# CVE-2026-84289: NousResearch hermes-agent denial of service in MCP Tool

Source: https://junglewise.ai/threats/cve-2026-84289-nousresearch-hermes-agent-denial-of-service-in-mcp-tool  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-01. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-84289 · Severity: medium · CVSS 4.3 · Published 2026-09-01

Hermes Agent is an AI agent framework that processes tool definitions and resource constraints. A flaw in how it handles MCP Tool resource boundaries allows an attacker to trigger uncontrolled memory allocation, causing the service to become unresponsive or crash. An attacker can exploit this remotely without authentication.

Technologies: [NousResearch Hermes Agent](https://junglewise.ai/threats/technologies/hermes-agent). Vendors: [NousResearch](https://junglewise.ai/threats/vendors/nousresearch).

## Quick answers

**Is CVE-2026-84289 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-84289 being exploited in the wild.

**How severe is CVE-2026-84289?** Medium severity, with a CVSS score of 4.3, published 1 September 2026.

**Which products and versions are affected?** NousResearch Hermes Agent up to 0.18.2.

**Is there a fix for CVE-2026-84289?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-84289 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-84289?** Check whether you run NousResearch Hermes Agent at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The vulnerability exists in the list_tools function of tools/mcp_tool.py within the MCP Tool component of Hermes Agent. The root cause is missing validation of MCP resource boundaries, allowing an attacker to craft malicious input that triggers uncontrolled memory allocation and resource exhaustion. The attack is network-accessible and does not require authentication or user interaction. Exploitation results in a denial of service (DoS) condition, making the agent unavailable. The vendor was contacted prior to public disclosure but did not respond; patches are not currently available.

## Affected products

- NousResearch hermes-agent up to 0.18.2

## Timeline

- 2026-09-01: disclosed: Vulnerability disclosed publicly; vendor previously contacted but did not respond
- 2026-09-01: other: Exploit code made public

## References

- [https://github.com/lche511/cve/blob/main/Hermes_MCP_Tool_Resource_Amplification_DoS_Report/REPORT.md](https://github.com/lche511/cve/blob/main/Hermes_MCP_Tool_Resource_Amplification_DoS_Report/REPORT.md)
- [https://vuldb.com/cve/CVE-2026-84289](https://vuldb.com/cve/CVE-2026-84289)

## Related threats

- [NousResearch hermes-agent resource exhaustion in resourceBufferFromUrl](https://junglewise.ai/threats/cve-2026-85107-nousresearch-hermes-agent-resource-exhaustion-in)
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- [NousResearch hermes-agent authorization bypass in Session Management](https://junglewise.ai/threats/cve-2026-85105-nousresearch-hermes-agent-authorization-bypass-in-session)
- [NousResearch hermes-agent denial of service in ACP Prompt Workflow](https://junglewise.ai/threats/cve-2026-84288-nousresearch-hermes-agent-denial-of-service-in-acp-prompt)
- [NousResearch hermes-agent denial of service in session chat](https://junglewise.ai/threats/cve-2026-84287-nousresearch-hermes-agent-denial-of-service-in-session-chat)