Service interruption via unescaped username regex
Published Jan 8, 2026 · Updated Jan 8, 2026
Regular expression denial of service in FlagForgeCTF flagForge before 2.3.3 allows remote attackers to interrupt service via a crafted username. The /api/user/[username] endpoint inserts the unescaped username path parameter into a RegExp used by MongoDB, allowing metacharacters to trigger CPU-intensive matching. No authentication or user interaction is required, and successful requests exhaust server CPU and disrupt access for other users.
Summary
What happened
Regular expression denial of service in FlagForgeCTF flagForge before 2.3.3 allows remote attackers to interrupt service via a crafted username. The /api/user/[username] endpoint inserts the unescaped username path parameter into a RegExp used by MongoDB, allowing metacharacters to trigger CPU-intensive matching. No authentication or user interaction is required, and successful requests exhaust server CPU and disrupt access for other users.
The record
- CVE
- CVE-2026-21868
- Published
- Jan 8, 2026
- Updated
- Jan 8, 2026
- Vendor
- FlagForgeCTF
- Product
- flagForge
- Classifications
- CWE-1333, T1499.003
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jan 8, 2026CVE publishedPublication date reported by the CVE source.
- Jan 8, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=< 2.3.3
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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