CVE-2026-23967

Curveball: Breaking Signature Integrity in sm-crypto

Alon Barad
Alon Barad
Software Engineer

Jan 22, 2026·5 min read·4 visits

Executive Summary (TL;DR)

The `sm-crypto` library, a popular JavaScript implementation of Chinese cryptographic standards (SM2), failed to enforce strict boundary checks during signature verification. This allows attackers to mathematically transform a valid signature into a new, equally valid signature without the private key. While this doesn't leak keys, it wreaks havoc on systems relying on signature uniqueness, such as blockchain ledgers and audit logs.

A signature malleability vulnerability in the sm-crypto JavaScript library allows attackers to forge secondary valid signatures for the same message, potentially bypassing replay protections in cryptographic applications.

Official Patches

JuneAndGreenGitHub Security Advisory and Patch Notes

Technical Appendix

CVSS Score
7.5/ 10
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
EPSS Probability
0.04%
Top 100% most exploited

Affected Systems

Node.js applications using sm-cryptoWeb applications (React/Vue) using sm-crypto for client-side signingWeChat Mini Programs using sm-cryptoFinancial systems implementing SM2 standards via JavaScript

Affected Versions Detail

Product
Affected Versions
Fixed Version
sm-crypto
JuneAndGreen
< 0.3.140.3.14
AttributeDetail
Vulnerability IDCVE-2026-23967
CWE IDCWE-347
CVSS7.5 (High)
Attack VectorNetwork
ImpactIntegrity (Signature Forgery)
Exploit StatusPoc Available (Theoretical)

MITRE ATT&CK Mapping

T1556Modify Authentication Process
Credential Access
T1212Exploitation for Credential Access
Credential Access
CWE-347
Improper Verification of Cryptographic Signature

The product does not verify or incorrectly verifies the cryptographic signature for data, allowing an attacker to spoof the signature or modify the data without detection.

Known Exploits & Detection

TheoreticalMathematical derivation of malleable signatures using modular arithmetic.

Vulnerability Timeline

Vulnerability Disclosed by XlabAI
2026-01-21
CVE-2026-23967 Assigned
2026-01-22
Patch Released in v0.3.14
2026-01-26

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