CVE-2026-53591 FreeScout Vulnerable to Unauthenticated Conversation Thread Injection via HMAC Length Bypass in FetchEmails
Unauthenticated attackers can inject messages into support conversations via crafted email headers, bypassing authentication and modifying data.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-53591?
CVE-2026-53591 is a high-severity authentication bypass vulnerability in freescout-help-desk freescout, published on 20 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
FreeScout is a free help desk and shared inbox built with PHP's Laravel framework. Prior to version 1.8.223, an unauthenticated attacker can inject messages into any existing support conversation by sending a single email to the helpdesk's public address with a crafted `In-Reply-To` header. No credentials, tokens, or prior access are required. The injected message is rendered in the agent UI as a legitimate customer reply, the conversation is automatically reopened, and the `last_reply_from` field is set to the attacker's identity. Version 1.8.223 contains a fix.
Weakness: CWE-287
Which versions are affected by CVE-2026-53591?
How do I fix CVE-2026-53591?
No fix has been published for CVE-2026-53591 yet. Limit exposure of the affected software until one is available.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-53591 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-53591 exists.
Sources
Also known as GHSA-8vm3-wwq4-ggfx.