Patched code sample

import re
import time

# This POC is based on the vulnerability description for CVE-2025-3264 (a placeholder for the real CVE-2024-3264).
# The vulnerable regex pattern can cause catastrophic backtracking with a crafted input,
# leading to a Denial of Service (DoS) due to high CPU usage.

def get_imports_vulnerable(code: str) -> str:
    """
    Simulates the vulnerable function from transformers<=4.49.0.
    The regex `.*?` can lead to catastrophic backtracking because it's too general
    and is used between two other patterns.
    """
    # VULNERABLE PATTERN
    vulnerable_pattern = r"\s*try\s*:.*?except.*?:"
    try:
        # The re.sub call is where the ReDoS occurs.
        return re.sub(vulnerable_pattern, "", code, flags=re.DOTALL)
    except re.error as e:
        # The regex engine may time out on very long inputs
        return f"Regex error: {e}"

def get_imports_fixed(code: str) -> str:
    """
    Simulates the fixed function from transformers>=4.51.0.
    The fix replaces the general `.*?` with a more specific `[^:]*?`, which means
    "match any character except a colon, non-greedily". This prevents the regex
    engine from backtracking across multiple `except` blocks or deeply nested structures,
    thus mitigating the ReDoS vulnerability.
    """
    # FIXED PATTERN
    # By excluding the colon ':', the pattern becomes non-backtracking for the intended use case.
    fixed_pattern = r"\s*try\s*:[^:]*?except[^:]*?:"
    return re.sub(fixed_pattern, "", code, flags=re.DOTALL)

if __name__ == "__main__":
    # A crafted string that triggers catastrophic backtracking in the vulnerable regex.
    # It starts with "try:", has a long sequence of characters that are not "except",
    # and ends with "except" but no final colon. The regex engine will try every
    # possible partition of the long string, leading to exponential complexity.
    print("Crafting malicious input string...")
    # A length of >25 is typically enough to show significant slowdown.
    malicious_part = "a" * 28
    evil_string = f"try:{malicious_part}except"
    print(f"Malicious string length: {len(evil_string)}\n")

    # --- Demonstrate Vulnerability ---
    print("Testing VULNERABLE function (transformers<=4.49.0)...")
    print("This may take a long time or hang indefinitely. Press CTRL+C to stop.")
    start_time_vuln = time.monotonic()
    try:
        get_imports_vulnerable(evil_string)
    except KeyboardInterrupt:
        print("\nExecution interrupted by user.")
    end_time_vuln = time.monotonic()
    print(f"Vulnerable function took: {end_time_vuln - start_time_vuln:.4f} seconds.\n")

    # --- Demonstrate Fix ---
    print("Testing FIXED function (transformers>=4.51.0)...")
    start_time_fixed = time.monotonic()
    get_imports_fixed(evil_string)
    end_time_fixed = time.monotonic()
    print(f"Fixed function took: {end_time_fixed - start_time_fixed:.4f} seconds.")

    # --- Conclusion ---
    if (end_time_vuln - start_time_vuln) > 1:
         print("\nDemonstration successful: The fixed function is orders of magnitude faster.")
    else:
         print("\nDemonstration note: Increase the multiplier in 'malicious_part' if the slowdown is not obvious on your machine.")