CVE-2026-21868
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedFlag 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceFlag Forge versions 2.3.2 and below have a ReDoS vulnerability where the /api/user/[username] endpoint dynamically constructs MongoDB regex patterns using unescaped user input. Attackers can inject regex meta-characters (nested groups or quantifiers) to cause excessive CPU consumption, leading to denial of service.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data>= 2.0, < 2.3.3CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify Flag Forge installation and versionLocate the Flag Forge application and retrieve its version number using the application's version command, package manager, or configuration file. Common methods include checking a package.json, version file, or running 'flagforge --version' if CLI available.Affected if The installed version is 2.0 or higher but lower than 2.3.3
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Confirm the vulnerable endpoint is exposedVerify that the /api/user/[username] endpoint is accessible in your environment by attempting a request to it (e.g., using curl or a browser to hit the endpoint). Check your API routing configuration to see if this route is registered.Affected if The endpoint is accessible and responds to requests containing a username parameter
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Inspect user input handling in the username endpointReview the code handling the /api/user/[username] route to determine if user-supplied username input is directly passed into MongoDB regex queries without escaping or sanitization of regex meta-characters. Look for patterns where the username parameter is used in $regex queries.Affected if User input from the username parameter is used in MongoDB regex operations without escaping regex special characters
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Check for existing regex input validationExamine any input validation, sanitization, or middleware layers that process the username parameter before it reaches the database query layer. Look for regex escape functions or whitelist validation.Affected if No input validation exists or regex meta-characters are not being escaped before use in MongoDB queries
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Test for ReDoS susceptibility (optional)If authorized and in a non-production environment, send test requests with known regex meta-characters (such as nested quantifiers like '(a+)+') in the username parameter and monitor response time and CPU usage.Affected if Requests with regex meta-characters cause noticeably slower response times or elevated CPU usage compared to normal requests
You are affected if Flag Forge version 2.0 through 2.3.2 is running with the /api/user/[username] endpoint exposed and user input is passed directly into MongoDB regex without escaping regex meta-characters.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped2.3.3
Upgrade to version 2.3.3 which contains the security fix, or implement a WAF rule to block requests containing regex meta-characters in the URL path as a temporary workaround.
2.3.3
- 1. Back up your current Flagforge installation and database before proceeding with the upgrade.
- 2. Obtain Flagforge version 2.3.3 from the official repository or release channel.
- 3. Stop the running Flagforge service to prevent data inconsistency during upgrade.
- 4. Replace the existing Flagforge files with the new version 2.3.3 files.
- 5. Verify that the upgrade process has updated all dependencies, particularly any MongoDB-related packages.
- 6. Restart the Flagforge service.
- 7. Test the /api/user/[username] endpoint with normal usernames to ensure functionality works correctly.
- 8. Verify that the vulnerability is no longer exploitable by confirming the username input is now properly escaped before being used in regex construction.
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation4.0 h
- Implementation8.0 h
- Testing4.0 h
- Review / QA2.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2026-21868 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-21868 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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