VU#728712: Konami’s Metal Gear Online 3 contains a heap-based buffer overflow

Overview

Konami’s Metal Gear Online 3 video game contains a heap-based buffer overflow that can be triggered by an input‑validation vulnerability that allows match hosts to remotely execute arbitrary code on lobby members’ machines through specially crafted data.

Description

Metal Gear Online 3 is an online 8 vs. 8 competitive shooter game that uses Steam Matchmaking to handle its multiplayer lobbies and matches. As detailed in CVE‑2026‑19874, version 1.1.2.8 of Metal Gear Online 3 (Steam AppID 287700) contains an input‑validation vulnerability in the processing of Steam lobby metadata related to the player‑removal feature. The game tracks a lobby field (kick_num) indicating the number of players designated for removal, along with corresponding Steam ID entries of each kicked player (kicked_id_%i). When joining a lobby, the player’s client parses these fields to check whether its own Steam ID is on the list, and if so, the player is prevented from joining the match.

The function responsible for parsing this lobby data does not validate the kick_num value against the size of the fixed‑length buffer allocated for kicked player identifiers. Supplying a kick_num value larger than the buffer capacity results in out‑of‑bounds writes into adjacent memory. The memory region immediately following this buffer contains internal Steamworks callback handler structures that store function pointers and callback arguments for processing lobby data changes, messages, and other related events. By manipulating the overflow, an attacker can corrupt these handler structures and redirect callback execution, resulting in control‑flow hijacking on affected client systems. The vulnerability can be triggered automatically when a client joins a lobby controlled by an attacker.

Impact

Exploitation of this vulnerability may allow remote code execution on affected clients. Initial control‑flow hijacking provides access only to existing in‑process code; however, the Metal Gear Online 3 binary includes Denuvo‑protected regions mapped with read‑write‑execute (RWX) permissions. These regions permit runtime injection of attacker‑supplied code, significantly increasing the severity of the issue. An attacker hosting a lobby can achieve code execution on any client that joins, without requiring further interaction from the victim. Additionally, because host privileges are automatically reassigned to another lobby participant when the current host exits, an attacker can obtain host control during an active match and subsequently deliver the malicious lobby data to all connected players. This enables compromise of multiple systems through a single exploitation event.

Solution

As of this writing, Konami has not released patch notes or an advisory that specifically addresses this vulnerability, but a fix was included in version 1.1.2.9 of the Metal Gear Online 3 executable, mgsvmgo.exe. The patch also iterated the server and lobby version numbers from 15 to 16 and 150 to 160, respectively, to prevent players on older versions from accessing the online services.

This is the latest patch that fixed the vulnerability:
https://steamdb.info/patchnotes/24176213/

The full patch list can be found here:
https://steamdb.info/app/287700/patchnotes/

Acknowledgements

Thank you to Alice Cecchetto for reporting this vulnerability. This document was written by Bob Kemerer.