Inefficient Algorithmic Complexity in hMailServer by Progressive Robot
CVE-2026-107576

7.5HIGH

Key Information:

Vendor

Progressive Robot Ltd

Status
Hmailserver
Vendor
CVE Published:
8 October 2026

What is CVE-2026-107576?

The hMailServer from Progressive Robot exhibits an inefficient algorithmic complexity in its inbound DKIM and ARC signature verification. This issue allows a remote attacker to potentially disrupt mail services by exploiting message headers with numerous fields or extensive folding. The flawed canonicalization processes delay header validation significantly, as the system faces challenges in handling headers with multiple signatures, leading to considerable resource consumption. Consequently, the mail server may become unavailable, impacting normal operations. The affected versions include 6.0.0 through 6.3.5. Immediate updates and rigorous configurations are recommended to mitigate possible threats.

Affected Version(s)

hMailServer 6.0.0 < 6.3.6

References

https://gitlab.com/hmailserver/hmailserver/-/work_items/73
https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.6

CVSS V3.1

Score:
7.5
Severity:
HIGH
Confidentiality:
None
Integrity:
None
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
None
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

Found in the hMailServer project's own security review (Progressive Robot Ltd)
.