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Vulnerability record · CVE-2026-21868 · published 8 January 2026

CVE-2026-21868: Flagforge inefficient regular expression (redos) vulnerability

Flagforge · Flagforge

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.

7.5 CVSS 3.1 High EPSS 0.31% · top 79.1% CWE-1333 · Inefficient regular expression (ReDoS)
7.5CVSS 3.1 base score
0.31%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
1References
17 Jun 2026Last modified by NVD

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.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

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

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Source: NIST National Vulnerability Database (record CVE-2026-21868), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.