While testing multiple Proton prefixes, I noticed that Windows applications running under Proton can enumerate significant parts of the host Linux filesystem, including:

Real user home directory paths

Desktop session logs (sddm, xsettingsd, KDE activity logs)

Baloo index database

Kernel and bootloader files from /boot

External drive paths under /run/media/

GPU and hardware details

Absolute Linux host paths included in Proton/Steam logs

This appears across multiple Proton versions and multiple game prefixes.

Although this does not provide write access, it exposes a large amount of host system metadata to any Windows application running inside Proton.

Affected Proton Versions

Proton Experimental

Likely applies to all Proton builds that map Z:\ to the full host filesystem

Environment

Linux (x86_64)

Steam library on secondary drive (/mnt/.../SteamLibrary)

GPU: NVIDIA GeForce RTX 3060

Prefixes tested: 489830, 1716740, and several others

Steps to Reproduce

Install any Windows game using Proton.

Launch once to generate the prefix.

On the host, run:

grep -R "/home" "<STEAM_LIB>/steamapps/compatdata/" | head
grep -R "Linux" "<STEAM_LIB>/steamapps/compatdata/" | head
grep -R "NVIDIA" "<STEAM_LIB>/steamapps/compatdata/" | head

Observe that the output includes host home directory content, session logs, /boot data, kernel configs, activity databases, and hardware info.

Repeat with other prefixes — exposure is consistent.

Redacted Evidence (Safe Examples)
Z:\home<USER>.local\share\kactivitymanagerd\resources\database
Z:\home<USER>.local\share\baloo\index
Z:\home<USER>.local\share\sddm/xorg-session.log
Z:\boot\vmlinuz-6.12.60
Z:\boot\config-6.12.60
Z:\boot/grub/x86_64-efi/.mod
sD3DDevice="NVIDIA GeForce RTX 3060"

Impact

A malicious or compromised Windows application could:

Fingerprint the host system

Enumerate personal user directory structure

Read session logs, activity metadata, and Baloo index files

Inspect kernel and bootloader metadata

Identify external drive mounts and user documents

This reduces Proton’s isolation and creates a Low-severity privacy / information disclosure condition.

Expected Behavior

Proton prefixes should ideally avoid exposing:

/home/ subdirectories unrelated to Steam or the game

/boot and bootloader files

External drive paths

Desktop session logs

unless absolutely required for game compatibility.

Suggested Mitigation

Restrict or filter Z:\ mapping

Provide a “hardened prefix” option with reduced host visibility

Limit exposure of non-essential host directories

Conclusion

This is a systemic privacy issue affecting multiple Proton prefixes.
It does not provide privilege escalation, but it exposes host metadata and user information to Windows applications in ways users may not expect.

Thanks for reviewing — I’m happy to provide more redacted samples if needed.