Non-Constant-Time SM2 Scalar Multiplication on ARM64 and RISC-V

Issue summary: A non-constant-time optimized implementation of scalar
point multiplication is used for SM2 private key operations on ARM64 and
RISC-V platforms.

Impact summary: An attacker able to measure the time taken by, or to observe
the cache-line access pattern of SM2 signing or decryption on an affected
platform can learn information about the secret scalar.

CWE: CWE-208: Observable Timing Discrepancy

Description: On ARM64 and RISC-V processors, the SM2 curve uses an optimized
scalar multiplication implementation whose conditional branches and table
look ups are chosen according to the bits of the secret scalar. The execution
time and the cache-access pattern therefore depend on the long-term private
key (during SM2 decryption) or the per-signature nonce (during SM2 signature
generation), forming a timing and cache side-channel.

FIPS Impact: no
SM2 is not a FIPS algorithm and the optimized SM2 implementation is not part
of the FIPS module.

OpenSSL 4.0, 3.6, 3.5 and 3.4 are vulnerable to this issue on AArch64 and
RISC-V.

OpenSSL 3.0, 1.1.1 and 1.0.2 are not affected by this issue.

OpenSSL 4.0 users should upgrade to OpenSSL 4.0.3.
OpenSSL 3.6 users should upgrade to OpenSSL 3.6.5.
OpenSSL 3.5 users should upgrade to OpenSSL 3.5.9.
OpenSSL 3.4 users should upgrade to OpenSSL 3.4.8.

This issue was reported on 2 May 2026 by Abhinav Agarwal.
It was independently reported on 6 June 2026 by Feng Xue.
The fix was developed by Igor Ustinov.

-- cut (non-publishing metadata for internal use) --
Reported by: Abhinav Agarwal, Feng Xue
Fixed by: Igor Ustinov