SHA2 Certificate Thumbprint Spoofing

TL;DR

Yes, a SHA2 x509 certificate thumbprint can be spoofed or specified, but it’s complex and depends heavily on the context. You can create certificates with specific thumbprints using tools like OpenSSL, but getting systems to *accept* that spoofed certificate requires controlling trust stores or employing man-in-the-middle techniques.

Understanding Certificate Thumbprints

A certificate thumbprint (also known as a hash) is a unique identifier for a digital certificate. It’s calculated by hashing the certificate’s contents using an algorithm like SHA256. Changing *anything* in the certificate will change the thumbprint.

How to Create Certificates with Specific Thumbprints

1. Using OpenSSL: You can generate a self-signed certificate and manipulate its serial number until you achieve your desired thumbprint. This is computationally intensive.

   openssl req -x509 -newkey rsa:2048 -keyout key.pem -out cert.pem -days 365 -nodes
   openssl x509 -noout -text -in cert.pem | grep Subject
   openssl x509 -hash -in cert.pem

   The last command will show you the current thumbprint. Adjusting the serial number is the main way to influence it, but there’s no direct ‘set thumbprint’ option.

2. Scripted Generation: Write a script (Python with cryptography library, for example) that iteratively generates certificates, calculates their thumbprints, and stops when the desired thumbprint is reached. This is more reliable than manual adjustments.

   # Example Python snippet (requires cryptography library)
   import hashlib
   from OpenSSL import crypto
   
   def calculate_thumbprint(cert):
     sha256_hash = hashlib.sha256(crypto.dump_certificate(crypto.load_certificate(crypto.FILETYPE_PEM, cert)).decode()).hexdigest()
     return sha256_hash

Spoofing Thumbprints in Practice

Creating the certificate is only half the battle. Getting a system to trust it with the spoofed thumbprint is much harder.

1. Trust Store Manipulation: If you control the target system’s trust store (e.g., Java keystore, Windows Certificate Store), you can add your self-signed certificate as a trusted root authority.
   • Java Keystore: Use keytool to import the certificate.

     keytool -importcert -file cert.pem -keystore mytruststore -alias myalias -noprompt

   • Windows Certificate Store: Use mmc.exe with the Certificates snap-in to import the certificate into the Trusted Root Certification Authorities store.
2. Man-in-the-Middle (MITM) Attacks: A MITM proxy can intercept TLS connections, present your spoofed certificate, and decrypt/re-encrypt traffic.
   • Tools like Burp Suite or mitmproxy allow you to generate certificates on the fly with specific thumbprints.
   • This requires compromising the network connection between the client and server.
3. Configuration Errors: In rare cases, misconfigured systems might accept any certificate without proper validation. This is a security vulnerability.

Limitations & Considerations

• Chain of Trust: If the target system expects a full certificate chain (intermediate certificates), you’ll need to generate those as well and ensure they are correctly configured.
• OCSP/CRL Checks: Online Certificate Status Protocol (OCSP) or Certificate Revocation Lists (CRL) checks can invalidate your spoofed certificate if it’s not properly signed by a trusted authority.
• Pinning: Many applications use certificate pinning, which hardcodes expected thumbprints. Spoofing will be ineffective against pinned certificates unless you can bypass the pinning mechanism.
• Cyber security implications: Spoofing certificates is a serious cyber security risk and should only be done in controlled environments for testing or legitimate penetration testing purposes.