NousResearch hermes-agent Slack Agent/Mattermost Agent escape output

published 2026-05-24 5 mo ago modified 2026-07-23 3 mo ago also EUVD-2026-31566

cvss: 6.5; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.3%; EPSS 0.3% chance of exploitation in 30 days, percentile 27%
public exploits: 0; none known

A vulnerability was detected in NousResearch hermes-agent up to 2026.4.16. The affected element is an unknown function of the component Slack Agent/Mattermost Agent. The manipulation of the argument format_message results in escaping of output. The attack can be executed remotely. The exploit is now public and may be used. The vendor was contacted early about this disclosure but did not respond in any way.

CISA SSVC

Exploitation: Proof of concept is public
Automatable: yes
Technical impact: partial
Assessed: 5 mo ago (2026-05-27)

Exploit prediction

0.34% chance of exploitation in the next 30 days, higher than 27% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

gist.github.com https://gist.github.com/YLChen-007/e90fb38ac03284176bae49898a3a46a4 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by	version	score	vector
CNA	4.0	6.9	CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:L/SC:N/SI:N/SA:N/E:P
CNA	3.1	6.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L/E:P/RL:X/RC:R
CNA	3.0	6.5	CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L/E:P/RL:X/RC:R
CNA	2.0	6.4	AV:N/AC:L/Au:N/C:N/I:P/A:P/E:POC/RL:ND/RC:UR
CNA	4.0	5.5	CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:L/SC:N/SI:N/SA:N/E:P/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
CNA	3.1	6.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L
CNA	2.0	6.4	AV:N/AC:L/Au:N/C:N/I:P/A:P

Affected products

NousResearch hermes-agent

Classification

Weakness

CWE-116 Improper Encoding or Escaping of Output
CWE-74 Improper Neutralization of Special Elements in Output Used by a Downstream Component

References

gist.github.com /YLChen-007/e90fb38ac03284176bae49898a3a46a4
exploit
vuldb.com /vuln/365317
vdb-entry, technical-description
vuldb.com /vuln/365317/cti
signature, permissions-required
vuldb.com /submit/812226
third-party-advisory

Sources

CVE List CVE-2026-9354
NVD CVE-2026-9354

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary