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TI-2026-75367 Aliases CVE-2026-86009

Mailu lets domain managers enable spoofing on other hosted domains

Priority HIGH 69CWE-285Published · Updated

Key facts

Attack vector
Network
Privileges required
High
User interaction
None
Attack complexity
Low
Attack requirements
None
Impact type
Security bypass
Confidentiality · integrity · availability
None · High · None
Fix status
No fix listed

Attack profile from CVSS 4.0 · CNA. Fix status from CNA. Impact type from CWE-285.

Summary

Mailu, a mail server distributed as a set of Docker images, is affected before commit f05aeb8322e2f61096af6b089d61ba6f40587d96. Domain managers can enable spoofing for hosted domains they do not control, which is an improper authorization flaw (CWE-285). A malicious domain administrator with high privileges can send emails authenticated with DKIM and SPF for domains they should not be able to send for.

Written with Claude Haiku 5.5 from the sources below · 2026-10-28

Description from the CNA

Mailu is a mail server distributed as a set of Docker images. Prior to f05aeb8322e2f61096af6b089d61ba6f40587d96, domain managers can enable spoofing for hosted domains they do not control. A potentially malicious domain administrator enables spoofing for another hosted domain and can send emails that are authenticated with DKIM/SPF for domains they should not be able to. This vulnerability is addressed by f05aeb8322e2f61096af6b089d61ba6f40587d96.

Why it matters now

Exploitation Proof of concept CISA SSVC · not in CISA KEV
EPSS 0.00378 31st percentileScored 2026-11-07
Fix No fix listed No fixed version in the sources yet

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
Poc
Automatable
No
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
MailuMailu< f05aeb8322e2f61096af6b089d61ba6f40587d96—No fix listedCNA
  • Mailu VendorMailu Affected < f05aeb8322e2f61096af6b089d61ba6f40587d96 Fixed in — No fix listed CNA

Remediation

Update Mailu to a build that includes commit f05aeb8322e2f61096af6b089d61ba6f40587d96, which addresses this flaw. The sources list no fixed version for Mailu.

Written with Claude Haiku 5.5 from the sources below · 2026-10-28

Timeline

  1. 2026-10-16 Threatist record created from CVE-2026-86009
  2. 2026-10-18 Priority changed: MEDIUM → LOW
  3. 2026-10-21 CISA SSVC decision points changed
  4. 2026-10-21 Priority changed: LOW → HIGH

References

Other 4

  • TI-2026-58835 Mailu: Authentication bypass in header-based proxy authentication via spoofable `X-Forwarded-By` trust CVSS 3.1CRITICAL 9.8
  • TI-2020-04295 Unspecified vulnerability in the fetchmail script in Mailu CVSS 3.1HIGH 8.8
  • TI-2026-36418 Mailu missing authentication on PATCH /api/v1/token/<id>, which allows unauthenticated removal of IP restrictions CVSS 3.1HIGH 7.5
  • TI-2026-75366 Mailu domain managers can take over higher-privileged accounts CVSS 3.1HIGH 7.2

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences