CVE-2026-40194Public exploitNetwork attack vector

Theoretical HMAC-prefix disclosure via variable-time comparison

Published Apr 10, 2026 · Updated Apr 10, 2026

Observable timing discrepancy in phpseclib allows remote attackers to infer HMAC prefix bits from SSH packet verification timing. SSH2::get_binary_packet() compares received and computed HMACs with PHP's short-circuiting != operator instead of a constant-time comparison. Pre-authentication reachability yields only one measurement per connection; MAC failure disconnects and new sessions use fresh keys, so no practical key or plaintext recovery is demonstrated.

CVSS severity3.7
Low
EPSS probability0.36%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Observable timing discrepancy in phpseclib allows remote attackers to infer HMAC prefix bits from SSH packet verification timing. SSH2::get_binary_packet() compares received and computed HMACs with PHP's short-circuiting != operator instead of a constant-time comparison. Pre-authentication reachability yields only one measurement per connection; MAC failure disconnects and new sessions use fresh keys, so no practical key or plaintext recovery is demonstrated.

The record

CVE
CVE-2026-40194
Published
Apr 10, 2026
Updated
Apr 10, 2026
Vendor
phpseclib
Product
phpseclib
Classifications
CWE-208
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 10, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 10, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 1.0.28
  2. Affected versionversion=>= 2.0.0, < 2.0.53
  3. Affected versionversion=>= 3.0.0, < 3.0.51

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

phpseclib · phpseclibversion=< 1.0.28; version=>= 2.0.0, < 2.0.53; version=>= 3.0.0, < 3.0.51

Published CVSS scores

3.7GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-208Observable Timing Discrepancy

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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