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Medium, CVSS 6.3amended

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

CVE-2026-84308GHSA-q97c-8qh3-fpc6

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (phpseclib/phpseclib 4.0.1, 3.0.57), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
6.3 · CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N
EPSS
  • CVE-2026-84308: 0.33% estimated exploitation activity in the next 30 days; percentile 25.84 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

The pure-PHP X25519 scalar multiplication in phpseclib is not constant-time. Field addition and subtraction each perform a data-dependent conditional modular reduction, so the cost of each Montgomery-ladder step is a linear function of that step's reduction count which is a quantity determined by the secret scalar's prefix. An observer with per-ladder-step resolution recovers the 251-bit clamped private scalar. This is a per-step leak, not an aggregate one: an instrumented code proof-of-concept recovers 20/20 test keys from 32 observed operations, and an observer that counts libgmp calls instead of timing them recovers a key from a single operation. This is not a low-order-input issue. Recovery works with the RFC 7748 base point u = 9, with no attacker-chosen input at all. Rejecting low-order public values does not close it. 2. Affected component Confirmed on phpseclib 3.0.56 (338 files under phpseclib/,sha256(sorted(relpath NUL filesha256 LF)) =…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from ghsa record GHSA-q97c-8qh3-fpc6

  2. Revision 1 · created ·

    Published from osv record GHSA-q97c-8qh3-fpc6

Agent brief

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# Patch Next advisory brief: CVE-2026-84308

Advisory: phpseclib — non-constant-time X25519 scalar multiplication permits full private-key recovery
IDs: CVE-2026-84308, GHSA-q97c-8qh3-fpc6
Record: https://patchnext.com/advisories/cve-2026-84308 (revision 2, source checked 26 Oct 2026, 14:53 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: phpseclib/phpseclib 4.0.1, 3.0.57. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist phpseclib/phpseclib: affected < 3.0.57; >= 4.0.0, < 4.0.1; fixed in 3.0.57, 4.0.1

## Suggested inspection (read-only)
- Packagist phpseclib/phpseclib: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on phpseclib/phpseclib.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-q97c-8qh3-fpc6 (OSV record GHSA-q97c-8qh3-fpc6 lists affected ranges: Packagist phpseclib/phpseclib >=0 =4.0.0 <4.0.1 (fixed))
- Verified: https://github.com/advisories/GHSA-q97c-8qh3-fpc6 (GitHub-reviewed advisory GHSA-q97c-8qh3-fpc6 lists affected ranges: Packagist phpseclib/phpseclib >=0 =4.0.0 <4.0.1)

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