Improve EAB JWS validation and increase test coverage

This commit is contained in:
Herman Slatman 2021-12-07 12:17:41 +01:00
parent d0c23973cc
commit 23898e9b76
No known key found for this signature in database
GPG key ID: F4D8A44EA0A75A4F
2 changed files with 906 additions and 43 deletions

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@ -9,8 +9,7 @@ import (
"github.com/smallstep/certificates/acme"
"github.com/smallstep/certificates/api"
"github.com/smallstep/certificates/logging"
squarejose "gopkg.in/square/go-jose.v2"
"go.step.sm/crypto/jose"
)
// ExternalAccountBinding represents the ACME externalAccountBinding JWS
@ -94,12 +93,6 @@ func (h *Handler) NewAccount(w http.ResponseWriter, r *http.Request) {
return
}
eak, err := h.validateExternalAccountBinding(ctx, &nar)
if err != nil {
api.WriteError(w, err)
return
}
prov, err := acmeProvisionerFromContext(ctx)
if err != nil {
api.WriteError(w, err)
@ -107,7 +100,7 @@ func (h *Handler) NewAccount(w http.ResponseWriter, r *http.Request) {
}
httpStatus := http.StatusCreated
acc, err := accountFromContext(r.Context())
acc, err := accountFromContext(ctx)
if err != nil {
acmeErr, ok := err.(*acme.Error)
if !ok || acmeErr.Status != http.StatusBadRequest {
@ -122,12 +115,19 @@ func (h *Handler) NewAccount(w http.ResponseWriter, r *http.Request) {
"account does not exist"))
return
}
jwk, err := jwkFromContext(ctx)
if err != nil {
api.WriteError(w, err)
return
}
eak, err := h.validateExternalAccountBinding(ctx, &nar)
if err != nil {
api.WriteError(w, err)
return
}
acc = &acme.Account{
Key: jwk,
Contact: nar.Contact,
@ -137,6 +137,7 @@ func (h *Handler) NewAccount(w http.ResponseWriter, r *http.Request) {
api.WriteError(w, acme.WrapErrorISE(err, "error creating account"))
return
}
if eak != nil { // means that we have a (valid) External Account Binding key that should be bound, updated and sent in the response
err := eak.BindTo(acc)
if err != nil {
@ -256,26 +257,27 @@ func (h *Handler) validateExternalAccountBinding(ctx context.Context, nar *NewAc
return nil, acme.NewError(acme.ErrorExternalAccountRequiredType, "no external account binding provided")
}
// TODO: extract the EAB in a similar manner as JWS, JWK, payload, etc? That would probably move a lot/all of
// the logic of this function into the middleware. Should not be too hard, because the middleware does know
// about the handler and thus about its dependencies.
eabJSONBytes, err := json.Marshal(nar.ExternalAccountBinding)
if err != nil {
return nil, acme.WrapErrorISE(err, "error marshaling externalAccountBinding into bytes")
}
eabJWS, err := squarejose.ParseSigned(string(eabJSONBytes))
eabJWS, err := jose.ParseJWS(string(eabJSONBytes))
if err != nil {
return nil, acme.WrapErrorISE(err, "error parsing externalAccountBinding jws")
}
// TODO: implement strategy pattern to allow for different ways of verification (i.e. webhook call) based on configuration
// TODO(hs): implement strategy pattern to allow for different ways of verification (i.e. webhook call) based on configuration?
keyID, acmeErr := validateEABJWS(ctx, eabJWS)
if acmeErr != nil {
return nil, acmeErr
}
keyID := eabJWS.Signatures[0].Protected.KeyID
externalAccountKey, err := h.db.GetExternalAccountKey(ctx, acmeProv.Name, keyID)
if err != nil {
if _, ok := err.(*acme.Error); ok {
return nil, err
return nil, acme.WrapError(acme.ErrorUnauthorizedType, err, "the field 'kid' references an unknown key")
}
return nil, acme.WrapErrorISE(err, "error retrieving external account key")
}
@ -294,14 +296,14 @@ func (h *Handler) validateExternalAccountBinding(ctx context.Context, nar *NewAc
return nil, err
}
var payloadJWK *squarejose.JSONWebKey
var payloadJWK *jose.JSONWebKey
err = json.Unmarshal(payload, &payloadJWK)
if err != nil {
return nil, acme.WrapError(acme.ErrorMalformedType, err, "error unmarshaling payload into jwk")
}
if !keysAreEqual(jwk, payloadJWK) {
return nil, acme.NewError(acme.ErrorMalformedType, "keys in jws and eab payload do not match") // TODO: decide ACME error type to use
return nil, acme.NewError(acme.ErrorUnauthorizedType, "keys in jws and eab payload do not match")
}
return externalAccountKey, nil
@ -309,7 +311,7 @@ func (h *Handler) validateExternalAccountBinding(ctx context.Context, nar *NewAc
// keysAreEqual performs an equality check on two JWKs by comparing
// the (base64 encoding) of the Key IDs.
func keysAreEqual(x, y *squarejose.JSONWebKey) bool {
func keysAreEqual(x, y *jose.JSONWebKey) bool {
if x == nil || y == nil {
return false
}
@ -320,3 +322,62 @@ func keysAreEqual(x, y *squarejose.JSONWebKey) bool {
}
return digestX == digestY
}
// validateEABJWS verifies the contents of the External Account Binding JWS.
// The protected header of the JWS MUST meet the following criteria:
// o The "alg" field MUST indicate a MAC-based algorithm
// o The "kid" field MUST contain the key identifier provided by the CA
// o The "nonce" field MUST NOT be present
// o The "url" field MUST be set to the same value as the outer JWS
func validateEABJWS(ctx context.Context, jws *jose.JSONWebSignature) (string, *acme.Error) {
if jws == nil {
return "", acme.NewErrorISE("no JWS provided")
}
if len(jws.Signatures) != 1 {
return "", acme.NewError(acme.ErrorMalformedType, "JWS must have one signature")
}
header := jws.Signatures[0].Protected
algorithm := header.Algorithm
keyID := header.KeyID
nonce := header.Nonce
if !(algorithm == jose.HS256 || algorithm == jose.HS384 || algorithm == jose.HS512) {
return "", acme.NewError(acme.ErrorMalformedType, "'alg' field set to invalid algorithm '%s'", algorithm)
}
if keyID == "" {
return "", acme.NewError(acme.ErrorMalformedType, "'kid' field is required")
}
if nonce != "" {
return "", acme.NewError(acme.ErrorMalformedType, "'nonce' must not be present")
}
jwsURL, ok := header.ExtraHeaders["url"]
if !ok {
return "", acme.NewError(acme.ErrorMalformedType, "'url' field is required")
}
outerJWS, err := jwsFromContext(ctx)
if err != nil {
return "", acme.WrapErrorISE(err, "could not retrieve outer JWS from context")
}
if len(outerJWS.Signatures) != 1 {
return "", acme.NewError(acme.ErrorMalformedType, "outer JWS must have one signature")
}
outerJWSURL, ok := outerJWS.Signatures[0].Protected.ExtraHeaders["url"]
if !ok {
return "", acme.NewError(acme.ErrorMalformedType, "'url' field must be set in outer JWS")
}
if jwsURL != outerJWSURL {
return "", acme.NewError(acme.ErrorMalformedType, "'url' field is not the same value as the outer JWS")
}
return keyID, nil
}

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