MongoDB
CVE-2026-21868
HIGH
Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain
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AccessSend crafted username with regex metacharacters
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ExploitMongoDB regex engine performs catastrophic backtracking
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ImpactCPU exhaustion causes service unavailability
Vulnerability Assessment
| Weakness |
CWE
OWASP Top 10:2025
A06:2021 - Vulnerable and Outdated Components
ASVS 5.0
V2 - Validation and Business Logic Regex complexity limits
MITRE ATT&CK
1 technique across 1 tactic: Impact The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up. |
| Exploitation Conditions | Flag Forge versions 2.3.2 and below with exposed /api/user/[username] endpoint accepting unescaped user input in regex patterns. No authentication required; default configurations vulnerable to remote ReDoS attacks. |
| Impact | CVSS 7.5 (HIGH). Attack vector: Network. Complexity: Low. Privileges required: None. User interaction: None. |
| Exploit ScenarioDerived | An attacker could exploit this vulnerability to Denial of Service for other users. |
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Alert meAnalysisAIv1 – Mar 12, 2026
Flagforge versions 2.3.2 and earlier suffer from a Regular Expression Denial of Service (ReDoS) vulnerability in the user profile API endpoint that accepts unvalidated usernames containing regex metacharacters, allowing unauthenticated remote attackers to trigger excessive CPU consumption and deny service to legitimate users. The MongoDB regex engine processes these malicious patterns inefficiently, making the platform unavailable without administrator intervention. No patch is currently available; users should implement WAF rules to filter regex metacharacters from username inputs as a temporary mitigation.
Earlier versions of this analysis (1)
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]). [CVSS 7.5 HIGH]
RemediationAI
Fixed in version 2.3.3.. Restrict network access to the affected service where possible.
Affects your stack
Find out whether the stacks and servers you run are on an affected version.
Checking this CVE against your own stacks and servers is part of the Basic plan and up.
Technical ContextAI · show details
Classified as CWE-1333 (Inefficient Regular Expression Complexity (ReDoS)). Affects the user profile API component of 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
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Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Primary rating from GitHub Advisory · only source for this CVE.
CVSS VectorGitHub Advisory
DescriptionGitHub Advisory
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.
Executive ViewAI
Organizations using Flag Forge face notable risk from CVE-2026-21868 rated CVSS 7.5. Exploitation could compromise the security of affected deployments. Organizations should apply vendor updates when available and consider compensating controls in the interim.
Apply vendor patch. If immediate patching is not feasible: (1) Implement network-level access controls to limit exposure; (2) Enable enhanced monitoring and alerting; (3) Apply principle of least privilege.
Threat Intelligence Overview
NVD
CVE.org
EPSS
GitHub
How this decision was reached
The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.
What every action level means
SSVC AssessmentENISA
Priority Score
Attack Technique
Denial of Service attacks render applications or systems unavailable by overwhelming resources or triggering failure conditions.
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