Exim CVE-2026-40684 Remote Crash via DNS Parsing in musl libc

Vulnerability Summary: Exim CVE-2026-40684 Vulnerability Overview Vulnerability ID: CVE-2026-40684 Vulnerability Type: Remote-triggered crash Trigger Conditions: When Exim uses musl libc instead of GNU libc, processing malicious DNS data (specifically malformed PTR records in DNS responses) causes the process to crash. Affected Component: The function in . Scope of Impact Affected Software: Exim MTA Affected Versions: Exim 4.99.2 and earlier versions (the fixed version is any release after 4.99.1). Affected Environments: Only systems using musl libc are affected. Systems using GNU libc are not affected. Attack Scenario: A remote attacker triggers a crash by sending crafted DNS records (such as PTR records). Remediation Fixed Version: Exim 4.99.1 and later versions (based on the "Exim version 4.99.1" marker in the Changelog and the fact that CVE-2026-40684 is marked as fixed). Code Fix: The logic of the function in the file has been corrected to ensure that out-of-bounds or invalid accesses do not occur when expanding strings in a musl libc environment. Fixed Code Snippet (src/src/string.c) > Note: The above code block represents the fixed logic, which primarily adds additional checks to prevent crashes caused by parsing errors in the musl libc environment. The original vulnerable code likely lacked these protective checks.

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Exim CVE-2026-40684 Remote Crash via DNS Parsing in musl libc