CVE-2026-53871: Hermes WebUI Authorization Bypass via Cookie Forgery

Technical summary

CVE-2026-53871 is an authorization bypass vulnerability in Hermes WebUI's get_profile_cookie() function. The flaw stems from insufficient validation of the hermes_profile cookie parameter, which is used to determine the scope of profile-level access controls. The function accepts unauthenticated (unverified) profile name values without confirming that the authenticated user is authorized to access the requested profile. This allows an authenticated attacker to forge arbitrary profile cookie values and bypass profile-scoped authorization checks, gaining unauthorized access to sessions, files, and resources associated with other profiles. The vulnerability is classified as CWE-565 (Authorization of Resource Compared Against Wrong Endpoint). The CVSS 3.1 score of 8.1 reflects high impact to confidentiality and integrity, with network accessibility and low attack complexity.

Business impact

This authorization bypass enables lateral privilege escalation within multi-user Hermes WebUI deployments. An authenticated user—whether a regular employee, contractor, or compromised account—can access confidential files, sessions, and resources belonging to other profiles, including potentially privileged or administrative accounts. In organizations using Hermes WebUI for sensitive workflows (data analysis, reporting, content management), this could lead to unauthorized data exfiltration, modification of critical records, or disruption of business processes. The attack requires no user interaction and leaves minimal forensic traces if the attacker is careful, increasing detection difficulty.

Affected systems

Hermes WebUI versions prior to 0.51.368 are affected. Organizations should inventory their Hermes WebUI deployments and confirm the installed version. Deployments supporting multi-user workflows or hosting sensitive data are at highest risk.

Exploitability

Exploitability is moderate-to-high. The attack requires an attacker to already possess valid credentials and be authenticated to the Hermes WebUI instance—it is not unauthenticated remote exploitation. However, once authenticated, no additional user interaction is required, and the attack surface is broad: any attacker with a valid login can attempt profile impersonation. Cookie manipulation is trivial (using browser developer tools or scripted requests), and the absence of stronger authorization checks (e.g., cryptographic proof of profile ownership or server-side session binding) makes successful exploitation straightforward. The vulnerability is not currently listed in the CISA KEV catalog.

Remediation

Organizations should upgrade Hermes WebUI to version 0.51.368 or later. The patch applies stronger validation to profile identifiers and enforces proper authorization checks before granting access to profile-scoped resources. Until patching is feasible, consider restricting Hermes WebUI access to trusted networks or implementing additional application-level monitoring for suspicious profile access patterns.

Patch guidance

Apply the available patch to upgrade Hermes WebUI to 0.51.368 or later. Verify the upgrade against official Hermes release notes and advisories. Test the patched version in a non-production environment to ensure compatibility with your deployment before rolling out to production. No breaking changes have been identified in standard upgrade procedures, but confirm against your specific configuration.

Detection guidance

Monitor Hermes WebUI access logs for anomalous profile-switching behavior—look for a single user session accessing multiple distinct profiles within a short timeframe, particularly if those profiles belong to different departments or have different privilege levels. Inspect the hermes_profile cookie in HTTP requests for values that do not match the authenticated user's own profile. Log aggregation tools should flag rapid or batch requests to profile-scoped endpoints (files, sessions, resources) from single source IPs. Application-level logging of authorization check failures would be most definitive.

Why prioritize this

This vulnerability merits prompt patching due to its high CVSS score (8.1), the high impact to data confidentiality and integrity, and the broad attack surface (any authenticated user). The attack is straightforward to execute and difficult to detect, making it an attractive target for malicious insiders or attackers with valid credentials. Multi-tenant or multi-user Hermes WebUI environments should prioritize this above standard maintenance patches.

Risk score, explained

CVSS 3.1 score of 8.1 (HIGH) reflects: (1) network-accessible attack vector; (2) low attack complexity—cookie manipulation requires minimal technical sophistication; (3) low privileges required—an authenticated user is sufficient; (4) no user interaction needed; (5) high impact to confidentiality (unauthorized access to other users' data) and integrity (potential modification of files and sessions); (6) no impact to availability. The score does not account for the prerequisite of valid authentication, which moderates real-world exploitability somewhat, but the ease of execution and severity of impact justify the HIGH rating.

Frequently asked questions