CVE-2026-40684 Denial of service in Exim
An attacker can crash Exim connections via malformed DNS PTR records on musl libc systems, requiring an upgrade to mitigate.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-40684?
CVE-2026-40684 is a medium-severity denial of service vulnerability in Exim Exim, published on 30 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
In Exim before 4.99.2, on systems using musl libc (not glibc), an attacker can crash the connection instance when malformed DNS data is present in PTR records. This is caused by a dn_expand oddity in octal printing.
Weakness: CWE-684
Which versions are affected by CVE-2026-40684?
It affects 1 product.
Products include Exim Exim.
How do I fix CVE-2026-40684?
Upgrade to a fixed version.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-40684 being exploited?
There is no known exploitation of CVE-2026-40684 in the wild and no public exploit code.
Other vulnerabilities in Exim Exim
Sources
Also known as CLSA-2026-1778174719, DEBIAN-CVE-2026-40684, GHSA-q496-q274-jgh9.