Vulnerability · Published Jan 8, 2026 · Assigned by GitHub_M
CVE-2026-21868Flag Forge has ReDoS Vulnerability in User Profile Lookup API
High 7.5
In plain English
A high-severity regular-expression denial of service in flagForge from FlagForgeCTF. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 24% of all known vulnerabilities).
Official 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 (as reported)
| Vendor | Product | Versions |
|---|---|---|
| FlagForgeCTF | flagForge | < 2.3.3 |
Weakness: CWE-1333 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H