CVE-2026-49440: Deno: Miller-Rabin Primality Test Allows Zero Rounds

node:crypto.checkPrime(candidate[, options][, callback]) and crypto.checkPrimeSync(candidate[, options]) ran no Miller-Rabin rounds at all when the caller left options.checks at its default of 0. In that mode, the only test applied to the candidate was trial division by the primes up to 17,863. Any composite whose smallest prime factor exceeds that bound — for example the product of two primes just above it, such as 17,881 × 17,891 — was reported as true (“probably prime”).

The same divergence affected the lower-level op_node_check_prime / op_node_check_prime_bytes paths that the polyfill calls into.

Node.js itself does not have this problem: it forwards checks = 0 to OpenSSL’s BN_check_prime, which substitutes a sensible default number of rounds based on the candidate’s bit length (per FIPS 186-4 Appendix C.3 Table C.1). Deno’s Rust implementation had no equivalent fallback, so count = 0 meant “skip the loop entirely.”