CVE-2026-21868VulnerabilityPublished 8 Jan 2026

CVE-2026-21868: FlagForgeCTF flagForge · ReDoS

Affects FlagForgeCTF flagForgeSource CVE Database V5

Not known to be exploited · CVSS 7.5 High : upgrade to 2.3.3 soon.

Data as of 25 Oct 2026, 19:02 UTC

CVSS 3.1 base score

7.5High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.3% chance of exploitation in 30 days – higher than 23% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.3.3

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 8 Jan 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 2026-10-25.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade flagForge and flagforgectf/flagForge to 2.3.3.

Affected software

Product	Affected	Fixed in
FlagForgeCTFflagForge	Affected <2.3.3	Fixed in 2.3.3
GitHubflagforgectf/flagForge`pkg:github/flagforgectf/flagForge`Other threats	Affected <2.3.3	Fixed in 2.3.3

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

1. Upgrade FlagForgeCTF instances to version 2.3.3 or later immediately to apply the official fix that properly escapes user input in regex construction. 2. Until patching is possible, implement Web Application Firewall (WAF) rules specifically designed to detect and block HTTP requests containing regex meta-characters (e.g., *, +, ?, {, }, (, ), [, ], |, ^, $, ., \) in the URL path, particularly targeting the /api/user/[username] endpoint. 3. Employ input validation and sanitization on the username parameter to reject or escape regex meta-characters before processing. 4. Monitor server CPU usage and application logs for unusual spikes or repeated requests containing suspicious patterns indicative of ReDoS attempts. 5. Restrict public access to the FlagForgeCTF platform where feasible, limiting exposure to trusted networks or VPNs. 6. Educate developers and administrators on the risks of dynamic regex construction from untrusted input and encourage secure coding practices. 7. Consider rate limiting or request throttling on the vulnerable API endpoint to reduce the impact of potential abuse. 8. Conduct regular security assessments and penetration tests focusing on input validation and denial of service vectors.

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.

Impact

For European organizations using FlagForgeCTF, particularly those involved in cybersecurity training, competitions, or educational programs, this vulnerability poses a significant risk of service disruption. An attacker exploiting this flaw can cause denial of service conditions by exhausting server CPU resources, leading to downtime or degraded performance of the CTF platform. This can interrupt training schedules, competitive events, or internal security exercises, potentially impacting organizational readiness and reputation. Since the vulnerability does not compromise confidentiality or integrity, the primary impact is availability. However, prolonged denial of service could indirectly affect operational continuity and user trust. Organizations relying on FlagForgeCTF as a critical component of their cybersecurity education or recruitment pipelines may face operational setbacks. Additionally, if the platform is exposed to the public internet without adequate protections, the risk of exploitation increases. The lack of authentication requirement for exploitation further amplifies the threat, making it accessible to remote attackers without credentials.

CWE-1333Inefficient Regular Expression Complexity (opens in a new tab)
Base weaknessLikelihood of exploit: High
The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.
Architecture and Design
Use regular expressions that do not support backtracking, e.g. by removing nested quantifiers.
System Configuration
Set backtracking limits in the configuration of the regular expression implementation, such as PHP's pcre.backtrack_limit. Also consider limits on execution time for the process.
Implementation
Do not use regular expressions with untrusted input. If regular expressions must be used, avoid using backtracking in the expression.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.1 (opens in a new tab)
Attack patterns
Regular Expression Exponential Blowup (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

github.com/FlagForgeCTF/flagForge/security/advisories/GHSA-949h-9824-xmcx (opens in a new tab)

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 15 Jan 2026, 04:37 UTC

WeaknessesCWE-1333

Technical details

References

Community reviews

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