Symlink Attack with Hmac Signatures

Symlink attacks in HMAC signature implementations exploit the way cryptographic signatures are verified against file paths and resource references. The attack pattern emerges when HMAC verification processes use file paths or resource identifiers that can be manipulated through symbolic links.

The core vulnerability occurs when HMAC implementations perform path resolution before signature verification. An attacker can create a symbolic link from a legitimate resource path to a sensitive file, causing the HMAC verification process to inadvertently validate against the wrong resource. This is particularly dangerous in systems that serve files or resources based on HMAC-signed URLs.

Consider a file download service that uses HMAC signatures to authenticate file access URLs. The service might construct URLs like:

https://example.com/download?file=report.pdf&signature=abc123

The server verifies the HMAC signature, then serves the requested file. However, if the file path resolution happens after signature verification, an attacker can create a symlink from report.pdf to /etc/passwd. The HMAC signature for report.pdf would still verify correctly, but the server would serve /etc/passwd instead.

Another manifestation occurs in temporary file handling. HMAC implementations often create temporary files for signature verification or processing. If these temporary files are created in predictable locations with predictable names, an attacker can pre-create symlinks to sensitive system files. When the HMAC process writes to the temporary file location, it actually overwrites the sensitive file.

Time-of-check-to-time-of-use (TOCTOU) race conditions compound this vulnerability. The system might verify the HMAC signature against one file, then later access a different file due to symlink manipulation between the verification and access steps. This creates a window where the signature is valid for one resource but the system accesses another.

HMAC-based API authentication can also be vulnerable when resource paths are included in the signed message. If the path resolution is not canonicalized before signing or verification, symlinks can redirect the request to unauthorized resources while maintaining a valid signature.

Detecting symlink attacks in HMAC implementations requires examining both the signature verification logic and the file/resource access patterns. The detection process focuses on identifying unsafe path handling and TOCTOU vulnerabilities.

Static code analysis should examine HMAC verification functions for path resolution patterns. Look for code that uses functions like os.path.join(), Path.resolve(), or similar path manipulation without proper canonicalization. The presence of os.symlink() or symlink() system calls in the same codebase as HMAC verification is a red flag.

Dynamic testing involves creating controlled symlink scenarios to test HMAC implementations. Create symlinks from valid resource paths to sensitive files and attempt to access them with valid HMAC signatures. If the system serves the sensitive content, it indicates a vulnerability.

Runtime monitoring can detect symlink attacks by tracking file access patterns. Monitor for unexpected file accesses that occur after successful HMAC verification. Look for access patterns where the accessed file differs from the file referenced in the signed message.

middleBrick's scanning approach for HMAC signature symlink vulnerabilities includes:

class HmacSymlinkScanner(Scanner):
    def scan(self, endpoint):
        # Test for path traversal in HMAC URLs
        test_cases = [
            ('valid_file', 'sensitive_file'),
            ('valid_path', '../sensitive_path'),
            ('temp_dir', '/etc/passwd')
        ]
        
        for valid_path, attack_path in test_cases:
            # Create test symlink
            create_symlink(attack_path, valid_path)
            
            # Generate HMAC for valid path
            signature = self.generate_hmac(valid_path)
            
            # Test endpoint with manipulated path
            response = self.request(endpoint, path=valid_path, signature=signature)
            
            if self.is_successful_response(response):
                self.report_vulnerability(
                    severity='high',
                    type='HMAC Symlink Attack',
                    description=f'HMAC signature valid for {valid_path} but accessed {attack_path}'
                )

The scanner tests multiple attack vectors including path traversal, temporary file manipulation, and TOCTOU scenarios. It verifies whether HMAC signatures remain valid when the underlying file paths are manipulated through symlinks.

middleBrick also analyzes the HMAC implementation's use of canonical path resolution. It checks whether the system uses functions like realpath(), canonicalize(), or similar before performing signature verification. Missing canonicalization is a strong indicator of symlink vulnerability.

Remediating symlink attacks in HMAC implementations requires a defense-in-depth approach that addresses both the cryptographic verification and the file access patterns. The primary defense is canonical path resolution before any HMAC operations.

The first remediation step is to always resolve paths to their canonical form before generating or verifying HMAC signatures. This ensures that the signature covers the actual physical file location, not a potentially manipulated logical path.

import hmac
import hashlib
from pathlib import Path

def generate_hmac_signature(file_path, secret_key):
    # Canonicalize path first
    canonical_path = str(Path(file_path).resolve())
    
    # Generate HMAC over canonical path
    message = f"file:{canonical_path}"
    signature = hmac.new(
        secret_key.encode(),
        message.encode(),
        hashlib.sha256
    ).hexdigest()
    
    return signature

def verify_hmac_signature(file_path, signature, secret_key):
    # Canonicalize path before verification
    canonical_path = str(Path(file_path).resolve())
    
    # Generate expected signature
    expected = generate_hmac_signature(canonical_path, secret_key)
    
    # Constant-time comparison
    return hmac.compare_digest(expected, signature)

This approach ensures that the HMAC signature is tied to the actual file location, making symlink manipulation ineffective. The canonical path includes the full absolute path, preventing attackers from substituting different files.

Implement TOCTOU protection by using atomic file operations. Instead of opening a file descriptor, reading data, and then closing it, use atomic operations that verify the file state remains unchanged throughout the operation.

import os
import fcntl

def secure_file_access(file_path, signature, secret_key):
    # Verify signature first
    if not verify_hmac_signature(file_path, signature, secret_key):
        raise PermissionError('Invalid signature')
    
    # Get canonical path
    canonical_path = Path(file_path).resolve()
    
    # Open file with O_NOFOLLOW to prevent symlink following
    fd = os.open(canonical_path, os.O_RDONLY | os.O_NOFOLLOW)
    
    try:
        # Lock file to prevent concurrent modification
        fcntl.flock(fd, fcntl.LOCK_SH)
        
        # Read file contents
        content = os.read(fd, os.path.getsize(canonical_path))
        
        return content
    finally:
        # Release lock and close
        fcntl.flock(fd, fcntl.LOCK_UN)
        os.close(fd)

The O_NOFOLLOW flag prevents the system from following symbolic links, ensuring that the file opened is the actual file, not a symlink target. Combined with file locking, this prevents both symlink attacks and concurrent modification attacks.

Implement strict directory whitelisting for HMAC-protected resources. Only allow HMAC signatures for files within pre-approved directories, and never allow traversal outside these boundaries.

middleBrick's remediation guidance includes specific recommendations for HMAC implementations:

• Always canonicalize paths before HMAC operations
• Use O_NOFOLLOW flag when opening files
• Implement file locking for atomic operations
• Validate that the canonical path is within allowed directories
• Log and monitor for unusual file access patterns

For systems that must handle temporary files, create them in secure, non-predictable locations with randomized names. Never use predictable temporary file paths that attackers could pre-create symlinks for.