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fix(protocol): ensure attca is parsed correctly #280

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fix(protocol): ensure attca is parsed correctly #280

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6 changes: 6 additions & 0 deletions protocol/attestation.go
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Original file line number Diff line number Diff line change
Expand Up @@ -249,6 +249,12 @@ func (a *AttestationObject) VerifyAttestation(clientDataHash []byte, mds metadat
return ErrInvalidAttestation.WithDetails("Unable to parse attestation certificate from x5c during attestation validation").WithInfo(fmt.Sprintf("Error returned from x509.ParseCertificate: %+v", err))
}

if attestationType == string(metadata.AttCA) {
if err = tpmParseSANExtension(x5c); err != nil {
return err
}
}

if x5c.Subject.CommonName != x5c.Issuer.CommonName {
if !entry.MetadataStatement.AttestationTypes.HasBasicFull() {
return ErrInvalidAttestation.WithDetails("Unable to validate attestation statement signature during attestation validation: attestation with full attestation from authenticator that does not support full attestation")
Expand Down
113 changes: 70 additions & 43 deletions protocol/attestation_tpm.go
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Original file line number Diff line number Diff line change
Expand Up @@ -147,15 +147,15 @@ func verifyTPMFormat(att AttestationObject, clientDataHash []byte, _ metadata.Pr
return "", nil, ErrAttestation.WithDetails("Error getting certificate from x5c cert chain")
}

aikCert, err := x509.ParseCertificate(aikCertBytes)
if err != nil {
var aikCert *x509.Certificate

if aikCert, err = x509.ParseCertificate(aikCertBytes); err != nil {
return "", nil, ErrAttestationFormat.WithDetails("Error parsing certificate from ASN.1")
}

sigAlg := webauthncose.SigAlgFromCOSEAlg(coseAlg)

err = aikCert.CheckSignature(x509.SignatureAlgorithm(sigAlg), certInfoBytes, sigBytes)
if err != nil {
if err = aikCert.CheckSignature(x509.SignatureAlgorithm(sigAlg), certInfoBytes, sigBytes); err != nil {
return "", nil, ErrAttestationFormat.WithDetails(fmt.Sprintf("Signature validation error: %+v\n", err))
}
// Verify that aikCert meets the requirements in §8.3.1 TPM Attestation Statement Certificate Requirements
Expand All @@ -164,23 +164,41 @@ func verifyTPMFormat(att AttestationObject, clientDataHash []byte, _ metadata.Pr
if aikCert.Version != 3 {
return "", nil, ErrAttestationFormat.WithDetails("AIK certificate version must be 3")
}

// 2/6 Subject field MUST be set to empty.
if aikCert.Subject.String() != "" {
return "", nil, ErrAttestationFormat.WithDetails("AIK certificate subject must be empty")
}

// 3/6 The Subject Alternative Name extension MUST be set as defined in [TPMv2-EK-Profile] section 3.2.9{}
var manufacturer, model, version string
var (
manufacturer, model, version string
ekuValid = false
eku []asn1.ObjectIdentifier
constraints tpmBasicConstraints
rest []byte
)

for _, ext := range aikCert.Extensions {
if ext.Id.Equal([]int{2, 5, 29, 17}) {
manufacturer, model, version, err = parseSANExtension(ext.Value)
if err != nil {
if ext.Id.Equal(oidExtensionSubjectAltName) {
if manufacturer, model, version, err = parseSANExtension(ext.Value); err != nil {
return "", nil, err
}
} else if ext.Id.Equal(oidExtensionExtendedKeyUsage) {
if rest, err = asn1.Unmarshal(ext.Value, &eku); len(rest) != 0 || err != nil || !eku[0].Equal(tcgKpAIKCertificate) {
return "", nil, ErrAttestationFormat.WithDetails("AIK certificate EKU missing 2.23.133.8.3")
}

ekuValid = true
} else if ext.Id.Equal(oidExtensionBasicConstraints) {
if rest, err = asn1.Unmarshal(ext.Value, &constraints); err != nil {
return "", nil, ErrAttestationFormat.WithDetails("AIK certificate basic constraints malformed")
} else if len(rest) != 0 {
return "", nil, ErrAttestationFormat.WithDetails("AIK certificate basic constraints contains extra data")
}
}
}

// 3/6 The Subject Alternative Name extension MUST be set as defined in [TPMv2-EK-Profile] section 3.2.9{}
if manufacturer == "" || model == "" || version == "" {
return "", nil, ErrAttestationFormat.WithDetails("Invalid SAN data in AIK certificate")
}
Expand All @@ -190,44 +208,10 @@ func verifyTPMFormat(att AttestationObject, clientDataHash []byte, _ metadata.Pr
}

// 4/6 The Extended Key Usage extension MUST contain the "joint-iso-itu-t(2) internationalorganizations(23) 133 tcg-kp(8) tcg-kp-AIKCertificate(3)" OID.
var (
ekuValid = false
eku []asn1.ObjectIdentifier
)

for _, ext := range aikCert.Extensions {
if ext.Id.Equal([]int{2, 5, 29, 37}) {
rest, err := asn1.Unmarshal(ext.Value, &eku)
if len(rest) != 0 || err != nil || !eku[0].Equal(tcgKpAIKCertificate) {
return "", nil, ErrAttestationFormat.WithDetails("AIK certificate EKU missing 2.23.133.8.3")
}

ekuValid = true
}
}

if !ekuValid {
return "", nil, ErrAttestationFormat.WithDetails("AIK certificate missing EKU")
}

// 5/6 The Basic Constraints extension MUST have the CA component set to false.
type basicConstraints struct {
IsCA bool `asn1:"optional"`
MaxPathLen int `asn1:"optional,default:-1"`
}

var constraints basicConstraints

for _, ext := range aikCert.Extensions {
if ext.Id.Equal([]int{2, 5, 29, 19}) {
if rest, err := asn1.Unmarshal(ext.Value, &constraints); err != nil {
return "", nil, ErrAttestationFormat.WithDetails("AIK certificate basic constraints malformed")
} else if len(rest) != 0 {
return "", nil, ErrAttestationFormat.WithDetails("AIK certificate basic constraints contains extra data")
}
}
}

// 6/6 An Authority Information Access (AIA) extension with entry id-ad-ocsp and a CRL Distribution Point
// extension [RFC5280] are both OPTIONAL as the status of many attestation certificates is available
// through metadata services. See, for example, the FIDO Metadata Service.
Expand Down Expand Up @@ -378,3 +362,46 @@ func isValidTPMManufacturer(id string) bool {

return false
}

func tpmParseSANExtension(attestation *x509.Certificate) (err error) {
var (
manufacturer, model, version string
)

for _, ext := range attestation.Extensions {
if ext.Id.Equal(oidExtensionSubjectAltName) {
if manufacturer, model, version, err = parseSANExtension(ext.Value); err != nil {
return ErrInvalidAttestation.WithDetails("Authenticator with invalid AIK SAN data encountered during attestation validation.").WithInfo(fmt.Sprintf("Error occurred parsing SAN extension: %s", err.Error()))
}
}
}

if manufacturer == "" || model == "" || version == "" {
return ErrAttestationFormat.WithDetails("Invalid SAN data in AIK certificate")
}

var unhandled []asn1.ObjectIdentifier

for _, uce := range attestation.UnhandledCriticalExtensions {
if uce.Equal(oidExtensionSubjectAltName) {
continue
}

unhandled = append(unhandled, uce)
}

attestation.UnhandledCriticalExtensions = unhandled

return nil
}

var (
oidExtensionSubjectAltName = []int{2, 5, 29, 17}
oidExtensionExtendedKeyUsage = []int{2, 5, 29, 37}
oidExtensionBasicConstraints = []int{2, 5, 29, 19}
)

type tpmBasicConstraints struct {
IsCA bool `asn1:"optional"`
MaxPathLen int `asn1:"optional,default:-1"`
}
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