CVE-2026-21868: Flag Forge ReDoS Denial of Service Issue

CVE-2026-21868 Overview

Flag Forge is a Capture The Flag (CTF) platform that enables security enthusiasts to compete in cybersecurity challenges. A Regular Expression Denial of Service (ReDoS) vulnerability exists in versions 2.3.2 and below, affecting the user profile API endpoint (/api/user/[username]). The application dynamically constructs a regular expression using unescaped user input from the username parameter, allowing attackers to craft malicious payloads that cause catastrophic backtracking in the MongoDB regex engine.

Critical Impact

Attackers can exploit this vulnerability remotely without authentication to exhaust server CPU resources, causing Denial of Service for legitimate users of the CTF platform.

Affected Products

• Flag Forge CTF Platform versions 2.3.2 and below
• Applications using the vulnerable /api/user/[username] API endpoint
• Deployments without Web Application Firewall (WAF) protection against regex meta-characters

Discovery Timeline

• 2026-01-08 - CVE-2026-21868 published to NVD
• 2026-01-08 - Last updated in NVD database

Technical Details for CVE-2026-21868

Vulnerability Analysis

This ReDoS vulnerability stems from improper handling of user-supplied input in regular expression construction. The Flag Forge application accepts a username parameter through the /api/user/[username] API endpoint and directly incorporates this value into a MongoDB regex query without proper sanitization or escaping.

When an attacker supplies a username containing regex meta-characters such as deeply nested groups ((((a+)+)+)+) or complex quantifiers, the regex engine enters a state of catastrophic backtracking. This exponential time complexity in pattern matching causes the MongoDB instance to consume excessive CPU cycles while attempting to evaluate the malicious pattern against stored data.

The vulnerability is particularly impactful because it requires no authentication to exploit. Any external attacker with network access to the Flag Forge API can send crafted requests that tie up server resources, degrading performance for all platform users or causing complete service unavailability.

Root Cause

The root cause of CVE-2026-21868 is the failure to escape or sanitize user input before incorporating it into a dynamically constructed regular expression (CWE-1333: Inefficient Regular Expression Complexity). The application treats the username parameter as a literal string when it should be treating it as potentially hostile input containing regex control characters.

MongoDB's regex engine, like many regex implementations, is susceptible to patterns that create exponential matching scenarios. Without input validation, attackers can inject these pathological patterns directly into queries.

Attack Vector

The attack vector is network-based, requiring no privileges or user interaction. An attacker sends HTTP requests to the /api/user/[username] endpoint with a crafted username parameter containing regex meta-characters. The malicious input triggers expensive regex evaluation operations on the server, consuming CPU resources and potentially blocking legitimate requests.

The vulnerability can be exploited by sending specially crafted usernames containing patterns such as nested quantifiers, alternation with overlapping character classes, or recursive group references. Each malicious request forces the server to expend significant computational resources, and sustained attack traffic can exhaust available processing capacity.

For technical details and proof-of-concept information, refer to the GitHub Security Advisory.

Detection Methods for CVE-2026-21868

Indicators of Compromise

• Unusual CPU spikes on servers hosting the Flag Forge application
• Elevated response times or timeouts on the /api/user/ API endpoint
• HTTP request logs containing regex meta-characters in URL paths (e.g., +, *, ?, (, ), {, }, |, ^, $, [, ])
• MongoDB slow query logs showing regex operations with abnormally long execution times

Detection Strategies

• Monitor web server access logs for requests to /api/user/ containing encoded regex operators such as %2B, %2A, %7C, or %28
• Implement application performance monitoring (APM) to detect latency anomalies on user profile API calls
• Configure MongoDB profiling to capture and alert on regex queries exceeding normal execution thresholds
• Deploy intrusion detection signatures to flag HTTP requests with suspicious character patterns in URL paths

Monitoring Recommendations

• Establish baseline metrics for CPU utilization and API response times to enable anomaly detection
• Configure alerting thresholds for MongoDB query execution times, particularly for regex operations
• Implement rate limiting on the /api/user/ endpoint to reduce the impact of sustained attacks
• Enable detailed logging for the user profile API to support forensic analysis of potential exploitation attempts

How to Mitigate CVE-2026-21868

Immediate Actions Required

• Upgrade Flag Forge to version 2.3.3 or later, which contains the fix for this vulnerability
• Deploy a Web Application Firewall (WAF) rule to block requests containing regex meta-characters in URL paths
• Implement rate limiting on the /api/user/[username] endpoint to reduce attack surface
• Review server logs for evidence of prior exploitation attempts

Patch Information

The Flag Forge development team has addressed this vulnerability in version 2.3.3. Organizations running affected versions (2.3.2 and below) should prioritize upgrading to the patched release. The fix implements proper escaping of user input before constructing regex patterns, preventing attackers from injecting malicious expressions.

For detailed patch information and upgrade instructions, refer to the GitHub Security Advisory.

Workarounds

• Implement a Web Application Firewall (WAF) rule to block requests containing regex meta-characters in the URL path
• Add input validation at the application layer to reject usernames containing characters such as +, *, ?, (, ), {, }, |, ^, $, [, ], and \
• Configure reverse proxy or load balancer rules to sanitize or reject suspicious username patterns before they reach the application
• Implement request timeouts for the user profile API to limit the impact of any individual malicious request

# Example WAF rule for NGINX to block regex meta-characters in user profile requests
location ~ ^/api/user/ {
    # Block requests containing common regex meta-characters
    if ($request_uri ~* "[\+\*\?\(\)\{\}\|\^\$\[\]\\]") {
        return 403;
    }
    proxy_pass http://flagforge_backend;
}