CVE-2026-40194VulnerabilityPublished 10 Apr 2026Updated 8 May 2026

SSH2::get_binary_packet() using != instead of hash_equals() · Phpseclib Has a Variable-time HMAC Comparison

Affects phpseclib/phpseclibSource GCVE Database

Authoritative record:CVE-2026-40194· Low 3.7

Not known to be exploited · CVSS 3.7 Low : upgrade to 1.0.28 when convenient.

Data as of 27 Oct 2026, 00:07 UTC

CVSS 3.1 base score

3.7Low

How severe the flaw is on the 0–10 CVSS scale.

Patch: Available – fixed in 1.0.28, 2.0.53, 3.0.51

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N

Attack Vector: Network
Attack Complexity: High
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: Low
Integrity: None
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 10 Apr 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade phpseclib/phpseclib to 1.0.28; phpseclib/phpseclib to 2.0.53; phpseclib/phpseclib to 3.0.51.

Affected software

Product	Affected	Fixed in
Packagistphpseclib/phpseclib	Affected >=0.1.1 <1.0.28	Fixed in 1.0.28
Packagistphpseclib/phpseclib	Affected >=2.0.0 <2.0.53	Fixed in 2.0.53
Packagistphpseclib/phpseclib	Affected >=3.0.0 <3.0.51	Fixed in 3.0.51

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

# 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`): ```php 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 != $this->hmac_check->hash(...)` | Verified with `git show <branch>:phpseclib/Net/SSH2.php`. ### Reachability The HMAC verification path at lines 3399-3415 is reached on **every received SSH packet** when the negotiated cipher is not AEAD — i.e., any of: - `aes128-cbc`, `aes192-cbc`, `aes256-cbc` - `aes128-ctr`, `aes192-ctr`, `aes256-ctr` - `3des-cbc`, `3des-ctr` - `blowfish-cbc`, `blowfish-ctr` - `twofish-*-cbc`, `twofish-*-ctr` - `arcfour`, `arcfour128`, `arcfour256` combined with any non-AEAD MAC (hmac-sha2-256, hmac-sha2-512, hmac-sha1, hmac-sha1-96, hmac-md5, hmac-md5-96, umac-64, umac-128, and their -etm variants — full list at `getSupportedMACAlgorithms()` around line 4754). AEAD ciphers (`aes128-gcm@openssh.com`, `aes256-gcm@openssh.com`, `chacha20-poly1305@openssh.com`) go through a different path (lines 3353-3381) and do not reach the `!=` comparison — their authentication tag is checked inside the AEAD implementation. `$this->hmac_check` is set during key exchange at `SSH2.php:1812`: ```php if (!$this->decrypt->usesNonce()) { [$this->hmac_check, $checkKeyLength] = self::mac_algorithm_to_hash_instance($mac_algorithm_in); $this->hmac_size = $this->hmac_check->getLengthInBytes(); } ``` So any SSH2 client session negotiating a non-AEAD cipher exercises the vulnerable code path starting from the first post-KEX packet. ### Contrast with existing code that does the right thing `hash_equals()` is already used in 9 other places in the phpseclib codebase, proving the maintainer knows the pattern: ``` phpseclib/File/CMS/DigestedData.php:186 phpseclib/File/CMS/SignedData/Signer.php:294 phpseclib/File/CMS/SignedData/Signer.php:406 phpseclib/Crypt/Common/Formats/Keys/PuTTY.php:236 phpseclib/Crypt/RSA/PublicKey.php:98, 105, 188, 229 phpseclib/Crypt/RSA/PrivateKey.php:386 ``` The SSH2 MAC comparison is the one place it was missed. ## Proof of concept All scripts under `poc/`. They prove three things: 1. PHP's `!=` on equal-length binary strings IS variable-time and short-circuits. 2. `hash_equals()

CWE-208Observable Timing Discrepancy (opens in a new tab)
Base weakness
Two separate operations in a product require different amounts of time to complete, in a way that is observable to an actor and reveals security-relevant information about the state of the product, such as whether a particular operation was successful or not.
OWASP Cheat Sheets
Error Handling (opens in a new tab)
Logging (opens in a new tab)
OWASP ASVS 4.0.3
V8.3.4 (opens in a new tab)
V14.3.3 (opens in a new tab)
V7.3.3 (opens in a new tab)
Attack patterns
Cross-Domain Search Timing (opens in a new tab)
Application Fingerprinting (opens in a new tab)
System Footprinting (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

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Exploitation evidence

What to do

Affected software

Description

WeaknessesCWE-208

Technical details

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