# CVE-2026-21868: Flag Forge has ReDoS Vulnerability in User Profile Lookup API

> An attacker can cause a denial of service in Flag Forge by sending crafted usernames that trigger excessive CPU usage in the regex engine.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 8 Jan 2026
- **Updated:** 8 Jan 2026
- **Also known as:** GHSA-949h-9824-xmcx

## What is CVE-2026-21868?

CVE-2026-21868 is a high-severity denial of service vulnerability in FlagForgeCTF flagForge, published on 8 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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.

Weakness: CWE-1333 (Inefficient Regular Expression Complexity)

## Which versions are affected by CVE-2026-21868?

It affects 1 product.


Products: Flagforge Flagforge.

## How do I fix CVE-2026-21868?

No fix has been published for CVE-2026-21868 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-21868 being exploited?

There is no known exploitation of CVE-2026-21868 in the wild and no public exploit code.

## Other vulnerabilities in FlagForgeCTF flagForge

- [CVE-2025-59841](https://insights.sbomify.com/CVE-2025-59841/): FlagForgeCTF's Improper Session Handling Allows Access After Logout (critical 9.8)
- [CVE-2025-61777](https://insights.sbomify.com/CVE-2025-61777/): FlagForge Allows Unauthenticated Badge Template API Access (critical 9.4)
- [CVE-2025-59932](https://insights.sbomify.com/CVE-2025-59932/): FlagForgeCTF Unauthenticated Resource Modification/Deletion (high 8.6)
- [CVE-2025-59827](https://insights.sbomify.com/CVE-2025-59827/): FlagForgeCTF is Missing Authorization in main-v2 (high 8.2)
- [CVE-2025-59826](https://insights.sbomify.com/CVE-2025-59826/): FlagForgeCTF Vulnerable to Unauthorized Problem Creation (high 7.6)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-21868 and fix it if it is affected.

What it is: CVE-2026-21868 is a high-severity denial of service vulnerability in FlagForgeCTF flagForge, published on 8 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-21868 in the wild and no public exploit code.

Affected product: FlagForgeCTF flagForge. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-21868/ (Markdown: https://insights.sbomify.com/CVE-2026-21868/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21868
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21868
- OSV: https://osv.dev/vulnerability/CVE-2026-21868

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-21868/
Summaries are generated and may contain mistakes; the official description above is the record.