CVE-2026-9354
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedA 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · low confidenceVulnerability in NousResearch hermes-agent's Slack/Mattermost Agent component where the format_message argument handling results in improper output escaping. This could allow injection of malicious content through crafted messages sent via these agents.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- Low
- Availability
- Low
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify hermes-agent installation and versionCheck for hermes-agent package installation using package manager or look for the application binary, then determine its version numberAffected if hermes-agent version is 2026.4.16 or earlier
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Verify Slack or Mattermost agent components are enabledExamine the agent configuration files or runtime settings to determine if Slack Agent or Mattermost Agent components are activeAffected if Slack Agent or Mattermost Agent components are enabled
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Locate format_message configuration in agent settingsSearch configuration files, environment variables, or runtime settings for format_message parameter usage in Slack/Mattermost agent configurationsAffected if format_message is configured and being used
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Inspect how user input flows through format_messageReview application logs, trace message processing, or examine the configuration to see if untrusted/user-controlled input is passed to format_messageAffected if User-controlled input is directly used in format_message without sanitization
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Verify output encoding for Slack/Mattermost messagesCheck the message rendering pipeline to confirm proper output encoding/escaping is applied before messages are sent to Slack or MattermostAffected if No output encoding/escaping is applied to format_message output
The environment is affected if hermes-agent version is 2026.4.16 or earlier, Slack or Mattermost agent is enabled, and format_message processes untrusted input without proper encoding.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataImplement proper output encoding/sanitization for the format_message function in the Slack/Mattermost agent components to ensure all content is safely escaped according to each platform's message formatting requirements.
- Consultation4.0 h
- Implementation8.0 h
- Testing6.0 h
- Review / QA4.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $6,176.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-9354 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-9354 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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