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Add additional check for id-kp-OCSPSigning EKU #745

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Add additional check for id-kp-OCSPSigning EKU #745

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20 changes: 19 additions & 1 deletion src/crypto/OCSP.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -34,6 +34,7 @@
#include <openssl/pem.h>
#include <openssl/rand.h>
#include <openssl/sha.h>
#include <openssl/x509v3.h>

using namespace digidoc;
using namespace std;
Expand Down Expand Up @@ -203,6 +204,10 @@ void OCSP::verifyResponse(const X509Cert &cert) const
}
}
auto store = X509CertStore::createStore(X509CertStore::OCSP, tm);
// OCSP_NOCHECKS skips ocsp_check_issuer() which requires the responder to be directly
// delegated by the cert's issuing CA. SK PKI uses a cross-CA OCSP responder issued by
// the root CA rather than the intermediate CA, so this check would always fail.
// We compensate by verifying the responder cert has id-kp-OCSPSigning EKU below.
if(OCSP_basic_verify(basic.get(), stack.get(), store.get(), OCSP_NOCHECKS | OCSP_PARTIAL_CHAIN) != 1)
{
unsigned long err = ERR_get_error();
Expand All @@ -217,7 +222,7 @@ void OCSP::verifyResponse(const X509Cert &cert) const
throw OpenSSLException(EXCEPTION_PARAMS("Failed to verify OCSP response."), err);
}

// Find issuer before OCSP validation to activate region TSL
// Find issuer before status validation
X509Cert issuer = X509CertStore::instance()->findIssuer(cert, X509CertStore::CA);
if(!issuer)
{
Expand All @@ -226,6 +231,19 @@ void OCSP::verifyResponse(const X509Cert &cert) const
throw e;
}

// Explicitly check the OCSP responder has id-kp-OCSPSigning EKU (normally done by
// ocsp_check_issuer which we skipped above).
if(X509Cert responder = responderCert(); responder)
{
bool hasOCSPSign = issuer == responder;
auto eku = make_unique_cast<EXTENDED_KEY_USAGE_free>(
X509_get_ext_d2i(responder.handle(), NID_ext_key_usage, nullptr, nullptr));
for(int i = 0; eku && i < sk_ASN1_OBJECT_num(eku.get()); i++)
hasOCSPSign |= OBJ_obj2nid(sk_ASN1_OBJECT_value(eku.get(), i)) == NID_OCSP_sign;
if(!hasOCSPSign)
THROW("OCSP responder certificate does not have id-kp-OCSPSigning EKU");
}

int status = V_OCSP_CERTSTATUS_UNKNOWN;
for(int i = 0, count = OCSP_resp_count(basic.get()); i < count; ++i)
{
Expand Down
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