Overview

CVE-2026-19874 is a critical vulnerability in Konami's Metal Gear Online 3, a popular multiplayer game. The vulnerability has a CVSS score of 9.1, indicating a high severity level. It is a heap-based buffer overflow vulnerability that occurs due to improper validation of lobby data fields related to kicked players. This vulnerability can potentially allow attackers to execute arbitrary code within the game process, making it a significant concern for players and administrators.

Understanding the Vulnerability / Threat

Root Cause Analysis

The root cause of CVE-2026-19874 is the improper validation of lobby data fields related to kicked players. Specifically, the affected function processes a list of kicked player identifiers using the lobby data key "kick_num" to determine the number of entries. However, the function does not validate that "kick_num" falls within the expected bounds. This allows an attacker to supply an oversized "kick_num" value and corresponding "kicked_id_%i" fields, leading to a heap-based buffer overflow. This vulnerability belongs to the CWE-122 category, which involves heap-based buffer overflows.

Attack Surface & Vector

The attack surface for CVE-2026-19874 is the network, as the vulnerability is exploitable via network-adjacent attacks. The attack vector involves sending malformed lobby data to the game server, which then processes the data without proper validation. The preconditions for exploitation are that the attacker must be able to send lobby data to the game server and that the server must process the data without proper validation.

Exploitation Mechanics — Scenario Walkthrough

Scenario: Compromising a Metal Gear Online 3 Game Server 1. Initial Position: An attacker is in a position to send lobby data to a Metal Gear Online 3 game server. 2. Triggering the Flaw: The attacker crafts a malicious lobby data packet with an oversized "kick_num" value and corresponding "kicked_id_%i" fields. The packet is sent to the game server, which processes the data without proper validation. 3. What Breaks: The game server processes the malformed lobby data, leading to a heap-based buffer overflow. The buffer overflow occurs because the server writes the provided player IDs past the end of the intended buffer and into adjacent memory regions. These adjacent regions contain Steam callback handler structures responsible for processing lobby data updates, lobby messages, and other related events. 4. Attacker's Prize: The attacker can overwrite fields within the callback handler structures, including function pointers and callback argument values. Successful exploitation may enable control-flow hijacking, potentially allowing arbitrary code execution within the game process.

Real-World Impact

The potential impact of CVE-2026-19874 is significant, as an attacker could exploit the vulnerability to execute arbitrary code within the game process. This could lead to a range of malicious activities, including but not limited to: - Arbitrary code execution - Data theft or manipulation - Lateral movement within the network - Deployment of malware or ransomware

Detection & Defense

Immediate Mitigations

The recommended mitigation for CVE-2026-19874 is to upgrade to Metal Gear Online 3 version 1.1.2.9 or later, which includes a patch for the vulnerability.

Detection Strategies

Detection of exploitation attempts for CVE-2026-19874 can be challenging, as the vulnerability is exploited via network-adjacent attacks and does not require authentication. However, defenders can monitor for suspicious network activity, such as unusual lobby data packets or anomalies in game server logs. Defenders can also implement network-based detection strategies, such as monitoring for traffic patterns that may indicate exploitation attempts.

Long-Term Hardening

To prevent vulnerabilities like CVE-2026-19874, defenders should implement a range of security best practices, including: - Regularly updating and patching software and systems - Implementing network segmentation and isolation - Monitoring network traffic and system logs - Implementing secure coding practices and code reviews

Key Takeaways

- CVE-2026-19874 is a critical heap-based buffer overflow vulnerability in Metal Gear Online 3. - The vulnerability is caused by improper validation of lobby data fields related to kicked players. - Exploitation of the vulnerability can lead to arbitrary code execution within the game process. - Defenders should upgrade to Metal Gear Online 3 version 1.1.2.9 or later and implement network-based detection strategies.

Sources

- National Vulnerability Database (NVD) - CVE-2026-19874