CVE-2026-40194

phpseclib is a PHP secure communications library. Starting in 0.1.1 and prior to 3.0.51, 2.0.53, and 1.0.28, 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. This vulnerability is fixed in 3.0.51, 2.0.53, and 1.0.28.

phpseclib is a PHP secure communications library. Starting in 0.1.1 and prior to 3.0.51, 2.0.53, and 1.0.28, 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. This vulnerability is fixed in 3.0.51, 2.0.53, and 1.0.28.

phpseclib SSH2: Variable-time comparison in HMAC verification

Summary

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)

Root cause

phpseclib/Net/SSH2.php lines 3399-3415 (master at e819a163c):

if ($this->hmac_check instanceof Hash) {
    $reconstructed = !$this->hmac_check_etm ?
        pack('Na*', $packet->packet_length, $packet->plain) :
        substr($packet->raw, 0, -$this->hmac_size);
    if (($this->hmac_check->getHash() & "\xFF\xFF\xFF\xFF") == 'umac') {
        $this->hmac_check->setNonce("\0\0\0\0" . pack('N', $this->get_seq_no));
        if ($hmac != $this->hmac_check->hash($reconstructed)) {                     // <-- line 3405
            $this->disconnect_helper(DisconnectReason::MAC_ERROR);
            throw new ConnectionClosedException('Invalid UMAC');
        }
    } else {
        if ($hmac != $this->hmac_check->hash(pack('Na*', $this->get_seq_no, $reconstructed))) {  // <-- line 3410
            $this->disconnect_helper(DisconnectReason::MAC_ERROR);
            throw new ConnectionClosedException('Invalid HMAC');
        }
    }
}

Both $hmac (read from the socket via Strings::pop($raw, $this->hmac_size) at line 3348) and the computed hash are equal-length binary strings. PHP's != operator on equal-length strings dispatches to zend_binary_strcmp() which internally calls memcmp(). Modern libc memcmp short-circuits on the first differing byte, producing a timing signal that scales linearly with the number of matching leading bytes.

The same bug exists on every supported branch

| Branch | File | Line(s) | Expression | |---|---|---|---| | master (@ e819a163c) | phpseclib/Net/SSH2.php | 3405, 3410 | $hmac != $this->hmac_check->hash(...) | | 3.0 | phpseclib/Net/SSH2.php | 3741, 3746 | $hmac != $this->hmac_check->hash(...) | | 2.0 | phpseclib/Net/SSH2.php | 3796 | $hmac != $this->hmac_check->hash(...) | | 1.0 | phpseclib/Net/SSH2.php | 3810 | $hmac != $this->hmac_check->hash(...) |

Verified with git show <branch>:phpseclib/Net/SSH2.php.

Reachability

The HMAC verification path at lines 3399-3415 is reached on every received SSH packet when the negotiated cipher is not AEAD — i.e., any of:

• aes128-cbc, aes192-cbc, aes256-cbc
• aes128-ctr, aes192-ctr, aes256-ctr
• 3des-cbc, 3des-ctr
• blowfish-cbc, blowfish-ctr
• twofish-*-cbc, twofish-*-ctr
• arcfour, arcfour128, arcfour256

combined with any non-AEAD MAC (hmac-sha2-256, hmac-sha2-512, hmac-sha1, hmac-sha1-96, hmac-md5, hmac-md5-96, umac-64, umac-128, and their -etm variants — full list at getSupportedMACAlgorithms() around line 4754).

AEAD ciphers ([email protected], [email protected], [email protected]) go through a different path (lines 3353-3381) and do not reach the != comparison — their authentication tag is checked inside the AEAD implementation.

$this->hmac_check is set during key exchange at SSH2.php:1812:

if…

phpseclib is a PHP secure communications library. Starting in 0.1.1 and prior to 3.0.51, 2.0.53, and 1.0.28, 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. This vulnerability is fixed in 3.0.51, 2.0.53, and 1.0.28.

phpseclib is a PHP secure communications library. Starting in 0.1.1 and prior to 3.0.51, 2.0.53, and 1.0.28, 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. This vulnerability is fixed in 3.0.51, 2.0.53, and 1.0.28.

phpseclib is a PHP secure communications library. Starting in 0.1.1 and prior to 3.0.51, 2.0.53, and 1.0.28, 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. This vulnerability is fixed in 3.0.51, 2.0.53, and 1.0.28.