CVE-2026-26717 in Richie
Summary
by MITRE • 02/25/2026
An issue in OpenFUN Richie (LMS) in src/richie/apps/courses/api.py. The application used the non-constant time == operator for HMAC signature verification in the sync_course_run_from_request function. This allows remote attackers to forge valid signatures and bypass authentication by measuring response time discrepancies
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Analysis
by VulDB Data Team • 02/28/2026
The vulnerability identified as CVE-2026-26717 resides within the OpenFUN Richie Learning Management System, specifically in the src/richie/apps/courses/api.py file. This flaw manifests in the sync_course_run_from_request function where the application employs a non-constant time comparison operator for HMAC signature verification. The implementation fails to use constant-time string comparison methods that are essential for preventing timing attacks. The vulnerability stems from the application's failure to properly validate cryptographic signatures using time-constant operations, creating a side-channel attack surface that adversaries can exploit.
The technical flaw represents a classic timing attack vulnerability that falls under CWE-203 - Observable Timing Discrepancy. When the application compares HMAC signatures using the standard == operator, it performs character-by-character comparison that reveals timing differences based on where the first mismatch occurs. This timing information can be measured by remote attackers to gradually reconstruct valid signatures through statistical analysis and repeated requests. The vulnerability directly impacts the authentication mechanism by allowing unauthorized parties to forge valid signatures without possessing the actual secret keys, effectively bypassing the entire signature verification process.
The operational impact of this vulnerability is significant as it compromises the integrity and authenticity guarantees provided by HMAC-based authentication. Attackers can exploit this weakness to impersonate legitimate users or systems, potentially gaining unauthorized access to course data, modifying course content, or executing unauthorized operations within the LMS environment. The vulnerability affects the core security controls of the application, undermining trust in the system's ability to verify the authenticity of incoming requests. This type of attack is particularly dangerous in educational platforms where sensitive student data and course materials are handled, as it could lead to unauthorized modifications or data breaches.
Mitigation strategies for this vulnerability involve implementing constant-time string comparison operations throughout the application's cryptographic verification processes. The solution requires replacing the standard == operator with a time-constant comparison function that takes equal time regardless of where differences occur in the compared strings. Security measures should include updating the sync_course_run_from_request function to utilize libraries or implementations that provide constant-time comparison such as the hmac.compare_digest() function in python. Additionally, organizations should implement comprehensive security testing including timing attack simulations to identify similar vulnerabilities in other cryptographic implementations. The fix aligns with ATT&CK technique T1211 - Exploitation for Defense Evasion, as attackers could leverage this vulnerability to bypass authentication mechanisms and maintain persistent access to the LMS system. Implementation of proper cryptographic practices and regular security audits are essential to prevent similar timing-based vulnerabilities from emerging in future code deployments.