CVE-2026-53591 in FreeScout
Summary
by MITRE • 07/20/2026
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.
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Analysis
by VulDB Data Team • 07/20/2026
This vulnerability represents a critical message injection flaw in FreeScout's email processing system that fundamentally undermines the integrity of support conversations. The issue stems from insufficient validation of email headers, specifically the In-Reply-To header field, which is commonly used by email clients to establish conversation threading. When an attacker sends an email to the helpdesk's public address with a crafted In-Reply-To header referencing an existing conversation thread, the system accepts this header without proper authentication or verification checks. This vulnerability falls under CWE-20, which addresses improper input validation, and specifically relates to CWE-79, input validation that allows cross-site scripting through email headers.
The technical implementation of this flaw occurs within FreeScout's Laravel framework where incoming emails are processed through the mail handling mechanism. When an email arrives with a valid In-Reply-To header pointing to an existing conversation, the system automatically associates the new message with that conversation thread without verifying the legitimacy of the sender or the authenticity of the header value. This creates a scenario where any unauthenticated user can manipulate existing support conversations by simply sending an email with a properly formatted header field that references an active ticket. The attacker's injected messages appear as legitimate customer replies within the agent interface, complete with proper formatting and conversation context.
The operational impact of this vulnerability extends beyond simple message injection to encompass full conversation manipulation capabilities. Once an attacker successfully injects a message, they can effectively control the narrative of any existing support thread by setting the last_reply_from field to their desired identity, causing the conversation to automatically reopen. This behavior creates false positive scenarios where agents might believe customers are actively responding to tickets when in fact the responses are fabricated by attackers. The vulnerability directly impacts the integrity of customer support operations and could be exploited for social engineering attacks, misinformation campaigns, or to disrupt legitimate support workflows.
The fix implemented in version 1.8.223 addresses this vulnerability through enhanced header validation mechanisms that verify the authenticity of In-Reply-To references before associating incoming messages with existing conversations. This mitigation strategy aligns with ATT&CK technique T1566, which covers social engineering through email manipulation, by implementing proper input validation and authentication checks for email headers. Organizations using FreeScout should immediately upgrade to version 1.8.223 or later to remediate this vulnerability, as the attack requires no credentials, tokens, or prior access to the system, making it particularly dangerous in environments where public email addresses are exposed. The fix represents a critical security enhancement that prevents unauthorized modification of support conversations and maintains the integrity of customer communication channels.