Security readout for executives and security teams
Plain-English summary
Flag Forge CTF platform versions 2.3.2 and earlier can be slowed or knocked offline through crafted username lookups. The issue affects availability, not data theft or modification. Public sources state the fix is version 2.3.3, with a WAF workaround for suspicious URL path characters.
Executive priority
Treat as a prompt availability-risk fix for public or event-critical CTF platforms. Prioritize upgrade before competitions, training events, or externally accessible deployments because a simple unauthenticated request pattern could degrade service for legitimate users.
Technical view
CVE-2026-21868 is a ReDoS flaw in /api/user/[username]. The application builds a MongoDB regular expression from unescaped username input. Regex metacharacters can trigger excessive CPU consumption, causing denial of service. CVSS 3.1 is 7.5: network exploitable, low complexity, no privileges, no user interaction, availability impact only.
Likely exposure
Exposure is likely where FlagForgeCTF flagForge 2.3.2 or below is deployed and the user profile API is reachable over a network, especially public CTF instances.
Exploitation context
The source bundle does not indicate active exploitation, and KEV is false. The vulnerability is remotely reachable without authentication according to the CVSS vector, but public evidence provided only supports the advisory description, not exploitation in the wild.
Researcher notes
Evidence is limited to the CVE record and GitHub advisory metadata in the provided bundle. The root issue is unescaped user input in a dynamically constructed regex passed to MongoDB. Validate defensively through code review, version checks, and monitoring rather than generating attack payloads.
Mitigation direction
- Upgrade FlagForgeCTF flagForge to version 2.3.3.
- If immediate upgrade is not possible, add WAF filtering for regex metacharacters in URL paths.
- Monitor CPU saturation and availability issues around user profile API traffic.
- Check the GitHub advisory for any updated vendor guidance.
Validation and detection
- Inventory Flag Forge deployments and confirm running versions.
- Verify whether /api/user/[username] is reachable from untrusted networks.
- Review application logs for unusual username path patterns and CPU spikes.
- Confirm version 2.3.3 or later after remediation.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
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CWE-1333: Exact CWE lookup
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Open ATT&CK lookupCVE-2026-21868 mapping review
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Open ATT&CK lookup- Severity
- High
- CVSS
- 7.5 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H3.93.6Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
7.5HighVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Source materials
- CVE List V5 sourceCVE List V5
- https://github.com/FlagForgeCTF/flagForge/security/advisories/GHSA-949h-9824-xmcxCVE reference · x_refsource_CONFIRM
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Inefficient Regular Expression Complexity
Inefficient Regular Expression Complexity represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.