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

CVSS 8.6 · High EPSS 0.21% · P12

Possible ATT&CK Techniques 1AI

T1505.003 · Web Shell

Affected Version Matrix 1

VendorProductVersion RangeStatus
freescout-help-deskfreescout< 1.8.223affected
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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
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。
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

VendorProductAffected VersionsCPESubscribe
freescout-help-deskfreescout < 1.8.223 -

II. Public POCs for CVE-2026-53591

#POC DescriptionSource LinkShenlong Link
AI-Generated POCPremium
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)

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

CVE-2026-535959.4 CRITICALFreeScout vulnerable to anonymous account takeover via /user-setup empty invite_hash on My
CVE-2026-535938.8 HIGHFreeScout Vulnerable to Authenticated Remote Code Execution via incomplete upload extensio
CVE-2026-488127.5 HIGHFreeScout Allows Unauthenticated Access to Legacy Attachment Files
CVE-2026-452956.5 MEDIUMFreeScout Vulnerable to Unauthenticated Thread Read-Status Manipulation and Conversation E
CVE-2026-535965.3 MEDIUMFreeScout has unrestricted file upload without rate limiting that leads to resource exhaus
CVE-2026-535944.9 MEDIUMFreeScout has Arbitrary File Read in App Logs Viewer via Forged Encrypted Path
CVE-2026-535924.6 MEDIUMFreeScout vulnerable to prototype pollution in getQueryParam

IV. Related Vulnerabilities

Same product: freescout
Same vendor: freescout-help-desk
Same weakness: CWE-287

V. Comments for CVE-2026-53591

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