What’s Vulnerable › Vulnerabilities
CVE-2026-21868: Flag Forge has ReDoS Vulnerability in User Profile Lookup API
- Severity: high (CVSS 7.5 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 0.3% probability of exploitation in the next 30 days
- Public exploit or detection template: none known
- Published: 2026-01-08 · updated 2026-06-17
- Weakness: CWE-1333
Description
Flag Forge is a Capture The Flag (CTF) platform. Versions 2.3.2 and below have a Regular Expression Denial of Service (ReDoS) vulnerability in the user profile API endpoint (/api/user/[username]). The application constructs a regular expression dynamically using unescaped user input (the username parameter). An attacker can exploit this by sending a specially crafted username containing regex meta-characters (e.g., deeply nested groups or quantifiers), causing the MongoDB regex engine to consume excessive CPU resources. This can lead to Denial of Service for other users. The issue is fixed in version 2.3.3. To workaround this issue, implement a Web Application Firewall (WAF) rule to block requests containing regex meta-characters in the URL path.
Affected products
- FlagForgeCTF flagForge: from 2.0 before 2.3.3 — fixed in 2.3.3
How to fix
Upgrade to a fixed version: FlagForgeCTF flagForge 2.3.3. Check the vendor advisory for mitigations.
References
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