CVE-2026-55197: Hermes WebUI Broken Access Control Leaks Cross-Profile Session Transcripts

Technical summary

CVE-2026-55197 is a broken access control vulnerability (CWE-639) in Hermes WebUI's /api/session endpoint. The flaw allows authenticated attackers to enumerate and retrieve session transcripts from arbitrary profiles by directly specifying foreign session IDs in GET requests. The vulnerability stems from insufficient authorization checks—the application validates that a user is authenticated but fails to verify that the requested session belongs to the authenticated user's own profile. A request to GET /api/session?session_id=<foreign_id>&messages=1 will return unauthorized conversation data if the session ID is guessable or enumerable. The CVSS 3.1 score of 6.5 (Medium) reflects high confidentiality impact limited to authenticated attack surface and no integrity or availability effects.

Business impact

This vulnerability poses a significant privacy and compliance risk. Unauthorized access to user conversation transcripts could violate data protection regulations (GDPR, CCPA, etc.) and damage customer trust. For organizations using Hermes WebUI to manage sensitive business communications or customer support interactions, the exposure of cross-profile conversations creates liability. Competitors, malicious insiders, or account-compromised attackers could harvest proprietary discussions, strategy notes, or personal information embedded in transcripts. The attack requires valid credentials, limiting blast radius to environments where account compromise is feasible or insider threats exist.

Affected systems

Hermes WebUI versions prior to 0.51.443 are vulnerable. The vulnerability does not affect later versions; confirm your deployment version against the vendor advisory or version history. The /api/session endpoint is the attack surface; any authenticated user of the system can potentially initiate the attack.

Exploitability

Exploitability is moderate. An attacker must possess valid Hermes WebUI credentials—either legitimate access or compromised credentials. No special exploitation tools are needed; a simple HTTP GET request with manipulated session_id parameters suffices. However, discovering valid foreign session IDs requires either enumeration (if IDs are sequential or predictable) or prior knowledge of target session identifiers. The lack of CVSS active exploitation data (KEV status: false) suggests this has not yet been weaponized in the wild, but the attack is straightforward once session IDs are known.

Remediation

Upgrade Hermes WebUI to version 0.51.443 or later to receive the access control fix. Verify the patch version against the official vendor release notes. Additionally, implement network segmentation to restrict /api/session access, enforce strong password policies to reduce credential compromise risk, monitor API logs for anomalous session_id parameter patterns, and consider multi-factor authentication to reduce the impact of stolen credentials.

Patch guidance

Apply the vendor patch by upgrading to Hermes WebUI 0.51.443 or later. Coordinate with your Hermes deployment owner for patch availability and testing in non-production environments first. Verify patching by confirming the installed version matches or exceeds 0.51.443 and by reviewing vendor release notes for any migration or configuration changes. Test the /api/session endpoint post-patch to ensure it rejects requests for foreign session IDs.

Detection guidance

Monitor API access logs for suspicious patterns: repeated GET requests to /api/session with varying or sequential session_id parameters from the same source IP or user account, particularly outside normal usage windows. Implement rate limiting on the /api/session endpoint to detect enumeration attempts. Log all session_id parameters and cross-reference them against user profile associations—mismatches indicate exploitation. Set alerts for unauthorized session access attempts. Query audit logs for profile cross-referencing activity. If available, enable API authentication logging to correlate user identity with requested session IDs.

Why prioritize this

This vulnerability merits near-term patching because it directly compromises data confidentiality in a way that affects compliance and user privacy. While it requires authentication, credential compromise is common, and the attack is trivial once credentials are obtained. However, it does not enable privilege escalation, data modification, or denial of service, placing it at Medium severity rather than Critical. Organizations handling sensitive conversations should prioritize this patch ahead of cosmetic or low-impact issues.

Risk score, explained

The CVSS 3.1 score of 6.5 (Medium) reflects: (1) Network accessibility (AV:N)—the endpoint is remotely reachable; (2) Low complexity (AC:L)—no special conditions needed to exploit; (3) Low privilege requirement (PR:L)—valid credentials are required, limiting scope; (4) High confidentiality impact (C:H)—entire conversation transcripts are disclosed; (5) No integrity or availability impact (I:N/A:N)—no data modification or service disruption. The Medium severity appropriately balances the serious privacy harm against the authentication prerequisite.

Frequently asked questions