crypto/rsa: bound public modulus size to prevent verify-path CPU-exhaustion DoS#81
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…ustion DoS checkKeySize is intentionally permissive about the RSA modulus size so that weak/odd keys found in the wild can still be parsed and inspected. That leaves the modular-exponentiation cost during public-key operations — O(bitlen(E) · bitlen(N)^2) — otherwise unbounded: a malicious certificate/key presenting a multi-megabit modulus can force tens of seconds to minutes of CPU per signature check (e.g. ~60s for a 2,000,000-bit modulus), which during a TLS scan can pin a worker on a single connection — a CPU-exhaustion DoS. Add MaxPublicModulusBitLen (default 16384), enforced in checkPublicKeySize alongside the existing MaxPublicExponentBitLen, so VerifyPKCS1v15 / VerifyPSS / EncryptPKCS1v15 / EncryptOAEP reject an oversized modulus *before* the modexp. Parsing functions and crypto/x509 do not consult it, so inspecting an oversized key remains safe; setting it to 0 restores the previous unbounded behavior. Addresses runZero platform audit finding TLS-01. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Summary
checkKeySizeis intentionally permissive about the RSA modulus size (so weak/odd keys found in the wild can still be parsed and inspected). That leaves the modular-exponentiation cost during public-key operations —O(bitlen(E) · bitlen(N)²)— otherwise unbounded: a malicious certificate/key presenting a multi-megabit modulus can force tens of seconds to minutes of CPU per signature check (≈60s for a 2,000,000-bit modulus). During a TLS scan that can pin a worker on a single connection — a CPU-exhaustion DoS.This adds
MaxPublicModulusBitLen(default 16384), enforced incheckPublicKeySizenext to the existingMaxPublicExponentBitLen, soVerifyPKCS1v15/VerifyPSS/EncryptPKCS1v15/EncryptOAEPreject an oversized modulus before the modexp runs.crypto/x509do not consult the variable — inspecting an oversized key remains safe (consistent withMaxPublicExponentBitLen).0to disable the bound and restore the previous unbounded behavior.Test
crypto/rsa/rsa_test.goaddsTestMaxPublicModulusBitLen: a ~2,000,000-bit modulus is rejected byVerifyPKCS1v15in <2s (vs ~60s unmitigated), andMaxPublicModulusBitLen = 0does not gate. Fullgo test ./crypto/rsa/passes.🤖 Generated with Claude Code