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During X509 path validation, return immediately if a signature is invalid #4045

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May 13, 2024
Merged

During X509 path validation, return immediately if a signature is invalid #4045

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96 changes: 63 additions & 33 deletions src/lib/x509/x509path.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -48,6 +48,68 @@ CertificatePathStatusCodes PKIX::check_chain(const std::vector<X509_Certificate>

CertificatePathStatusCodes cert_status(cert_path.size());

// Before anything else verify the entire chain of signatures
for(size_t i = 0; i != cert_path.size(); ++i) {
std::set<Certificate_Status_Code>& status = cert_status.at(i);

const bool at_self_signed_root = (i == cert_path.size() - 1);

const X509_Certificate& subject = cert_path[i];
const X509_Certificate& issuer = cert_path[at_self_signed_root ? (i) : (i + 1)];

// Check the signature algorithm is known
if(!subject.signature_algorithm().oid().registered_oid()) {
status.insert(Certificate_Status_Code::SIGNATURE_ALGO_UNKNOWN);
} else {
std::unique_ptr<Public_Key> issuer_key;
try {
issuer_key = issuer.subject_public_key();
} catch(...) {
status.insert(Certificate_Status_Code::CERT_PUBKEY_INVALID);
}

if(issuer_key) {
if(issuer_key->estimated_strength() < restrictions.minimum_key_strength()) {
status.insert(Certificate_Status_Code::SIGNATURE_METHOD_TOO_WEAK);
}

const auto sig_status = subject.verify_signature(*issuer_key);

if(sig_status.first != Certificate_Status_Code::VERIFIED) {
status.insert(sig_status.first);
} else {
// Signature is valid, check if hash used was acceptable
const std::string hash_used_for_signature = sig_status.second;
BOTAN_ASSERT_NOMSG(!hash_used_for_signature.empty());
const auto& trusted_hashes = restrictions.trusted_hashes();

// Ignore untrusted hashes on self-signed roots
if(!trusted_hashes.empty() && !at_self_signed_root) {
if(!trusted_hashes.contains(hash_used_for_signature)) {
status.insert(Certificate_Status_Code::UNTRUSTED_HASH);
}
}
}
}
}
}

// If any of the signatures were invalid, return immediately; we know the
// chain is invalid and signature failure is always considered the most
// critical result. This does mean other problems in the certificate (eg
// expired) will not be reported, but we'd have to assume any such data is
// anyway arbitrary considering we couldn't verify the signature chain

for(size_t i = 0; i != cert_path.size(); ++i) {
for(auto status : cert_status.at(i)) {
// This ignores errors relating to the key or hash being weak since
// these are somewhat advisory
if(static_cast<uint32_t>(status) >= 5000) {
return cert_status;
}
}
}

if(!hostname.empty() && !cert_path[0].matches_dns_name(hostname)) {
cert_status[0].insert(Certificate_Status_Code::CERT_NAME_NOMATCH);
}
Expand Down Expand Up @@ -83,6 +145,7 @@ CertificatePathStatusCodes PKIX::check_chain(const std::vector<X509_Certificate>
status.insert(Certificate_Status_Code::CHAIN_LACKS_TRUST_ROOT);
}

// This should never happen; it indicates a bug in path building
if(subject.issuer_dn() != issuer.subject_dn()) {
status.insert(Certificate_Status_Code::CHAIN_NAME_MISMATCH);
}
Expand Down Expand Up @@ -127,39 +190,6 @@ CertificatePathStatusCodes PKIX::check_chain(const std::vector<X509_Certificate>
status.insert(Certificate_Status_Code::CA_CERT_NOT_FOR_CERT_ISSUER);
}

auto issuer_key = issuer.subject_public_key();

// Check the signature algorithm is known
if(!subject.signature_algorithm().oid().registered_oid()) {
status.insert(Certificate_Status_Code::SIGNATURE_ALGO_UNKNOWN);
} else {
// only perform the following checks if the signature algorithm is known
if(!issuer_key) {
status.insert(Certificate_Status_Code::CERT_PUBKEY_INVALID);
} else {
const auto sig_status = subject.verify_signature(*issuer_key);

if(sig_status.first == Certificate_Status_Code::VERIFIED) {
const std::string hash_used_for_signature = sig_status.second;
BOTAN_ASSERT_NOMSG(!hash_used_for_signature.empty());
const auto& trusted_hashes = restrictions.trusted_hashes();

// Ignore untrusted hashes on self-signed roots
if(!trusted_hashes.empty() && !at_self_signed_root) {
if(!trusted_hashes.contains(hash_used_for_signature)) {
status.insert(Certificate_Status_Code::UNTRUSTED_HASH);
}
}
} else {
status.insert(sig_status.first);
}

if(issuer_key->estimated_strength() < restrictions.minimum_key_strength()) {
status.insert(Certificate_Status_Code::SIGNATURE_METHOD_TOO_WEAK);
}
}
}

// Check cert extensions

if(subject.x509_version() == 1) {
Expand Down
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