MatrixAdapter._markdown_to_html processes user–supplied Markdown into HTML for display in Matrix clients. The function does not correctly sanitize or escape malicious content, allowing an attacker to inject JavaScript or other executable code into the HTML output. When a victim loads a Matrix message that contains the injected content, the script runs in their browser. The vulnerability is classified as CWE‑79 (Cross‑Site Scripting) and CWE‑94 (Code Injection), and the CVSS base score of 5.1 indicates a moderate severity.

Any installation of NousResearch’s hermes‑agent component that is version 2026.5.29.2 or earlier is affected, because the vulnerable MatrixAdapter module resides in gateway/platforms/matrix.py. Deployments exposing the MatrixAdapter endpoint to external or untrusted users are at risk. No other assets are mentioned.

The EPSS score is reported as <1 %, indicating a low likelihood of exploitation, yet the exploit is publicly available on GitHub and can be executed remotely. An attacker can craft malicious Markdown that is rendered as HTML by a Matrix client, enabling script execution in the victim’s browser. Damage is contingent on end users viewing the compromised content, so the risk is moderate but should not be ignored. The vulnerability is not listed in CISA’s KEV catalog, but its remote nature and client‑side impact warrant monitoring and timely remediation.