CVE-2026-27656

Summary

Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to properly validate user identity in the OpenID {{IsSameUser()}} comparison logic, which allows an attacker to take over arbitrary user accounts via an overly permissive substring matching flaw in the user discovery flow.. Mattermost Advisory ID: MMSA-2026-00590

Affected Software

VendorProductVersion RangeStatus
MattermostMattermost11.4.0 <= 11.4.0affected
MattermostMattermost11.3.0 <= 11.3.1affected
MattermostMattermost11.2.0 <= 11.2.3affected
MattermostMattermost10.11.0 <= 10.11.11affected
MattermostMattermost11.5.0unaffected
MattermostMattermost11.4.1unaffected
MattermostMattermost11.3.2unaffected
MattermostMattermost11.2.4unaffected
MattermostMattermost10.11.12unaffected

Weaknesses

  • CWE-303: CWE-303: Incorrect Implementation of Authentication Algorithm

ADP Enrichment

CISA ADP Vulnrichment

  • SSVC:
  • Exploitation: none
    • Automatable: no
    • Technical Impact: total

References