CVE-2026-29143 S/MIME Decryption Impersonation

High 7.8 Published 2 Apr 2026 Updated 2 Apr 2026

Attackers can impersonate senders by controlling trusted headers in S/MIME encrypted messages due to improper authentication of inner MIME entities.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: None listed

What is CVE-2026-29143?

CVE-2026-29143 is a high-severity vulnerability in SEPPmail Secure Email Gateway, published on 2 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

SEPPmail Secure Email Gateway before version 15.0.3 does not properly authenticate the inner message of S/MIME-encrypted MIME entities, allowing an attacker to control trusted headers.

Weakness: CWE-20

Which versions are affected by CVE-2026-29143?

It affects 1 product.

Products include Seppmail Secure Email Gateway.

How do I fix CVE-2026-29143?

No fix has been published for CVE-2026-29143 yet. Limit exposure of the affected software until one is available.

Is CVE-2026-29143 being exploited?

There is no known exploitation of CVE-2026-29143 in the wild and no public exploit code.

Sources

Also known as GHSA-q7v7-25qx-fcxf.