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CVE-2026-53591— FreeScout Vulnerable to Unauthenticated Conversation Thread Injection via HMAC Length Bypass in FetchEmails

Quick assessment

Affected

freescout-help-desk freescout

Exploitation

Public or AI PoC available; prioritize validation

Recommended action

Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

FreeScout是FreeScout公司开源的一个使用 PHP(Laravel 框架)构建的超轻量级且功能强大的免费开源帮助台和共享收件箱。 FreeScout 1.8.223之前版本存在授权问题漏洞，该漏洞源于存在授权问题，可能导致未经身份验证的攻击者通过向公共地址发送带有特制In-Reply-To标头的电子邮件，注入消息到任何现有支持对话中。

CVSS 8.6 · High EPSS 0.27% · P19 Updated Jul 21, 2026

Possible ATT&CK Techniques 1 AI

T1505.003 · Web Shell

Affected Version Matrix 1

Vendor	Product	Version Range	Status
freescout-help-desk	freescout	`< 1.8.223`	affected

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I. Basic Information for CVE-2026-53591

Vulnerability Information

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Vulnerability Title

FreeScout Vulnerable to Unauthenticated Conversation Thread Injection via HMAC Length Bypass in FetchEmails

Source: CVE Program / CVE List V5

Vulnerability 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.

Source: CVE Program / CVE List V5

CVSS Information

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L

Source: CVE Program / CVE List V5

Vulnerability Type

认证机制不恰当

Source: CVE Program / CVE List V5

Vulnerability Title

FreeScout 授权问题漏洞

Source: CNNVD (China National Vulnerability Database)

Vulnerability Description

Source: CNNVD (China National Vulnerability Database)

CVSS Information

N/A

Source: CNNVD (China National Vulnerability Database)

Vulnerability Type

N/A

Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor	Product	Affected Versions	CPE	Subscribe
freescout-help-desk	freescout	< 1.8.223	-

II. Public POCs for CVE-2026-53591

#	POC Description	Source Link	Shenlong Link

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Qwen3.6-35B-A3B · 8958 chars

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III. Intelligence Information for CVE-2026-53591

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Vendor Advisories for CVE-2026-53591 (1)

🔗 Unauthenticated Conversation Thread Injection via HMAC Length Bypass in FetchEmails · Advisory · freescout-help-desk/freescout · GitHub Read full article x_refsource_CONFIRM

https://www.cve.org/CVERecord?id=CVE-2026-53591

Same Patch Batch · freescout-help-desk · 2026-07-20 · 8 CVEs total

CVE-2026-53595	9.4 CRITICAL	FreeScout vulnerable to anonymous account takeover via /user-setup empty invite_hash on My
CVE-2026-53593	8.8 HIGH	FreeScout Vulnerable to Authenticated Remote Code Execution via incomplete upload extensio
CVE-2026-48812	7.5 HIGH	FreeScout Allows Unauthenticated Access to Legacy Attachment Files
CVE-2026-45295	6.5 MEDIUM	FreeScout Vulnerable to Unauthenticated Thread Read-Status Manipulation and Conversation E
CVE-2026-53596	5.3 MEDIUM	FreeScout has unrestricted file upload without rate limiting that leads to resource exhaus
CVE-2026-53594	4.9 MEDIUM	FreeScout has Arbitrary File Read in App Logs Viewer via Forged Encrypted Path
CVE-2026-53592	4.6 MEDIUM	FreeScout vulnerable to prototype pollution in getQueryParam

IV. Related Vulnerabilities

Same product: freescout

Same vendor: freescout-help-desk

Same weakness: CWE-287