How does TLS handshake analysis reveal server details

Question

During a TLS handshake, servers exchange certificates and configuration details. How can this information be used to identify the server and its software?

Answer 1

Analyzing a TLS handshake can reveal critical details about a server's identity and software configuration. This process is invaluable for security assessments, network diagnostics, and penetration testing. Here's how it works:

Key Elements Revealed During a TLS Handshake

1. Server Certificate (X.509)

   • Purpose: Authenticates the server to the client.

   • Details Extracted:

     • Common Name (CN) and Subject Alternative Names (SANs): Indicate the domain names the certificate secures.

     • Issuer Information: Identifies the Certificate Authority (CA) that issued the certificate.

     • Validity Period: Specifies the certificate's active dates.

     • Public Key: Used for encrypting data sent to the server.

   • Use Case: By examining the certificate, one can determine the server's domain, the CA hierarchy, and the encryption standards in use.

2. TLS Version and Cipher Suites

   • Purpose: Establishes the security protocols and algorithms for the session.

   • Details Extracted:

     • TLS Version: Indicates the protocol version (e.g., TLS 1.2, TLS 1.3).

     • Cipher Suites: List of supported encryption algorithms.

   • Use Case: Analyzing these can reveal the server's security posture and compatibility with various clients.

3. Server Name Indication (SNI)

   • Purpose: Allows a client to specify the hostname it's trying to connect to during the handshake.

   • Details Extracted:

     • Hostname: The specific domain the client wishes to access.

   • Use Case: Useful for identifying the target domain, especially in environments where multiple domains are hosted on a single IP address.

4. TLS Extensions and Parameters

   • Purpose: Provide additional capabilities and preferences for the TLS session.

   • Details Extracted:

     • Supported Groups: Elliptic curves and other groups used in key exchange.

     • Signature Algorithms: Preferred algorithms for digital signatures.

     • ALPN (Application-Layer Protocol Negotiation): Indicates supported application protocols like HTTP/2.

   • Use Case: These parameters can hint at the server's software stack and its configuration.

Tools for TLS Handshake Analysis

• Wireshark: A network protocol analyzer that can capture and dissect TLS handshakes, revealing certificates, cipher suites, and more.

• OpenSSL: A toolkit for the TLS and SSL protocols; it can initiate handshakes and display certificate details.

• JA3 Fingerprinting: Generates fingerprints of TLS clients and servers based on handshake parameters, aiding in identifying specific software implementations.

Practical Example

Suppose you're analyzing traffic to a web server. By capturing the TLS handshake:

• Certificate Analysis: You discover the certificate is issued to www.example.com by Let's Encrypt, valid for 90 days.

• Cipher Suites: The server supports modern cipher suites like TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, indicating up-to-date security practices.

• ALPN Extension: The presence of h2 suggests the server supports HTTP/2.

From this information, you infer that the server is likely running a modern web server like Nginx or Apache with HTTP/2 enabled and uses Let's Encrypt for SSL certificates.