phpseclib — non-constant-time X25519 scalar multiplication permits full private-key recovery

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-84308
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-09-01 20:17:24 UTC
Updated2026-09-01 20:17:24 UTC
Descriptionphpseclib is a PHP secure communications library. Prior to 3.0.57 and 4.0.1, pure-PHP X25519 scalar multiplication in phpseclib/Math/PrimeField/Integer.php performs data-dependent conditional modular reductions in add() and subtract(). During the Montgomery ladder in phpseclib/Crypt/EC/BaseCurves/Montgomery.php, the reduction behavior of each step depends on the secret scalar prefix, creating per-step timing and libgmp call-count observations that can reveal a reused 251-bit clamped private scalar. The phpseclib/Crypt/EC/Formats/Keys/MontgomeryPrivate.php derivation path invokes the pure-PHP multiplication without a native-engine check, while phpseclib/Crypt/EC/Formats/Keys/PKCS8.php reaches it when ext-sodium is unavailable. Exploitation requires a reused or long-lived X25519 private key, knowledge of the corresponding public key, execution of the pure-PHP path, and a local observer capable of resolving individual ladder steps or libgmp entry-point calls. Ephemeral X25519 keys, including phpseclib's normal SSH exchange path, are not affected. Recovery of the scalar permanently compromises operations that reuse that key. This issue is fixed in versions 3.0.57 and 4.0.1.

Risk And Classification

Primary CVSS: v3.1 6.3 MEDIUM from [email protected]

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

Problem Types: CWE-208 | CWE-385 | CWE-208 CWE-208: Observable Timing Discrepancy | CWE-385 CWE-385: Covert Timing Channel

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary6.3MEDIUMCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N
3.1CNADECLARED6.3MEDIUMCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N

CVSS v3.1 Breakdown

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

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

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Phpseclib Phpseclib affected < 3.0.57 Not specified
CNA Phpseclib Phpseclib affected >= 4.0.0, < 4.0.1 Not specified

References

ReferenceSourceLinkTags
github.com/phpseclib/phpseclib/commit/fb56bc5bb9009b54a6c26b31aeec8ed944... [email protected] github.com
github.com/phpseclib/phpseclib/releases/tag/4.0.1 [email protected] github.com
github.com/phpseclib/phpseclib/releases/tag/3.0.57 [email protected] github.com
github.com/phpseclib/phpseclib/security/advisories/GHSA-q97c-8qh3-fpc6 [email protected] github.com
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