CVE: CVE-2026-21868
Name: Flag Forge (FlagForgeCTF) ReDoS Vulnerability
CVSS v3.1 Score: 7.5 (High)
Severity: High
Exploitability: Easily triggered over the network with no authentication and no user interaction.
Exploit Availability: No known public exploit code, but the pattern is trivial to abuse.
Impact: Availability / Denial of Service (CPU exhaustion).
Mitigation: Patch by upgrading to Flag Forge v2.3.3 or later
What Is This Vulnerability
Flag Forge is a platform used to run Capture The Flag competitions — essentially web software where people log in, view challenges, and submit flags.
One of its API endpoints lets anyone fetch public user profiles by username — something like:
GET /api/user/<username>The problem is that internally, the application turns that username into a regular expression (regex) without sanitizing or escaping special characters first. So if someone sends a username that is not just plain letters and numbers — but contains regex symbols like (, ), +, *, ?, {}, |, \, etc. — the server builds a regex that can cause massive CPU use while matching it.
Because regex engines can explode exponentially in work under certain patterns, this means a remote attacker can make the platform spend huge amounts of CPU on a simple request. As a result, legitimate users slow down or can’t use the platform — classic Denial of Service.
The core mistake is trusting untrusted input and building dangerous regex patterns from it.
How This Could Be Exploited
This is educational, defensive information only — never use it on systems you don’t own or are not authorized to test.
Here’s what an attacker could do:
1. Send specially crafted usernames
Instead of a normal name like:
/api/user/aliceThey could send:
/api/user/((a+)+)or
/api/user/(a+)+bThose aren’t real usernames — they are regex patterns. The server builds a regex from them and asks the database/search engine to match it.
2. Regex engines can backtrack a lot
Certain nested patterns like (a+)+ cause the regex engine to try huge numbers of permutations internally. As the regex engine backtracks, it uses more and more CPU time.
3. Remote execution without login
Because the endpoint is public and accepts anything after /api/user/, no login or privilege is needed. An attacker can script many such requests from anywhere.
4. CPU exhaustion = Denial of Service
Even without flooding with traffic, a single request with a pathological regex can peg CPU for seconds or longer. Repeatedly sending such requests — especially at scale — makes the application unresponsive for everyone else.
This is not a database hack, data breach, or code injection. It’s a resource exhaustion issue that hits availability.
What Part of the Stack Is Vulnerable
- The user profile lookup API (
/api/user/<username>). - The server code that builds a JavaScript (or backend) RegExp object from a raw username.
- The logic that sends that regex off to the database/search layer (like MongoDB), which applies it inefficiently.
- No input filtering or escaping before regex construction.
All of these together mean CPU can be consumed unexpectedly.
MITRE / Weakness Mapping
The underlying software weakness is:
- CWE-1333 | Inefficient Regular Expression Complexity — code uses regex patterns with exponential worst case, and input containing special regex constructs makes the system do huge amounts of work when trying to resolve them.
Is There a Public Exploit / PoC?
There’s no widely published exploit script or proof of concept floating around yet.
However, the abuse pattern is predictable given the nature of regex engines: feeding crafted regex strings that cause catastrophic backtracking will make the server consume large amounts of CPU.
Even without a specific “exploit script,” this is easy to trigger once you know the endpoint and that it builds regex from whatever you send.
How to Detect Exploitation Attempts
To know if someone is probing or exploiting this vulnerability, watch for patterns in logs and metrics like:
1. Web Server / Application Logs
Look for unusually long or weird username segments in requests to /api/user/.
Examples:
/api/user/(a+)+
/api/user/((.*)+)+
/api/user/%28a%2B%29%2BWatch especially for requests containing:
( ) + * { } | \ [ ] ?either raw or URL-encoded (%28, %29, %2B, etc.).
2. Performance Metrics
If CPU usage spikes on the server when those requests happen, or response times for /api/user/… suddenly jump, that’s a red flag.
3. Database Logs
If your database (e.g., MongoDB) logs regex query times or slow queries, an unusual pattern of $regex lookups taking a long time could indicate abuse.
4. Network / IDS Alerts
Systems that inspect HTTP paths can flag patterns with regex metacharacters hitting a user lookup endpoint.
Detection Rules You Can Use
Below are examples you can adapt to your environment:
Web Server Filter Example
Match dangerous characters in the username section:
^/api/user/.*[\(\)\{\}\+\*\?\|\[\]\\].*$Any request matching that could be flagged for review or rate-limited.
SIEM/Splunk Rule Example
Extract the username path and look for meta characters:
index=web_logs "/api/user/"
| rex field=uri_path "/api/user/(?<userpart>.*)$"
| where userpart like "%(%" OR userpart like "%+%" OR userpart like "%{%"
| stats count by userpart, clientip
This shows which weird values are coming in and from where.
What a Defender Should Do
Before Patch
If you can’t immediately upgrade:
1. WAF Rule
In your Web Application Firewall, block or challenge requests to /api/user/* when the path part contains regex metacharacters.
2. Rate Limiting
Cap how many times a single client can hit /api/user/* per minute to contain abuse.
3. Monitor CPU and Latency
Set alerts for spikes in CPU or app response times linked to this endpoint.
4. Regex Input Filtering
At the app layer, drop or sanitize requests where the username part contains any regex special characters.
These are temporary mitigations until you patch the software itself.
Definitive Fix
The only real fix is to upgrade Flag Forge so that usernames are handled safely — never built into regex patterns without escaping. That’s what the official mitigation does: it escapes special characters so user input can’t influence the regex logic.
Official Patch / Upgrade Link:
https://github.com/FlagForgeCTF/flagForge/security/advisories/GHSA-949h-9824-xmcx
Upgrade to Flag Forge v2.3.3 or later as soon as possible.
Final Takeaway
- This vulnerability lets a bad guy send specially crafted usernames that look like regex patterns.
- The system turns those into real regexes and asks the database to match them.
- Some regex patterns take huge amounts of CPU to resolve — so the server slows down or stops serving real users.
- You don’t need a login to trigger it — just an HTTP request to the public user API.
- There’s no public exploit script yet, but it’s easy to generate these patterns once you understand regex.
- Defenders should watch logs for weird usernames, watch CPU and latency, and apply temporary blocks on suspicious inputs.
- The permanent solution is to upgrade to the patched version.
