sgbett secp256k1-native up to 0.17.x Montgomery ladder extconf.rb Point#mul/scalar_multiply_ct timing discrepancy

Summaryinfo

A vulnerability labeled as problematic has been found in sgbett secp256k1-native up to 0.17.x. The affected element is the function Point#mul/scalar_multiply_ct of the file extconf.rb of the component Montgomery ladder. Such manipulation leads to timing discrepancy. This vulnerability is documented as CVE-2026-63410. The attack can be executed remotely. There is not any exploit available. The affected component should be upgraded.

Detailsinfo

A vulnerability, which was classified as problematic, has been found in sgbett secp256k1-native up to 0.17.x. Affected by this issue is the function Point#mul/scalar_multiply_ct of the file extconf.rb of the component Montgomery ladder. The manipulation with an unknown input leads to a timing discrepancy vulnerability. Using CWE to declare the problem leads to CWE-208. Two separate operations in a product require different amounts of time to complete, in a way that is observable to an actor and reveals security-relevant information about the state of the product, such as whether a particular operation was successful or not. Impacted is confidentiality. CVE summarizes:

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.

The advisory is available at github.com. This vulnerability is handled as CVE-2026-63410 since 07/16/2026. The exploitation is known to be difficult. The attack may be launched remotely. Technical details are known, but there is no available exploit. This vulnerability is assigned to T1592 by the MITRE ATT&CK project.

Upgrading to version 0.18.0 eliminates this vulnerability.

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Productinfo

Vendor

• sgbett

Name

• secp256k1-native

Version

• 0.1
• 0.2
• 0.3
• 0.4
• 0.5
• 0.6
• 0.7
• 0.8
• 0.9
• 0.10
• 0.11
• 0.12
• 0.13
• 0.14
• 0.15
• 0.16
• 0.17

Website

• Product: https://github.com/sgbett/secp256k1-native/

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