CVE-2026-29143 PUBLISHED

S/MIME Decryption Impersonation

Assigner: NCSC.ch
Reserved: 04.03.2026 Published: 02.04.2026 Updated: 02.04.2026

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.

Metrics

CVSS 4.0

CVSS Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:L/SI:H/SA:N
CVSS Score: 7.8

Exploitability Metrics		Vulnerable System Impact Metrics		Subsequent System Impact Metrics
Attack Vector	Network	Confidentiality	None	Confidentiality	Low
Attack Complexity	Low	Integrity	Low	Integrity	High
Attack Requirements	None	Availability	None	Availability	None
Privileges Required	None
User Interaction	None

CVSS 4.0

Product Status

Vendor	SEPPmail
Product	Secure Email Gateway
Versions	Default: unaffected affected from 0 to 15.0.3 (excl.)

Credits

Andris Suter-Dörig finder
Matteo Scarlata coordinator
Kenny Paterson coordinator

References

https://downloads.seppmail.com/extrelnotes/150/ERN15.0.html#seppmail-vulnerability-disclosure-1503

Problem Types

CWE-20 Improper Input Validation CWE

Impacts

CAPEC-43 Exploiting Multiple Input Interpretation Layers
CAPEC-194 Fake the Source of Data