CVE-2026-21868: Flag Forge is a Capture The Flag (CTF) platform

Routine

HIGH 7.5REMOTEUPGRADE

CVE-2026-21868 is a high-severity flaw (CVSS 7.5) affecting flagforge. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 quantif

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.3% (top 76%)
CISA exploited list	No
Weakness	CWE-1333
Published	2026-01-08
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: flagforge

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-01-08 01:15 UTCPublished CVE record published
2026-06-17 10:19 UTCUpdated Record last modified

flagforge flagforge UPGRADE

Affected: >= 2.0 and < 2.3.3

Fixed in: 2.3.3

Safe to downgrade to: a release before 2.0

1. Check installed version

apt-cache policy flagforge || rpm -q flagforge || flagforge --version

2. UPGRADE to 2.3.3 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade flagforge

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade flagforge

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 2.0 (Debian / Ubuntu)

apt-cache madison flagforge sudo apt install flagforge=<VERSION_BEFORE_2.0> sudo apt-mark hold flagforge

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list flagforge sudo dnf downgrade flagforge-<VERSION_BEFORE_2.0> sudo dnf versionlock add flagforge

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-21868?

flagforge: upgrade to 2.3.3. Exact commands for each OS are on this page.

Is CVE-2026-21868 being exploited?

CVE-2026-21868 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-21868 affect?

CVE-2026-21868 affects flagforge. Severity is high (CVSS 7.5). 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 expres

How urgent is CVE-2026-21868?

Priority 37/100. High severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2025-59841 CRITICAL 9.8 UPGRADE Flag Forge is a Capture The Flag (CTF) platform
CVE-2025-59826 HIGH 7.6 DOWNGRADE Flag Forge is a Capture The Flag (CTF) platform
CVE-2025-61777 CRITICAL 9.4 UPGRADE Flag Forge is a Capture The Flag (CTF) platform
CVE-2025-59932 HIGH 8.6 UPGRADE Flag Forge is a Capture The Flag (CTF) platform
CVE-2025-59843 MEDIUM 5.3 UPGRADE Flag Forge is a Capture The Flag (CTF) platform

References

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