CVE-2026-110247: libmime Buffer Overflow Vulnerability

CVE-2026-110247 is a buffer overflow flaw in libmime that enables heap corruption through integer overflow in size calculations. Attackers can exploit this to write controlled data across heap allocations. This article covers technical details, affected Thunderbird versions, security impact, and mitigation steps.

CVE-2026-110247 Overview

CVE-2026-110247 is an integer overflow vulnerability in Mozilla Thunderbird's libmime component. Several buffer-growth sites performed size arithmetic using 32-bit integer types without overflow checks. When processing a sufficiently large message, the growth guard can fail open or allocate a buffer smaller than required. A subsequent memcpy then writes attacker-controlled data across adjacent heap allocations. Mozilla fixed the issue in Thunderbird 158.

Critical Impact
Processing a crafted email can trigger a heap buffer overflow in libmime, enabling memory corruption and potential code execution within the Thunderbird process.

Affected Products

Mozilla Thunderbird versions prior to 158
libmime MIME parsing component bundled with Thunderbird
Downstream distributions shipping unpatched Thunderbird builds

Discovery Timeline

2026-10-13 - CVE-2026-110247 published to NVD
2026-10-13 - Last updated in NVD database
Thunderbird 158 - Mozilla releases fixed version addressing the overflow

Technical Details for CVE-2026-110247

Vulnerability Analysis

The flaw resides in libmime, the MIME parser Thunderbird uses to decode email bodies and attachments. Multiple buffer-growth call sites compute allocation sizes using 32-bit arithmetic. When a message is large enough, the computed size wraps around before the growth guard evaluates it.

Two failure modes result from the wrap. The guard can incorrectly approve the allocation, failing open on an unsafe size. Alternatively, the reallocation returns a buffer smaller than the data that follows. In both cases, the subsequent memcpy writes beyond the intended allocation into adjacent heap regions.

Because the copied bytes originate from message content, the overflow is attacker-controlled. Corruption of adjacent heap metadata or object pointers can be leveraged for arbitrary code execution within the Thunderbird process context.

Root Cause

The root cause is unchecked 32-bit size arithmetic [CWE-190: Integer Overflow or Wraparound] in libmime buffer-growth routines. Size computations should use 64-bit types or explicit overflow-safe helpers, and growth guards must validate the pre-truncation value.

Attack Vector

An attacker delivers a crafted email containing MIME content sized to trigger the overflow during parsing. Thunderbird processes the message when it is opened, previewed, or indexed, invoking the vulnerable libmime path without additional user interaction beyond normal email handling.

The vulnerability manifests in libmime buffer-growth sites during MIME decoding. See the Mozilla Security Advisory MFSA 2026-109 and Mozilla Bug Report #2071621 for implementation details.

Detection Methods for CVE-2026-110247

Indicators of Compromise

Thunderbird process crashes or unexpected terminations correlated with inbound mail processing
Receipt of unusually large MIME messages or attachments with malformed headers
Unexpected child processes spawned by thunderbird.exe or thunderbird-bin
Heap corruption signatures recorded by host memory protection or crash reporters

Detection Strategies

Inventory endpoints running Thunderbird and flag versions below 158 for remediation
Monitor mail gateways for messages with abnormally large MIME parts or recursive multipart structures
Alert on anomalous process behavior originating from the Thunderbird binary, including network connections and shell spawns
Correlate crash telemetry with MIME parsing stack frames referencing libmime symbols

Monitoring Recommendations

Collect endpoint process, file, and network telemetry from systems where Thunderbird is installed
Forward Thunderbird crash reports to a central location for review by the security team
Track outbound connections initiated by the Thunderbird process to detect post-exploitation activity

How to Mitigate CVE-2026-110247

Immediate Actions Required

Upgrade all Thunderbird installations to version 158 or later
Identify unmanaged installations through software inventory and schedule patching
Restrict automatic preview of remote content and large attachments on unpatched clients
Instruct users to avoid opening messages from untrusted senders until patches are applied

Patch Information

Mozilla addressed CVE-2026-110247 in Thunderbird 158. Review the Mozilla Security Advisory MFSA 2026-109 for release details and the Mozilla Bug Report #2071621 for the underlying fix.

Workarounds

Disable automatic message preview in Thunderbird to reduce exposure during triage of unpatched hosts
Enforce mail gateway size limits to reject messages large enough to trigger the overflow
Strip or quarantine messages with malformed MIME structures at the perimeter until patching completes

bash

# Verify installed Thunderbird version on Linux
thunderbird --version

# Example apt upgrade on Debian/Ubuntu
sudo apt update && sudo apt install --only-upgrade thunderbird