CVE-2026-53591Public exploitNetwork attack vector

Unauthenticated conversation injection via skipped HMAC validation

Published Jul 20, 2026 · Updated Jul 20, 2026

Improper authentication in FreeScout before 1.8.223 allows remote attackers to inject messages into existing conversations by email. The FetchEmails command accepts an In-Reply-To thread ID without verifying its HMAC whenever the hash component is not exactly 16 characters. Access only to the public helpdesk address is required; injected mail appears as a customer reply, reopens the ticket, and changes its last-reply identity.

CVSS severity8.6
High
EPSS probability0.38%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper authentication in FreeScout before 1.8.223 allows remote attackers to inject messages into existing conversations by email. The FetchEmails command accepts an In-Reply-To thread ID without verifying its HMAC whenever the hash component is not exactly 16 characters. Access only to the public helpdesk address is required; injected mail appears as a customer reply, reopens the ticket, and changes its last-reply identity.

The record

CVE
CVE-2026-53591
Published
Jul 20, 2026
Updated
Jul 20, 2026
Vendor
FreeScout
Product
FreeScout
Classifications
CWE-287, T1659
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 20, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 20, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 1.8.223

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

FreeScout · FreeScoutversion=< 1.8.223

Published CVSS scores

8.6GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-287Improper Authentication
T1659Content Injection

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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