A recently patched vulnerability in MECCHA CHAMELEON allowed attacker-controlled Steam Workshop maps to write files to arbitrary locations on Windows systems, potentially resulting in remote code execution once the victim restarted their device.
Security researchers at Aikido Security disclosed a delayed remote code execution (RCE) vulnerability affecting the online hide-and-seek game MECCHA CHAMELEON, which reportedly sold 15 million copies in its first month.
The flaw enabled a malicious community map to exploit an exposed Unreal Engine audio recording function to place attacker-controlled files on a player’s system.
MECCHA CHAMELEON Flaw
The game’s developer addressed this issue in MECCHA CHAMELEON version 4.0.0, released on August 20, 2026. According to Aikido Security researcher Robbe Van Roey, the update is automatically installed before the game launches.
MECCHA CHAMELEON supports community-created levels distributed through the Steam Workshop. When a lobby host selects a map, other players are prompted to download it before joining the match.
Unlike traditional game levels that contain only models, textures, and other assets, the game’s Unreal Engine 5.6.1 maps can include Blueprints, Unreal’s visual scripting system.
These Blueprint graphs run within the MECCHA CHAMELEON process and can invoke native functions exposed by developers to Blueprint.
This architecture created a significant trust boundary problem: an untrusted map downloaded from a community platform could execute game-authorized Blueprint logic on every player’s machine.
Aikido’s researchers examined accessible Blueprint nodes, focusing on functions associated with operating system interactions, including saving, exporting, writing, recording, opening, and launching files.
The investigation identified Unreal’s Finish Recording Output function, which saves recorded audio to a user-supplied path. Researchers found that a malicious map could either provide an absolute path or construct a relative path to escape the game’s default BouncedWavFiles directory.
This created an arbitrary file-write primitive, enabling an attacker to target user-writable locations, including the Windows Startup folder, where files can run automatically after a system restart.
Initially, Unreal Engine’s automatic addition of a .wav extension prevented a straightforward executable drop. However, researchers found a parsing discrepancy between Unreal’s filename handling and lower-level Windows file APIs.
Including a null byte in the supplied name could cause the Windows API to disregard the appended suffix under the tested conditions.
To achieve code execution, more than just selecting a risky destination was required; the file’s content also had to be executable. The researchers used uncompressed 16-bit PCM WAV audio, in which each audio sample directly represents the bytes written to the WAV data section.
Since Unreal generated the RIFF/WAVE header, conventional PE executables could not be used. Instead, the proof of concept embedded HTML Application (HTA) content within the PCM audio samples.
Windows can execute .hta files via mshta.exe; unlike browser-hosted HTML, HTA scripts can access Windows COM objects and initiate processes.
The final malicious map activated the recording workflow upon level load, played the prepared PCM audio, and then saved the resulting payload into a Startup-folder path. The payload would execute only after the victim restarted Windows, making this a delayed RCE rather than immediate code execution.
Aikido reported the issue to MECCHA CHAMELEON’s maintainers, who disabled file creation through the affected StopRecordingOutput functionality in version 4.0.0. Researchers also reviewed Steam Workshop maps and stated they found no evidence that existing maps were exploiting the vulnerability.
However, the researchers cautioned that other alleged flaws have been reported privately but were not independently confirmed.
Until the security posture of custom maps becomes clearer, players should prefer official maps, apply updates promptly, and treat unsolicited Workshop content as potentially untrusted code rather than harmless game content.
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