TI-2026-17569 Aliases CVE-2026-21868

Flag Forge has ReDoS Vulnerability in User Profile Lookup API

LOW 39CWE-1333Published 2026-01-08 · Updated 2026-01-08

Key facts

Attack vector: Network
Privileges required: None
User interaction: None
Attack complexity: Low
Impact type: Denial of service
Confidentiality · integrity · availability: None · None · High
Fix status: Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA. Impact type from CWE-1333.

Summary

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.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV

EPSS 0.00312 24th percentileScored 2026-11-07

Fix Fixed flagForge (FlagForgeCTF): 2.3.3

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation: None
Automatable: Yes
Technical impact: Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

Vendor	Product	Affected versions	Fixed in	Status	Source
FlagForgeCTF	flagForge	before 2.3.3	2.3.3	Patch available	CNA

flagForge VendorFlagForgeCTF Affected before 2.3.3 Fixed in 2.3.3 Patch available CNA

CPE configurations NVD · 1

CPE	Versions	Role	Vendor
cpe:2.3:a:flagforge:flagforge::::::::	2.0 to before 2.3.3	Vulnerable	FlagForgeCTF

cpe:2.3:a:flagforge:flagforge:*:*:*:*:*:*:*:* Versions 2.0 to before 2.3.3 Vulnerable ·FlagForgeCTF

Remediation

Fix · CNA flagForge (FlagForgeCTF) Fixed in 2.3.3

Timeline

2026-10-02 Threatist record created from CVE-2026-21868

References

Advisory 1

https://github.com/FlagForgeCTF/flagForge/security/advisories/GHSA-949h-9824-xmcx

TI-2025-38170 FlagForge Allows Unauthenticated Badge Template API Access CVSS 3.1CRITICAL 9.4
TI-2025-37280 FlagForgeCTF's Improper Session Handling Allows Access After Logout CVSS 3.1CRITICAL 9.8
TI-2025-37266 FlagForgeCTF is Missing Authorization in main-v2 CVSS 3.1CRITICAL 9.8
TI-2025-37320 FlagForgeCTF Unauthenticated Resource Modification/Deletion CVSS 3.1HIGH 8.6
TI-2025-37272 FlagForgeCTF Hint Exposure via API CVSS 3.1HIGH 7.5
TI-2025-37282 FlagForgeCTF Exposes User Emails via Public /api/user/[username] API CVSS 4.0MEDIUM 6.9

See all 7 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences