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CVE-2026-55666 – Authentication Bypass / Account Takeover – Rocket.Chat prior to 8.5.1 / 8.4.4 / …

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Summary

CVE-2026-55666 is a critical authentication bypass and account takeover vulnerability in Rocket.Chat’s Apple Sign-In OAuth flow, rated 9.3 (Critical) under CVSS 4.0. The flaw exists in apps/meteor/app/apple/server/loginHandler.ts, where a missing validation on an email-fallback path allows an unauthenticated attacker to forge Apple JWTs and take over arbitrary user accounts. No authentication and no user interaction are required to exploit this vulnerability.

Technical details

  • Root cause: In apps/meteor/app/apple/server/loginHandler.ts, the handleIdentityToken function parses JWTs issued by Apple during the OAuth flow and attempts to extract an email claim. When the JWT does not contain an email field, the code falls back to accepting an arbitrary email value supplied directly by the requester, with no validation. This is classified as CWE-287 (Improper Authentication) and CWE-288 (Authentication Bypass Using an Alternate Path or Channel).
  • Trigger condition: Apple Sign-In (Apple OAuth) must be enabled on the Rocket.Chat instance. The attacker crafts a forged Apple JWT that deliberately omits the email claim, then includes an arbitrary target email address in the OAuth request body.
  • Attack vector: Fully remote and unauthenticated over the network. The attacker needs only to know (or enumerate) a valid user’s email address on the target instance.
  • Impact: Complete account takeover — the attacker authenticates as any targeted user, gaining full access to that user’s messages, channels, files, and permissions. In instances where administrative accounts use Apple Sign-In, this can escalate to full platform compromise.

Affected software

  • Rocket.Chat versions prior to 7.10.13
  • Rocket.Chat versions 8.0.x prior to 8.0.7
  • Rocket.Chat versions 8.1.x prior to 8.1.6
  • Rocket.Chat versions 8.2.x prior to 8.2.6
  • Rocket.Chat versions 8.3.x prior to 8.3.6
  • Rocket.Chat versions 8.4.x prior to 8.4.4
  • Rocket.Chat versions 8.5.x prior to 8.5.1

Severity

CVSS 4.0 Base Score: 9.3 (Critical)
Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate action — patch: Upgrade to one of the fixed releases: 8.5.1, 8.4.4, 8.3.6, 8.2.6, 8.1.6, 8.0.7, or 7.10.13, depending on your current release branch. Patches are available from the official Rocket.Chat release channel.
  • If patching is not immediately possible: Disable Apple Sign-In / Apple OAuth in the Rocket.Chat administration panel (Administration → OAuth → Apple) to eliminate the vulnerable code path until the upgrade can be applied.
  • General hardening: Restrict public internet access to Rocket.Chat login endpoints where not operationally required, and review OAuth provider configurations to ensure only necessary providers are enabled.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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