CVE-2026-33934: OpenEMR Authorization Bypass Vulnerability

CVE-2026-33934 Overview

CVE-2026-33934 is an Authorization Bypass vulnerability in OpenEMR, a widely-used free and open source electronic health records (EHR) and medical practice management application. The vulnerability exists due to a missing authorization check in portal/sign/lib/show-signature.php that allows any authenticated patient portal user to retrieve the drawn signature images of any staff member by supplying an arbitrary user value in the POST body.

Critical Impact

Authenticated patient portal users can access confidential staff signature images, potentially enabling signature forgery, identity theft, or social engineering attacks against healthcare organization personnel.

Affected Products

• OpenEMR versions prior to 8.0.0.3
• OpenEMR patient portal signature functionality
• Deployments utilizing the portal/sign/lib/show-signature.php endpoint

Discovery Timeline

• 2026-03-26 - CVE-2026-33934 published to NVD
• 2026-03-26 - Last updated in NVD database

Technical Details for CVE-2026-33934

Vulnerability Analysis

This vulnerability represents a classic Insecure Direct Object Reference (IDOR) scenario within the OpenEMR patient portal. The show-signature.php endpoint fails to verify that the requesting user has appropriate permissions to view the signature associated with a given user parameter. An authenticated patient portal user can manipulate the user value in POST requests to enumerate and retrieve signature images belonging to any staff member in the system.

The vulnerability is particularly notable because the companion write endpoint (save-signature.php) was already hardened against this exact authorization bypass issue, but the corresponding read endpoint was overlooked during the security hardening process. This asymmetric protection created a gap that exposes sensitive staff signature data.

In healthcare environments, staff signatures are considered sensitive personal information and are often used for document authentication, prescription validation, and legal attestation purposes. Unauthorized access to these signatures poses significant risks for forgery and fraud.

Root Cause

The root cause is a missing authorization check (CWE-639: Authorization Bypass Through User-Controlled Key) in the portal/sign/lib/show-signature.php file. The endpoint accepts a user-controlled user parameter without validating whether the authenticated session has permission to access that specific user's signature data. The fix involves implementing proper access control using the AccessDeniedHelper class from OpenEMR's ACL framework.

Attack Vector

The attack is network-based and requires low privileges—specifically, valid authentication to the OpenEMR patient portal. An attacker with legitimate patient portal access can craft POST requests to the vulnerable endpoint with arbitrary user values to enumerate and retrieve staff signature images. No user interaction is required beyond the attacker's own actions.

// Security patch in portal/sign/lib/show-signature.php
// Source: https://github.com/openemr/openemr/commit/ae7ee1872d2e6300b165e24687cc90cf6847a4e5

 $signer = '';
 $ignoreAuth = false;
 
+use OpenEMR\Common\Acl\AccessDeniedHelper;
 use OpenEMR\Common\Session\SessionUtil;
 use OpenEMR\Common\Session\SessionWrapperFactory;

The patch introduces the AccessDeniedHelper class to enforce proper authorization checks before returning signature data, ensuring users can only access signatures they are authorized to view.

Detection Methods for CVE-2026-33934

Indicators of Compromise

• Unusual POST requests to /portal/sign/lib/show-signature.php with varying user parameter values
• High-volume requests to the signature endpoint from a single authenticated patient session
• Access logs showing patient portal users requesting signature data for user IDs that do not match their own
• Sequential or enumeration patterns in user parameter values across multiple requests

Detection Strategies

• Implement web application firewall (WAF) rules to monitor and alert on suspicious patterns in requests to /portal/sign/lib/show-signature.php
• Configure application logging to capture all POST requests to signature-related endpoints with full parameter details
• Establish baseline behavior for signature endpoint usage and alert on anomalies such as bulk requests or unusual user parameter values
• Review access logs for patient portal sessions making requests with staff-level user identifiers

Monitoring Recommendations

• Enable detailed HTTP request logging for all patient portal signature endpoints
• Deploy intrusion detection system (IDS) rules to identify IDOR attack patterns against healthcare applications
• Implement real-time alerting for any signature retrieval attempts where the requesting user differs from the target user parameter
• Conduct periodic log analysis to identify potential exploitation attempts that may have occurred before patching

How to Mitigate CVE-2026-33934

Immediate Actions Required

• Upgrade OpenEMR to version 8.0.0.3 or later immediately to address this vulnerability
• Review access logs for signs of exploitation, particularly POST requests to show-signature.php with suspicious user parameters
• Consider temporarily restricting access to the patient portal signature functionality until patching is complete
• Notify affected staff members if evidence of signature exfiltration is discovered

Patch Information

OpenEMR has released version 8.0.0.3 which patches this vulnerability. The fix implements proper authorization checks using the AccessDeniedHelper class to ensure users can only access signature data they are authorized to view. The security patch is available in commit ae7ee1872d2e6300b165e24687cc90cf6847a4e5.

For detailed patch information, refer to the GitHub Security Advisory GHSA-w9w5-7x6h-657q and the GitHub Release v8.0.0.3.

Workarounds

• If immediate patching is not possible, restrict network access to the patient portal to trusted IP ranges only
• Implement WAF rules to block POST requests to /portal/sign/lib/show-signature.php from patient portal sessions
• Disable the signature functionality in the patient portal configuration until the patch can be applied
• Apply the manual code fix from the GitHub commit if upgrading is delayed

# Upgrade OpenEMR to patched version
cd /var/www/openemr
git fetch --tags
git checkout v8_0_0_3

# Alternatively, apply the specific security patch
git cherry-pick ae7ee1872d2e6300b165e24687cc90cf6847a4e5

# Restart web server to apply changes
sudo systemctl restart apache2