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Metal Gear Online 3 Flaw Let a Steam Lobby Execute Code on Your PC
Metal Gear Online 3, the multiplayer mode of Metal Gear Solid V: The Phantom Pain, contained a major vulnerability. It allowed the host of a Steam lobby to execute arbitrary code on the machines of players who joined it, with no action required on their part. A patch has been developed by Konami.
A banned-player counter that checks nothing
Released in 2015, Metal Gear Online 3 is the online 8 vs 8 mode available to those who own Metal Gear Solid V: The Phantom Pain. Konami's famous game relies on Steam Matchmaking to organize its multiplayer lobbies. In this case, it was precisely in the processing of lobby metadata that the security issue was identified.
According to Carnegie Mellon University's CERT/CC, the player exclusion mechanism relies on two fields:
kick_num, which indicates the number of players marked to be removed from the game.kicked_id_%i, which contains the corresponding Steam IDs.
When a client joins a lobby, it reads these values to check whether its own ID is on the list. That makes sense for determining whether the player is allowed to join the lobby or not. But in the code of the function responsible for this, there is a problem: this analysis never compares kick_num to the size of the fixed-length buffer allocated to store those IDs. A malicious host only has to advertise a number larger than the expected capacity to trigger what are known as out-of-bounds writes in memory.
According to CERT/CC, the memory area located right after this buffer is sensitive because it contains internal Steamworks structures. By exploiting this vulnerability, now tracked as CVE-2026-19874, an attacker can therefore hijack the program's execution flow.
Denuvo, the ingredient that makes everything worse
Hijacking execution flow only gives access, in theory, to code already present in the process. But to go further and inject then execute their own code, an attacker can rely on Denuvo. Indeed, the Metal Gear Online 3 binary includes regions protected by Denuvo, an anti-cheat and anti-piracy system, which has full access to certain areas (read, write, execute).
That's not all. When the host of a lobby leaves the match, the role is automatically transferred to another participant. An attacker can therefore reclaim that status during a match, then broadcast the poisoned metadata to all players already connected. This makes it possible to compromise multiple machines. Most importantly, this is what makes the attack possible, because the lobby host must be the one exploiting the flaw.
A fix delivered by Konami
In its bulletin dated August 24, 2026, CERT/CC states that it notified Konami on July 15, 2026. Even so, Konami never responded to the researchers, although the vulnerability is now patched. In fact, the issue affects game version 1.1.2.8 and was fixed in version 1.1.2.9 of the mgsvmgo.exe executable.
Besides, on SteamDB, the corresponding build 24176213 is dated August 4, 2026, with a note that no official patch notes accompanied this update, aside from the list of modified files in two repositories. Konami clearly lacked transparency on this one.
Even so, players will be forced to install the fix in order to access the online mode. The patch increased the server version number from 15 to 16 and the lobby version from 150 to 160. So, a client on an older version will not be able to connect until it updates.
Video games as an entry point to a workstation are not an isolated case. Remember, Microsoft fixed an RCE flaw in Age of Empires II: Definitive Edition in July 2026. Now, another classic title is affected by a flaw.
