---
id: CVE-2026-63410
title: >-
  secp256k1-native: Compiler-reconstructed secret-dependent branch in native
  scalar multiplication
summary: >-
  secp256k1-native is a native C secp256k1 implementation for Ruby. In affected
  native builds, native constant-time scalar multiplication through `Point#mul`
  or `scalar_multiply_ct` can leak timing information about a secret scalar when
  co…
severity: medium
cvss: 5.9
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N'
cvssSource: cna
cwe:
  - CWE-208
  - CWE-1255
vendor: sgbett
product: secp256k1-native
affected:
  - 'secp256k1-native >= 0.17.0, < 0.18.0'
ssvc:
  exploitation: poc
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-11-02T14:49:39.327340Z'
exploitAvailable: true
published: '2026-10-30'
updated: '2026-11-02'
sourceUpdated: '2026-11-02T14:50:09.239Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-63410'
references:
  - url: >-
      https://github.com/sgbett/secp256k1-native/security/advisories/GHSA-vp2j-gqfm-r3cf
    label: >-
      https://github.com/sgbett/secp256k1-native/security/advisories/GHSA-vp2j-gqfm-r3cf
  - url: 'https://github.com/sgbett/secp256k1-native/pull/32'
    label: 'https://github.com/sgbett/secp256k1-native/pull/32'
  - url: >-
      https://github.com/sgbett/secp256k1-native/commit/a89bea1617a2d063c6545ad44565dc22098c880e
    label: >-
      https://github.com/sgbett/secp256k1-native/commit/a89bea1617a2d063c6545ad44565dc22098c880e
  - url: >-
      https://github.com/sgbett/secp256k1-native/blob/master/docs/advisories/0001-compiler-reconstructed-ct-branch.md
    label: >-
      https://github.com/sgbett/secp256k1-native/blob/master/docs/advisories/0001-compiler-reconstructed-ct-branch.md
  - url: 'https://github.com/sgbett/secp256k1-native/releases/tag/v0.18.0'
    label: 'https://github.com/sgbett/secp256k1-native/releases/tag/v0.18.0'
tags:
  - cve.org
  - exploit-available
epss: 0.00281
epssPercentile: 0.20722
ingestedAt: '2026-11-05T19:30:42.674Z'
---

## Overview

secp256k1-native is a native C secp256k1 implementation for Ruby. In affected native builds, native constant-time scalar multiplication through `Point#mul` or `scalar_multiply_ct` can leak timing information about a secret scalar when compiled by a toolchain that reconstructs branchless selection into a branch. The `Montgomery ladder` uses `uint256_select` with an all-zeros or all-ones mask, but affected GCC builds at `-O2` recover the original condition and emit secret-dependent `je` or `jne` jumps whose timing correlates with scalar bits. This defeats the assumption behind the 0.17.0 `jp_add_internal` infinity-branch fix. A consumer must multiply a secret private key or nonce in a context where an attacker can observe timing; repeated measurements can statistically recover scalar bits. The defect is in the compiled binary, so source review or `ctgrind` on a non-reconstructing toolchain may miss it. GCC 14.3.0, 15.1.0, and 15.2.0 reproduced the branch reconstruction, and `extconf.rb` appends `-O2`. Tested GCC versions through 12.4.0 used bitwise operations and GCC 13.3.0 used constant-time `cmov`; the pure-Ruby path is not affected by this flaw and separately makes no constant-time claim. GCC 16 and clang were untested, and point releases, targets, and flag combinations were not exhaustively bisected. `Earlier pre-0.17.0 versions had different constant-time issues rather than this introduced select idiom.` Version 0.18.0 contains the verified fix. The advisory's broad pre-0.18.0 guidance also covers earlier, different constant-time issues; whether this CVE groups those issues or only the select idiom introduced in 0.17.0 remains unresolved.

## Affected

- `secp256k1-native >= 0.17.0, < 0.18.0`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.