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CVE-2026-21868— Flag Forge has ReDoS Vulnerability in User Profile Lookup API

Quick assessment

Affected
FlagForgeCTF flagForge
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Flag Forge是FlagForge开源的一个易于使用的CTF平台。 Flag Forge 2.3.2及之前版本存在安全漏洞，该漏洞源于用户资料API端点存在正则表达式拒绝服务问题。

CVSS 7.5 · High EPSS 0.31% · P22

Possible ATT&CK Techniques 1 AI

T1499 · Endpoint Denial of Service
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I. Basic Information for CVE-2026-21868

Vulnerability Information
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Vulnerability Title
Flag Forge has ReDoS Vulnerability in User Profile Lookup API
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
CWE-1333
Source: CVE Program / CVE List V5
Vulnerability Title
Flag Forge 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Flag Forge是FlagForge开源的一个易于使用的CTF平台。 Flag Forge 2.3.2及之前版本存在安全漏洞，该漏洞源于用户资料API端点存在正则表达式拒绝服务问题。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
FlagForgeCTF flagForge < 2.3.3 -

II. Public POCs for CVE-2026-21868

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III. Intelligence Information for CVE-2026-21868

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Vendor Advisories for CVE-2026-21868 (1)

IV. Related Vulnerabilities

Same product: flagForge
Same vendor: FlagForgeCTF
Same weakness: CWE-1333

V. Comments for CVE-2026-21868

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