# 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.

- **Severity:** Medium 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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 24 May 2026
- **Updated:** 27 May 2026
- **Also known as:** GHSA-7pxq-5rjc-m2wr

## 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.

Weakness: CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component), CWE-116 (Improper Encoding or Escaping of Output)

## 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.

## Is CVE-2026-9354 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-9354 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-9354 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-9354 exists.

Affected product: NousResearch hermes-agent. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-9354/ (Markdown: https://insights.sbomify.com/CVE-2026-9354/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-9354
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-9354
- OSV: https://osv.dev/vulnerability/CVE-2026-9354

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-9354/
Summaries are generated and may contain mistakes; the official description above is the record.