Secure Socket Tunneling Protocol (SSTP) is widely used to carry PPP frames over an SSL/TLS channel (typically TCP 443). For site operators, enterprise security engineers and developers, ensuring that SSTP traffic is actually encrypted end-to-end is critical for compliance and threat mitigation. This article provides a practical, technically rich guide to validating SSTP encryption—covering capture methods, protocol analysis, TLS validation, active tests, and automation approaches. The steps and tools described here are applicable to lab environments and production audits.

Understanding SSTP and where encryption occurs

Before validating encryption, it helps to understand SSTP’s layering. SSTP encapsulates PPP frames inside an HTTPS-like TLS session. The TLS channel protects the SSTP payloads, and the PPP session carries authentication (PAP/CHAP/MSCHAPv2), IP configuration, and data. Thus, the security of the VPN is reliant on the underlying TLS session:

  • TLS handshake negotiates version and cipher suite and authenticates the server (and possibly client) via certificates.
  • TLS record layer encrypts SSTP control and data packets (Application Data records).
  • PPP payloads (e.g., LCP, IPCP, authentication) are transported only after the TLS tunnel is established.

High-level validation methodology

A systematic validation consists of five phases:

  • Capture network traffic containing the SSTP session.
  • Identify and isolate the SSTP/TLS session(s).
  • Analyze the TLS handshake and cipher suite negotiation.
  • Confirm that application data is encrypted (no plaintext PPP/authentication visible).
  • Perform active tests to confirm client/server behavior under invalid certificates or downgraded ciphers.

Phase 1 — Capture: where and how to collect packets

Choose a capture point with full visibility of client-server traffic. Options include:

  • Port mirroring (SPAN) on enterprise switches—useful in production.
  • Host-level captures using tcpdump/tshark on Linux, or Microsoft Network Monitor/Packet Capture on Windows server.
  • Azure/AWS VPC traffic mirroring for cloud-hosted SSTP servers.

Basic tcpdump example (Linux):

sudo tcpdump -i eth0 -s 0 -w sstp_capture.pcap tcp port 443 and host CLIENT_IP and host SERVER_IP

For Windows hosts, use Network Monitor or the built-in netsh trace: netsh trace start capture=yes tracefile=c:capturessstp.etl (convert ETL to pcapng for Wireshark analysis if needed).

Phase 2 — Isolate the TLS/SSTP session in a packet analyzer

Open the capture in Wireshark or use tshark for CLI. Useful Wireshark display filters:

  • tcp.port == 443
  • tls.handshake.type == 1 (Client Hello)
  • tls.record.version (shows TLS record versions)
  • sstp (Wireshark has an SSTP dissector; if not present, analyze TLS payload)

Example tshark command to extract TLS handshakes:

tshark -r sstp_capture.pcap -Y "tls.handshake" -V

If the capture is large, filter by the client IP before analysis:

tshark -r sstp_capture.pcap -Y "ip.addr == CLIENT_IP && tcp.port == 443"

Examining the TLS handshake and ciphers

The handshake reveals certificate chain, TLS version, and negotiated cipher suite—these determine strength and forward secrecy properties.

Key items to verify

  • Certificate chain: valid chain to a trusted CA, correct CN/SAN and no expired certificates.
  • TLS version: prefer TLS 1.2 and 1.3; disallow SSL 3.0/TLS 1.0/1.1.
  • Cipher suite: ensure use of AEAD ciphers like AES-GCM or ChaCha20-Poly1305 and that RSA key exchange (non-PFS) is avoided. Look for ECDHE/ECDH for Perfect Forward Secrecy (PFS).
  • Signature algorithms: prefer SHA-256 or stronger; avoid SHA-1 signatures.

Using openssl s_client to inspect the server

From a trusted machine, connect to the SSTP server’s TCP port to inspect certs and supported ciphers:

openssl s_client -connect sstp.example.com:443 -showcerts -servername sstp.example.com

To test server behavior for TLS versions or ciphers:

openssl s_client -connect sstp.example.com:443 -tls1_2

or limit ciphers:

openssl s_client -connect sstp.example.com:443 -cipher "ECDHE-ECDSA-AES256-GCM-SHA384"

This helps confirm the server will negotiate secure ciphers and reject weak ones.

Confirming encryption of application data

After the handshake, application data records should be encrypted. In Wireshark, look for TLS Application Data records rather than plaintext PPP frames. If PPP authentication messages (e.g., PAP, CHAP) appear in cleartext, encryption is missing or has been bypassed.

Wireshark indications

  • Packets labeled “TLSv1.2 Application Data” (or TLSv1.3) — these are encrypted records.
  • Absence of dissectors showing PPP LCP/PAP/CHAP within the capture before decrypting indicates encryption.
  • Presence of “ChangeCipherSpec” followed by “Application Data” indicates the switch to encrypted traffic.

Attempting to decrypt captured TLS

Decryption is only possible if you have relevant secrets:

  • Server private key (works only for RSA key exchange; fails for ECDHE/PFS).
  • Session keys exported to an SSL key log file (RFC 5705) — many clients/libraries can export via SSLKEYLOGFILE (e.g., NSS/OpenSSL-based apps).

To configure Wireshark with an RSA private key:

Preferences > Protocols > TLS > (RSA keys list) — add server IP, port, protocol and PEM private key. Then Wireshark can decrypt RSA-based sessions.

However, modern SSTP deployments should use ECDHE (PFS) so private-key decryption will not work—this is actually a sign of good practice. If decryption fails but the handshake used ECDHE, the signal is that your traffic had forward secrecy.

Active testing: certificate and downgrade tests

Passive captures prove encryption exists; active tests verify client/server behaviors under attack scenarios.

1) Invalid certificate test

Set up an intercepting proxy with a self-signed certificate or a certificate signed by an untrusted CA and attempt to connect a client. Expected behavior:

  • Client should reject or prompt/inform about the certificate; automated VPN clients should fail to connect.
  • If the client accepts silently, certificate validation is broken.

2) TLS downgrade attack test

Attempt to force the server to negotiate older TLS versions or weak ciphers using openssl s_client or test suites such as testssl.sh. Confirm server refuses obsolete versions and weak ciphers.

3) MITM with SSL proxy (in lab)

For a controlled testbed, use an SSL proxy (e.g., Burp, mitmproxy) installed with a CA trusted only on the test client. This demonstrates whether the client performs certificate pinning or strict checks. If the client refuses, pinning/strict validation is effective.

Logging and host-level indicators

Server and client logs add context to packet data:

  • Windows Event Viewer: check RasClient, RasServer, and System for SSTP-related entries and SChannel events.
  • Enable SChannel event logging for deeper TLS errors (registry changes required).
  • On Windows clients, rasdial/rasphone logs give connection status but not packet-level crypto details.
  • On Linux-based SSTP implementations, syslog or systemd-journal entries and strongSwan logs (if using IPSec alternatives) are helpful.

Automation and continuous validation

Large deployments benefit from automated checks that detect regressions (e.g., accidental enabling of weak ciphers). Consider these approaches:

  • Periodic openssl s_client checks combined with parsing scripts to assert TLS version and cipher suite.
  • Automated testssl.sh runs with alerts on regression.
  • SIEM ingest of SChannel and VPN logs to alert on cert expirations or unexpected TLS failures.
  • Scheduled packet captures in a staging environment to confirm PPP payloads remain encrypted after changes.

Common pitfalls and mitigations

Be aware of these issues when validating SSTP encryption:

  • Misinterpreting TLS record layers — seeing TCP 443 traffic doesn’t equal encryption; inspect TLS records not just ports.
  • Private key decryption false security — if you can decrypt traffic using the private key, the server probably uses RSA key exchange: consider enabling ECDHE for PFS.
  • Client-side exceptions — some clients may permit deprecated ciphers for compatibility; enforce server-side policies and use client configuration profiles.
  • Certificate chain issues — expired or misconfigured certs lead to silent failures or fallback to less secure mechanisms in some stacks; monitor certificate health.

Sample checklist for an SSTP encryption audit

  • Capture a representative session at the network edge.
  • Verify TLS handshake and confirm TLS 1.2+ (prefer TLS 1.3) was negotiated.
  • Ensure ECDHE (or another PFS key exchange) is used—no RSA key-exchange-only suites.
  • Confirm an AEAD cipher (AES-GCM/Chacha20-Poly1305) is in use.
  • Ensure certificate chain is valid and uses strong signature algorithms.
  • Validate that PPP authentication (e.g., MSCHAPv2) is not visible in plaintext.
  • Run active tests to confirm the client rejects invalid certs and weak ciphers.
  • Set up automated periodic scans and logging to detect regressions.

Validating SSTP VPN encryption requires a combination of packet analysis, TLS inspection, active testing, and host-level logging. Use Wireshark/tshark for deep handshake and record-layer inspection, openssl s_client and testssl.sh for server-side checks, and controlled MITM tests in a lab to confirm client behavior. The presence of TLS Application Data records and the absence of readable PPP authentication frames in a capture are strong indicators that SSTP encryption is functioning properly—while ECDHE-based handshakes demonstrate that forward secrecy is in place.

For ongoing deployment security, integrate these checks into CI/CD and monitoring workflows, and ensure certificate management and cipher policy are part of routine audits.

Published by Dedicated-IP-VPN