CVE-2026-21868
Flagforge 2.0 – 2.3.3
Raw vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:HSummary
CVE-2026-21868 is a high-severity Inefficient Regular Expression Complexity (CWE-1333) vulnerability in Flagforge Flagforge. Its CVSS base score is 7.5 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Endpoint Denial of Service (T1499); ranked at the 20th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to SA-11 (Developer Testing and Evaluation) and SA-15 (Development Process, Standards, and Tools) — see the control section below for these in your framework.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
CVE-2026-21868 is a Regular Expression Denial of Service (ReDoS) vulnerability (CWE-1333) affecting Flag Forge, a Capture The Flag (CTF) platform, in versions 2.3.2 and prior. The issue resides in the user profile API endpoint (/api/user/[username]), where the application dynamically constructs a regular expression using unescaped user input from the username parameter. This allows specially crafted input containing regex meta-characters, such as deeply nested groups or quantifiers, to trigger excessive CPU consumption in the MongoDB regex engine.
The vulnerability has a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H), indicating high severity due to its network accessibility and lack of prerequisites. Any unauthenticated remote attacker can exploit it by sending a malicious request to the affected endpoint with a crafted username, causing significant resource exhaustion. This results in denial of service for other users of the platform as the server becomes unresponsive.
The vulnerability is fixed in Flag Forge version 2.3.3. As a workaround, operators can implement a Web Application Firewall (WAF) rule to block requests containing regex meta-characters in the URL path. Additional details are available in the GitHub Security Advisory at https://github.com/FlagForgeCTF/flagForge/security/advisories/GHSA-949h-9824-xmcx.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-1664
Vulnerability Data
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…
more
(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.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Developer testing and evaluation can discover inefficient regex patterns via performance or static analysis.
Development standards and tools can require safe regex construction and forbid known exponential patterns.
Denial-of-service protections limit resource exhaustion caused by expensive regex evaluation.
Input validation can constrain data that would otherwise trigger worst-case regex complexity.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Security testing can detect and reject regex patterns with exponential worst-case complexity.
Secure development lifecycle mandates review of algorithmic efficiency, directly addressing ReDoS-prone regex.
Application security requirements can specify input-validation rules that limit regex complexity.
Secure architecture principles encourage avoidance of computationally expensive constructs such as catastrophic backtracking.
Secure coding standards explicitly prohibit or limit the use of inefficient regular expressions.