Symlink Attack with Mutual Tls

Remediating symlink attacks in Mutual TLS environments requires a defense-in-depth approach that combines certificate validation, input sanitization, and secure file handling practices.

First, implement strict certificate validation that goes beyond basic trust chain verification. Validate that certificate attributes used in file paths contain only expected characters and don't include path traversal sequences:

public String sanitizeCertificateAttribute(String attribute) {
    if (attribute == null) return null;
    // Remove any path traversal attempts
    attribute = attribute.replaceAll("\\.\\./", "");
    // Allow only alphanumeric, hyphen, and underscore
    if (!attribute.matches("^[a-zA-Z0-9-_]+$") { 
        throw new SecurityException("Invalid certificate attribute");
    }
    return attribute;
}

Second, avoid using certificate attributes directly in file paths. Instead, use a secure mapping system that translates certificate information to internal identifiers:

public File getUserDataFile(X509Certificate cert) {
    String userId = getUserIdFromCertificate(cert);
    int userIdInt = Integer.parseInt(userId); // Validate numeric ID
    return new File(getUserDirectory(userIdInt), "data.json");
}

Third, implement proper file system permissions and avoid following symbolic links when accessing user data. Use Java's Files.newByteChannel with NOFOLLOW_LINKS option:

public byte[] readFileSecurely(File file) throws IOException {
    try (SeekableByteChannel channel = Files.newByteChannel(file.toPath(),
            StandardOpenOption.READ, LinkOption.NOFOLLOW_LINKS)) {
        ByteBuffer buffer = ByteBuffer.allocate((int) channel.size());
        channel.read(buffer);
        return buffer.array();
    }
}

Fourth, implement certificate attribute whitelisting. Only allow specific, known certificate attributes to be used in your application logic:

public boolean isCertificateAttributeSafe(String attributeName) {
    return Arrays.asList("OU", "O", "CN").contains(attributeName);
}

Finally, use middleBrick's CLI tool to continuously scan your Mutual TLS endpoints for symlink vulnerabilities:

middlebrick scan https://api.example.com --client-cert client.pem --client-key client.key

This command tests the Mutual TLS endpoint with your client certificate and identifies any symlink-related vulnerabilities in the authentication and authorization flow.