CVE-2026-26717: OpenFUN Richie LMS Auth Bypass Vulnerability

CVE-2026-26717 Overview

An HMAC timing attack vulnerability has been identified in OpenFUN Richie, an open-source Learning Management System (LMS). The flaw exists in src/richie/apps/courses/api.py, specifically within the sync_course_run_from_request function. The application uses the non-constant time == operator for HMAC signature verification, which allows remote attackers to forge valid signatures and bypass authentication by measuring response time discrepancies.

Critical Impact

Remote attackers can exploit timing side-channels to gradually reconstruct valid HMAC signatures byte-by-byte, ultimately enabling authentication bypass and unauthorized access to protected API endpoints.

Affected Products

• OpenFUN Richie LMS (versions prior to security patch)
• Applications using src/richie/apps/courses/api.py with vulnerable signature verification

Discovery Timeline

• 2026-02-25 - CVE-2026-26717 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-26717

Vulnerability Analysis

This vulnerability falls under CWE-208 (Observable Timing Discrepancy), a well-documented class of side-channel attacks. The root issue lies in how Python's standard equality operator (==) performs string comparison. When comparing two strings, Python uses a byte-by-byte comparison that returns False immediately upon encountering a mismatch. This early termination creates measurable timing differences that attackers can exploit.

In the context of HMAC signature verification, this means an attacker can systematically brute-force each byte of a valid signature by analyzing response times. A correct byte will result in slightly longer processing time as the comparison proceeds to the next character, while an incorrect byte causes immediate rejection.

The vulnerability requires network-based exploitation with high attack complexity due to the precision timing measurements needed. However, successful exploitation could compromise both confidentiality and integrity of the affected system by enabling signature forgery.

Root Cause

The vulnerability stems from the use of Python's non-constant time == operator for comparing HMAC signatures in the sync_course_run_from_request function. Secure cryptographic implementations require constant-time comparison functions that always take the same amount of time regardless of where a mismatch occurs, preventing timing-based information leakage.

Attack Vector

The attack is network-accessible and requires no authentication or user interaction. An attacker would:

1. Send multiple requests with systematically varied signature bytes
2. Measure response times with high precision
3. Identify the correct byte based on timing discrepancies
4. Repeat for each byte position until a valid signature is constructed
5. Use the forged signature to bypass authentication and access protected endpoints

The security patch addresses this by importing Python's hmac module and using its built-in hmac.compare_digest() function for constant-time comparison:

# Security patch in src/richie/apps/courses/api.py
API endpoints for the courses app.
"""

+import hmac
+
from django.conf import settings
from django.core.cache import caches
from django.db.models import Q

Source: GitHub Commit Update

Detection Methods for CVE-2026-26717

Indicators of Compromise

• Unusual volume of API requests to the sync_course_run_from_request endpoint with varied signature values
• Rapid successive requests from the same source IP targeting authentication endpoints
• Requests with incrementally changing signature byte patterns
• Network traffic patterns indicative of timing analysis (precise request spacing)

Detection Strategies

• Monitor API endpoints for high-frequency requests with failed authentication attempts followed by eventual success
• Implement rate limiting on signature verification endpoints to detect and block timing attack attempts
• Deploy web application firewall (WAF) rules to identify patterns consistent with timing oracle attacks
• Analyze server logs for unusual request patterns targeting the courses API endpoint

Monitoring Recommendations

• Enable detailed logging for the sync_course_run_from_request function including request timing data
• Set up alerts for authentication bypass attempts or unusual signature verification patterns
• Monitor network latency metrics for API endpoints that could indicate timing attack reconnaissance
• Review access logs for repeated failed signature validations from single sources

How to Mitigate CVE-2026-26717

Immediate Actions Required

• Update OpenFUN Richie to the latest patched version that includes the security fix
• Review and audit any custom code using non-constant time comparison for cryptographic operations
• Implement rate limiting on affected API endpoints as a temporary defense measure
• Enable enhanced logging to detect potential exploitation attempts

Patch Information

The vulnerability has been addressed in commit a1b5bbda3403d7debb466c303a32852925fcba5f. The fix replaces the vulnerable == comparison with Python's hmac.compare_digest() function, which provides constant-time comparison to prevent timing attacks. Additional details are available in the Medium Article on Timing Attack.

Workarounds

• Implement strict rate limiting on the affected API endpoint to make timing measurements impractical
• Add random timing jitter to API responses to obscure timing discrepancies (not a complete fix)
• Deploy network-level controls to limit request frequency from individual sources
• Consider temporarily disabling the affected endpoint if not critical to operations until patching is complete

# Configuration example - Add rate limiting in nginx
location /api/courses/sync {
    limit_req zone=api_limit burst=5 nodelay;
    limit_req_status 429;
    proxy_pass http://richie_backend;
}