CVE-2026-9354 NousResearch hermes-agent Slack Agent/Mattermost Agent escape output
Remote attackers can inject malicious scripts via the format_message argument in the Slack or Mattermost agent, potentially leading to code execution.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-9354?
CVE-2026-9354 is a medium-severity cross-site scripting vulnerability in NousResearch hermes-agent, published on 24 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Which versions are affected by CVE-2026-9354?
How do I fix CVE-2026-9354?
No fix has been published for CVE-2026-9354 yet. Limit exposure of the affected software until one is available.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-9354 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-9354 exists.
Sources
Also known as GHSA-7pxq-5rjc-m2wr.