Cyber Resilience

CVE-2026-40194

Phpseclib ≤ 1.0.27

Published
10 April 2026
Modified
08 May 2026
Patch / advisory
View advisory
CVSS Score v3.1 3.7
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
EPSS Score 0.0036 29th percentile
Risk Priority 16 floored blend · peak EPSS

Summary

CVE-2026-40194 is a low-severity Observable Timing Discrepancy (CWE-208) vulnerability in Phpseclib Phpseclib. Its CVSS base score is 3.7 (Low).

Operationally, exploitation aligns with the MITRE ATT&CK technique Adversary-in-the-Middle (T1557); ranked at the 29th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified map to SC-13 (Cryptographic Protection) and SA-11 (Developer Testing and Evaluation) — see the control section below for these in your framework.

EU & UK References

Vulnerability Data

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…

more

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.

CWE(s)
CWE-208

Related Threats

MITRE ATT&CK Enterprise TechniquesAI

T1557 Adversary-in-the-Middle Credential Access
Adversaries may attempt to position themselves between two or more networked devices using an adversary-in-the-middle (AiTM) technique to support follow-on behaviors such as [Network Sniffing](https://attack.
attack.mitre.org →
Why these techniques?

Timing side-channel in SSH HMAC verification enables an on-path adversary to forge valid packets and perform undetected injection/modification, directly facilitating Adversary-in-the-Middle attacks against SSH sessions.

Confidence: MEDIUM · MITRE ATT&CK Enterprise v19.0

CVEs Like This One

CVE-2026-32935Same product: Phpseclib Phpseclib
CVE-2023-27560Same product: Phpseclib Phpseclib
CVE-2026-55599Same product: Phpseclib Phpseclib
CVE-2023-52892Same product: Phpseclib Phpseclib
CVE-2023-49316Same product: Phpseclib Phpseclib
CVE-2024-27355Same product: Phpseclib Phpseclib
CVE-2024-27354Same product: Phpseclib Phpseclib
CVE-2026-44368Shared CWE-208
CVE-2025-52457Shared CWE-208
CVE-2026-27882Shared CWE-208

Affected Assets

phpseclib
phpseclib
≤ 1.0.27 · 2.0.0 — 2.0.53 · 3.0.0 — 3.0.51

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)
  • SC-13 Cryptographic Protection
  • SA-11 Developer Testing and Evaluation
  • SA-8 Security and Privacy Engineering Principles
Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V11.2.4

Mitigating Controls (NIST 800-53 r5) AI

SC-13 Cryptographic Protection good match
prevent

Directly requires correct implementation of cryptographic integrity mechanisms (HMAC comparison) rather than allowing variable-time operators.

SA-11 Developer Testing and Evaluation partial match
prevent

Mandates developer security testing that can detect timing side-channel flaws such as non-constant-time HMAC verification.

SA-8 Security and Privacy Engineering Principles partial match
prevent

Requires application of secure engineering principles that include constant-time algorithms for sensitive comparisons.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly require constant-time implementations that eliminate observable timing discrepancies.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.17 Clock synchronization none match
none

Consistent reference clocks limit the attacker's ability to measure or manipulate timing differences that could reveal internal state or processing paths.

References