CVE-2026-54872

Issue summary: The generic elliptic-curve scalar multiplication used for

ECDSA and SM2 signature operations with curves that do not have a dedicated

implementation leaks information about the secret nonce through timing.

Impact summary: An attacker able to measure signing times may learn

information about the per-signature secret nonce, which over many signatures

can, via a lattice / Hidden Number Problem attack, lead to recovery of the

private key.

CWE: CWE-208: Observable Timing Discrepancy

Description: The generic elliptic-curve scalar multiplication used for

curves that do not have a dedicated constant-time implementation pads the

secret scalar with non-constant-time BIGNUM operations, so the time taken

depends on the value of the secret scalar derived from the ECDSA and SM2 nonce.

The leak is very small; observing it requires a large number of

measurements. The effect is largest for curves whose group order lies

on a machine-word boundary, such as brainpoolP384r1.

Applications using ECDSA signing over the Brainpool and other generic prime

curves, and SM2 signing on platforms that use the generic implementation,

are vulnerable to this issue.

The NIST curves P-256, P-384 and P-521 use dedicated constant-time

implementations and are not affected.

FIPS Impact: no

The FIPS modules are not affected: the approved NIST curves used in the FIPS

provider have dedicated constant-time implementations and do not use the

affected code path.