VulnerabilityPublished 24 Mar 2026

CVE-2026-33250: longturn freeciv21 · Improper Input Validation

Affects longturn freeciv21Source CVE Database V5

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Not known to be exploited · CVSS 7.5 High : upgrade to 3.1.1 soon.

Data as of 30 Oct 2026, 11:52 UTC

CVSS 3.1 base score

7.5High

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

EPSS: 0.8% chance of exploitation in 30 days – higher than 58% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 3.1.1

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 24 Mar 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: 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 30 Oct 2026.

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.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity 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 freeciv21 and longturn/freeciv21 to 3.1.1.

Affected software

Product	Affected	Fixed in
longturnfreeciv21	Affected <3.1.1	Fixed in 3.1.1
GitHublongturn/freeciv21`pkg:github/longturn/freeciv21`Other threats	Affected <3.1.1	Fixed in 3.1.1

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

AI-suggested

To mitigate CVE-2026-33250, users and administrators should immediately upgrade all Freeciv21 installations to version 3.1.1 or later, which contains the necessary input validation fixes to prevent stack overflow. For public servers, deploying network-level protections such as firewalls and intrusion prevention systems to restrict access to trusted IP ranges can reduce exposure. Monitoring network traffic for anomalous or malformed packets targeting Freeciv21 ports may help detect exploitation attempts. Configuring enhanced logging or employing external monitoring tools can improve visibility since default logs lack useful information. For client users, connecting only to trusted servers minimizes risk of client crashes. Server operators should consider implementing rate limiting and connection throttling to mitigate potential denial-of-service attacks. Regularly reviewing and applying security updates from the Freeciv21 project and maintaining good patch management practices are essential. Additionally, educating users about the risks of connecting to untrusted servers can reduce client-side impact.

Description

Freeciv21 is a free open source, turn-based, empire-building strategy game. Versions prior to 3.1.1 crash with a stack overflow when receiving specially-crafted packets. A remote attacker can use this to take down any public server. A malicious server can use this to crash the game on the player's machine. Authentication is not needed and, by default, logs do not contain any useful information. All users should upgrade to Freeciv21 version 3.1.1. Running the server behind a firewall can help mitigate the issue for non-public servers. For local games, Freeciv21 restricts connections to the current user and is therefore not affected.

Impact

The primary impact of CVE-2026-33250 is a denial-of-service condition affecting Freeciv21 servers and clients. Public servers can be remotely crashed by unauthenticated attackers, disrupting gameplay and potentially causing reputational damage to server operators. For players, connecting to a malicious server can result in client crashes, degrading user experience and trust. The lack of effective logging complicates incident detection and response. Organizations or communities hosting Freeciv21 servers may face service outages, loss of player engagement, and increased support costs. While the vulnerability does not directly compromise confidentiality or integrity, the availability impact can be significant, especially for popular public servers. The ease of exploitation and network accessibility increase the risk of widespread disruption. Since Freeciv21 is open source and used globally, the threat affects a broad user base, particularly in regions with active gaming communities and public server hosting. The vulnerability could be leveraged in coordinated denial-of-service campaigns against gaming communities or as a nuisance attack vector.

Summary

Freeciv21, an open-source turn-based strategy game, suffers from a critical vulnerability identified as CVE-2026-33250, stemming from improper input validation (CWE-20) and resulting in a stack-based buffer overflow (CWE-121). Versions earlier than 3.1.1 fail to properly validate incoming network packets, allowing specially-crafted packets to overflow the stack and crash the server or client application. This vulnerability requires no authentication or user interaction, making exploitation straightforward over the network. The attack surface includes any public Freeciv21 server exposed to untrusted networks, where an attacker can remotely trigger a denial-of-service by sending malicious packets. Conversely, a malicious server can exploit this flaw to crash clients connecting to it. The default logging configuration does not capture meaningful data to aid in detection or forensic analysis. Local games are unaffected due to connection restrictions limiting communication to the current user. The vulnerability has a CVSS v3.1 base score of 7.5, reflecting high severity primarily due to its network attack vector, lack of required privileges, and complete denial-of-service impact on availability. No known exploits are currently reported in the wild. Mitigation involves upgrading to Freeciv21 version 3.1.1, which addresses the input validation and overflow issues. Additional network-level protections such as firewalls can reduce exposure for non-public servers.

CWE-20Improper Input Validation (opens in a new tab)
Class weaknessLikelihood of exploit: HighCWE Top 25 (2025): #18OWASP Top 10 A05:2025: Injection (opens in a new tab)
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.
MITRE discourages mapping vulnerabilities to this broad entry; the root cause is usually a more specific weakness.
Architecture and Design · Attack Surface Reduction
Consider using language-theoretic security (LangSec) techniques that characterize inputs using a formal language and build "recognizers" for that language. This effectively requires parsing to be a distinct layer that effectively enforces a boundary between raw input and internal data representations, instead of…
Architecture and Design · Libraries or Frameworks
Use an input validation framework such as Struts or the OWASP ESAPI Validation API. Note that using a framework does not automatically address all input validation problems; be mindful of weaknesses that could arise from misusing the framework itself (CWE-1173).
Architecture and Design · Implementation · Attack Surface Reduction
Understand all the potential areas where untrusted inputs can enter the product, including but not limited to: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any…
OWASP Cheat Sheets
XML Security (opens in a new tab)
Mass Assignment (opens in a new tab)
Injection Prevention (opens in a new tab)
SQL Injection Prevention (opens in a new tab)
Injection Prevention in Java (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.1 (opens in a new tab)
V5.1.4 (opens in a new tab)
V5.1.3 (opens in a new tab)
Attack patterns
Buffer Overflow via Environment Variables (opens in a new tab)
Server Side Include (SSI) Injection (opens in a new tab)
Cross Zone Scripting (opens in a new tab)
Command Line Execution through SQL Injection (opens in a new tab)
Object Relational Mapping Injection (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-121Stack-based Buffer Overflow (opens in a new tab)
Variant weaknessLikelihood of exploit: HighCWE Top 25 (2025): #14
The product writes past the end of a buffer that was allocated on the stack.
Operation · Build and Compilation · Environment Hardening
Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index…
Architecture and Design
Use an abstraction library to abstract away risky APIs. Not a complete solution.
Implementation
Implement and perform bounds checking on input.
OWASP Cheat Sheets
XML Security (opens in a new tab)
Mass Assignment (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.1 (opens in a new tab)
V5.1.4 (opens in a new tab)
V5.1.3 (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 24 Mar 2026, 00:15 UTC

WeaknessesCWE-20, CWE-121

Technical details

References

Community reviews

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