CVE-2026-40194: phpseclib has a variable-time HMAC comparison in SSH2::get_binary_packet() using != instead of hash_equals()

phpseclib\Net\SSH2::get_binary_packet() uses PHP’s != operator to compare a received SSH packet HMAC against the locally computed HMAC. != on equal-length binary strings in PHP uses memcmp(), which short-circuits on the first differing byte. This is a real variable-time comparison (CWE-208), proven by scaling benchmarks.

The finding is Low severity (defense-in-depth), not Critical. Practical exploitation over the network is prevented by SSH’s disconnect-on-MAC-failure behavior combined with per-connection session keys. The fix is a one-liner: replace != with hash_equals(), which the codebase already uses in 9 other places.

• Target: phpseclib/phpseclib
• File: phpseclib/Net/SSH2.php
• Lines (master e819a163c): 3405 and 3410
• CWE: CWE-208 (Observable Timing Discrepancy)
• CVSS v3.1: 3.7 — CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
• Severity: Low (cryptographic hygiene / defense-in-depth)
• Affected branches: master, 3.0, 2.0, 1.0 (all supported versions)