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fix: reject substituted ASN.1 certificate tags #51

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fix: reject substituted ASN.1 certificate tags #51

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cb5a4b6
fix: require exact cert ASN.1 tags
leopoldjoy Jun 26, 2026
b8941c3
refactor: trim duplicate ASN.1 tag checks
leopoldjoy Jun 26, 2026
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24 changes: 23 additions & 1 deletion src/CertManager.sol
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,8 @@ contract CertManager is ICertManager {
using LibAsn1Ptr for Asn1Ptr;
using LibBytes for bytes;

error InvalidAsn1Tag();

event CertVerified(bytes32 indexed certHash);
event CertRevoked(bytes32 indexed certHash);
event CertUnrevoked(bytes32 indexed certHash);
Expand Down Expand Up @@ -163,7 +165,10 @@ contract CertManager is ICertManager {
/// {revokeCerts}; the same value is recorded on-chain when the cert is first verified, so a
/// revocation applies to every byte-encoding of that certificate. Reverts on malformed DER.
function computeCertId(bytes memory cert) external pure returns (bytes32) {
Asn1Ptr tbsCertPtr = cert.firstChildOf(cert.root());
Asn1Ptr root = cert.root();
_requireAsn1Tag(cert, root, 0x30);
Asn1Ptr tbsCertPtr = cert.firstChildOf(root);
_requireAsn1Tag(cert, tbsCertPtr, 0x30);
return _certIdentity(cert, tbsCertPtr);
}

Expand Down Expand Up @@ -290,11 +295,17 @@ contract CertManager is ICertManager {
}

Asn1Ptr root = certificate.root();
_requireAsn1Tag(certificate, root, 0x30);
require(root.totalLength() == certificate.length, "invalid cert length");
Asn1Ptr tbsCertPtr = certificate.firstChildOf(root);
_requireAsn1Tag(certificate, tbsCertPtr, 0x30);
return certificate.keccak(tbsCertPtr.header(), tbsCertPtr.totalLength());
}

function _requireAsn1Tag(bytes memory der, Asn1Ptr ptr, bytes1 tag) internal pure {
if (der[ptr.header()] != tag) revert InvalidAsn1Tag();
}

function _isPinnedRootAlias(bytes32 certHash, VerifiedCert memory cert) internal pure returns (bool) {
return certHash != ROOT_CA_CERT_HASH && cert.ca && cert.subjectHash == ROOT_CA_CERT_SUBJECT_HASH
&& cert.pubKey.length == ROOT_CA_CERT_PUB_KEY.length
Expand Down Expand Up @@ -349,10 +360,13 @@ contract CertManager is ICertManager {
view
returns (uint64 notAfter, int64 maxPathLen, bytes32 issuerHash, bytes32 subjectHash, bytes memory pubKey)
{
_requireAsn1Tag(certificate, ptr, 0x30);
Asn1Ptr versionPtr = certificate.firstChildOf(ptr);
_requireAsn1Tag(certificate, versionPtr, 0xa0);
Asn1Ptr vPtr = certificate.firstChildOf(versionPtr);
Asn1Ptr serialPtr = certificate.nextSiblingOf(versionPtr);
Asn1Ptr sigAlgoPtr = certificate.nextSiblingOf(serialPtr);
_requireAsn1Tag(certificate, sigAlgoPtr, 0x30);

require(certificate.keccak(sigAlgoPtr.content(), sigAlgoPtr.length()) == CERT_ALGO_OID, "invalid cert sig algo");
uint256 version = certificate.uintAt(vPtr);
Expand All @@ -368,11 +382,15 @@ contract CertManager is ICertManager {
returns (uint64 notAfter, int64 maxPathLen, bytes32 issuerHash, bytes32 subjectHash, bytes memory pubKey)
{
Asn1Ptr issuerPtr = certificate.nextSiblingOf(sigAlgoPtr);
_requireAsn1Tag(certificate, issuerPtr, 0x30);
issuerHash = certificate.keccak(issuerPtr.content(), issuerPtr.length());
Asn1Ptr validityPtr = certificate.nextSiblingOf(issuerPtr);
_requireAsn1Tag(certificate, validityPtr, 0x30);
Asn1Ptr subjectPtr = certificate.nextSiblingOf(validityPtr);
_requireAsn1Tag(certificate, subjectPtr, 0x30);
subjectHash = certificate.keccak(subjectPtr.content(), subjectPtr.length());
Asn1Ptr subjectPublicKeyInfoPtr = certificate.nextSiblingOf(subjectPtr);
_requireAsn1Tag(certificate, subjectPublicKeyInfoPtr, 0x30);
Asn1Ptr extensionsPtr = certificate.nextSiblingOf(subjectPublicKeyInfoPtr);

if (certificate[extensionsPtr.header()] == 0x81) {
Expand All @@ -395,6 +413,7 @@ contract CertManager is ICertManager {
returns (bytes memory subjectPubKey)
{
Asn1Ptr pubKeyAlgoPtr = certificate.firstChildOf(subjectPublicKeyInfoPtr);
_requireAsn1Tag(certificate, pubKeyAlgoPtr, 0x30);
Asn1Ptr pubKeyAlgoIdPtr = certificate.firstChildOf(pubKeyAlgoPtr);
Asn1Ptr algoParamsPtr = certificate.nextSiblingOf(pubKeyAlgoIdPtr);
Asn1Ptr subjectPublicKeyPtr = certificate.nextSiblingOf(pubKeyAlgoPtr);
Expand Down Expand Up @@ -435,13 +454,15 @@ contract CertManager is ICertManager {
{
require(certificate[extensionsPtr.header()] == 0xa3, "invalid extensions");
extensionsPtr = certificate.firstChildOf(extensionsPtr);
_requireAsn1Tag(certificate, extensionsPtr, 0x30);
Asn1Ptr extensionPtr = certificate.firstChildOf(extensionsPtr);
uint256 end = extensionsPtr.content() + extensionsPtr.length();
bool basicConstraintsFound = false;
bool keyUsageFound = false;
maxPathLen = -1;

while (true) {
_requireAsn1Tag(certificate, extensionPtr, 0x30);
Asn1Ptr oidPtr = certificate.firstChildOf(extensionPtr);
bytes32 oid = certificate.keccak(oidPtr.content(), oidPtr.length());

Expand Down Expand Up @@ -544,6 +565,7 @@ contract CertManager is ICertManager {
bytes memory signatureHints
) internal view {
Asn1Ptr sigAlgoPtr = certificate.nextSiblingOf(ptr);
_requireAsn1Tag(certificate, sigAlgoPtr, 0x30);
require(certificate.keccak(sigAlgoPtr.content(), sigAlgoPtr.length()) == CERT_ALGO_OID, "invalid cert sig algo");
Asn1Ptr sigPtr = certificate.nextSiblingOf(sigAlgoPtr);
require(sigPtr.header() + sigPtr.totalLength() == certificate.length, "trailing cert fields");
Expand Down
29 changes: 29 additions & 0 deletions test/CertManager.t.sol
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Original file line number Diff line number Diff line change
Expand Up @@ -166,6 +166,35 @@ contract CertManagerTest is Test {
cm.verifyCACertWithHints(rootTwin, rootHash, hints);
}

function test_VerifyCACertWithHints_RejectsOuterTagSubstitution() public {
vm.warp(1775145600);
CertManager cm = new CertManager(new P384Verifier());

bytes32 rootHash = keccak256(CB0);
bytes memory mutated = bytes.concat(CB1);
mutated[0] = 0x31; // constructed SET with the same children is not an X.509 Certificate SEQUENCE.

vm.expectRevert(CertManager.InvalidAsn1Tag.selector);
cm.verifyCACertWithHints(mutated, rootHash, "");
}

function test_VerifyCACertWithHints_RejectsTbsAlgorithmTagSubstitution() public {
vm.warp(1775145600);
CertManager cm = new CertManager(new P384Verifier());

bytes32 rootHash = keccak256(CB0);
bytes memory mutated = bytes.concat(CB1);
Asn1Ptr root = mutated.root();
Asn1Ptr tbsPtr = mutated.firstChildOf(root);
Asn1Ptr versionPtr = mutated.firstChildOf(tbsPtr);
Asn1Ptr serialPtr = mutated.nextSiblingOf(versionPtr);
Asn1Ptr sigAlgoPtr = mutated.nextSiblingOf(serialPtr);
mutated[sigAlgoPtr.header()] = 0x31; // constructed, but not AlgorithmIdentifier SEQUENCE.

vm.expectRevert(CertManager.InvalidAsn1Tag.selector);
cm.verifyCACertWithHints(mutated, rootHash, "");
}

function _verifyCA(CertManager cm, P384HintCollector collector, bytes memory cert, bytes32 parentHash)
internal
returns (bytes32)
Expand Down
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