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CVE-2026-53591 – Unauthenticated Message Injection – FreeScout Help Desk prior to 1.8.223

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Summary

CVE-2026-53591 is a high-severity improper authentication vulnerability (CWE-287) in FreeScout, the open-source PHP/Laravel help desk and shared inbox platform, affecting all versions prior to 1.8.223. An unauthenticated remote attacker can inject arbitrary messages into any existing support conversation by sending a single email to the helpdesk’s public address with a crafted In-Reply-To header, requiring no credentials or prior access. The vulnerability carries a CVSS v3.1 base score of 8.6 (AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L).

Technical details

  • Root cause: The FetchEmails artisan command verifies incoming message authenticity via HMAC validation, but the check is conditional — it is only enforced when the hash component in the In-Reply-To header is exactly 16 characters long. A "backward compatibility" code path skips HMAC verification entirely for hashes of any other length, allowing the thread ID to be accepted without authentication.
  • Trigger conditions: An attacker crafts an In-Reply-To header of the form <FS_reply-{thread_id}[email protected]>, where the hash component (x) is a single character (or any length other than 16). This triggers the unprotected code path.
  • Attack vector: Network — the attacker sends a standard email to the helpdesk’s publicly advertised inbox address. No network adjacency, authentication tokens, or prior account access are required.
  • Impact: The injected message is rendered in the agent UI as a legitimate customer reply; the targeted conversation is automatically reopened; and the last_reply_from field is set to the attacker’s identity. This enables spoofing of customer identities within internal support workflows, manipulation of conversation history, forced reopening of closed tickets, and social engineering of support agents — all without any victim interaction.

Affected software

  • FreeScout (freescout-help-desk/freescout) — all versions prior to 1.8.223

Severity

  • CVSS v3.1 Base Score: 8.6 (HIGH)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L

Mitigation and recommended actions

  • Immediate: Upgrade FreeScout to version 1.8.223 or later. This release removes the backward-compatible fallback for the old Message-ID format, enforcing HMAC validation regardless of hash length (references GHSA-8vm3-wwq4-ggfx).
  • If immediate patching is not possible, consider restricting inbound email processing by applying strict sender validation controls at the mail server or MTA level, and limit network exposure of the FreeScout web interface to trusted sources only.

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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