE-2026-4371 Out of bounds read in IMAP parsing
A malicious mail server can trigger an out-of-bounds read in Thunderbird, potentially crashing the application or leaking sensitive data.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-4371?
CVE-2026-4371 is a high-severity memory corruption vulnerability in Mozilla Thunderbird, published on 24 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
A malicious mail server could send malformed strings with negative lengths, causing the parser to read memory outside the buffer. If a mail server or connection to a mail server were compromised, an attacker could cause the parser to malfunction, potentially crashing Thunderbird or leaking sensitive data. This vulnerability was fixed in Thunderbird 149 and Thunderbird 140.9.
Which versions are affected by CVE-2026-4371?
It affects 1 product.
Products include Mozilla Thunderbird.
How do I fix CVE-2026-4371?
Upgrade to a fixed version.
Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.
Is CVE-2026-4371 being exploited?
There is no known exploitation of CVE-2026-4371 in the wild and no public exploit code.
Other vulnerabilities in Mozilla Thunderbird
Sources
Also known as DEBIAN-CVE-2026-4371, GHSA-4p49-pghr-968w, UBUNTU-CVE-2026-4371.