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CVE PROGRAM SOURCE RECORD5.7 Medium

CVE-2026-27656

Account Takeover via Substring Matching in OpenID Connect Authentication

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Status
Published
Published
Updated
Assigning organization
Mattermost

Description

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 products

Version statuses are supplied by the assigning organization. Consult the original record for detailed ranges and platform conditions.

Mattermost Mattermost

Default version status: unaffected

  • 11.4.0 to ≤ 11.4.0affected
  • 11.3.0 to ≤ 11.3.1affected
  • 11.2.0 to ≤ 11.2.3affected
  • 10.11.0 to ≤ 10.11.11affected
  • 11.5.0unaffected
  • 11.4.1unaffected
  • 11.3.2unaffected
  • 11.2.4unaffected
  • 10.11.12unaffected

References