CVE-2026-110247

Several buffer-growth sites in libmime performed size arithmetic in 32-bit types without overflow checks. On a sufficiently large message the growth guard could fail open, or the buffer could be reallocated smaller than required, and a subsequent memcpy would then write attacker-controlled data across adjacent heap allocations. This vulnerability was fixed in Thunderbird 158.

• https://bugzilla.mozilla.org/show_bug.cgi?id=2071621
• https://www.mozilla.org/security/advisories/mfsa2026-109/