A flaw in Node.js HMAC verification uses a non-constant-time comparison when validating user-provided signatures, potentially leaking timing information proportional to the number of matching bytes. Under certain threat models where high-resolution timing measurements are possible, this behavior could be exploited as a timing oracle to infer HMAC values.

Node.js already provides timing-safe comparison primitives used elsewhere in the codebase, indicating this is an oversight rather than an intentional design decision.

This vulnerability affects 20.x, 22.x, 24.x, and 25.x.

NOTE: https://nodejs.org/en/blog/vulnerability/march-2026-security-releases#timing-side-channel-in-hmac-verification-via-memcmp-in-crypto_hmaccc-leads-to-potential-mac-forgery-cve-2026-21713---medium