CVE-2026-42499: replace the quadratic string concatenation in net/mail
consumePhrase with a strings.Builder so phrases containing many adjacent
RFC 2047 encoded-words are coalesced in linear time, preventing a denial
of service from pathological RFC 5322 address inputs
CVE-2026-42501: reject a checksum database (sumdb) /lookup/ response that
lacks a hash for the requested module in cmd/go checkSumDB instead of
silently treating it as verified, preventing a malicious module proxy from
bypassing checksum database validation and serving a corrupted module