Output Escaping Vulnerability in NousResearch Slack and Mattermost Agent
CVE-2026-9354

6.9MEDIUM

Key Information:

Vendor

Nousresearch

Status
Hermes-agent
Vendor
CVE Published:
24 May 2026

Badges

👾 Exploit Exists🟡 Public PoC

What is CVE-2026-9354?

A security vulnerability has been identified in NousResearch hermes-agent versions up to 2026.4.16, specifically within an unrecognized function of the Slack and Mattermost Agents. This flaw allows for the manipulation of the 'format_message' argument, leading to improper escaping of output. As a result, attackers can exploit this flaw remotely, making it a significant security risk. Although efforts were made to inform the vendor about this issue prior to its public disclosure, there was no response from them. Organizations using the affected versions should prioritize reviewing their security posture and consider updating to address this vulnerability.

Affected Version(s)

hermes-agent 2026.4.0

hermes-agent 2026.4.1

hermes-agent 2026.4.2

Exploit Proof of Concept (PoC)

PoC code is written by security researchers to demonstrate the vulnerability can be exploited. PoC code is also a key component for weaponization which could lead to ransomware.

CVE-2026-9354 - Proof of Concept

References

https://vuldb.com/vuln/365317
vdb-entrytechnical-description
https://vuldb.com/vuln/365317/cti
signaturepermissions-required
https://vuldb.com/submit/812226
third-party-advisory

CVSS V4

Score:
6.9
Severity:
MEDIUM
Confidentiality:
None
Integrity:
Low
Availability:
Low
Attack Vector:
Network
Attack Complexity:
Low
Attack Required:
None
Privileges Required:
Undefined
User Interaction:
None

Timeline

  • 🟡

    Public PoC available

  • 👾

    Exploit known to exist

  • Vulnerability published

  • Vulnerability Reserved

Credit

Eric-i (VulDB User)
VulDB CNA Team
.