oidc: add Config.InsecureSkipSignatureCheck

Includes a big warning for why this is usually a bad idea.

Fixes #350

oidc/verify.go

@@ -107,6 +107,17 @@ type Config struct {
 
 	// Time function to check Token expiry. Defaults to time.Now
 	Now func() time.Time
+
+	// InsecureSkipSignatureCheck causes this package to skip JWT signature validation.
+	// It's intended for special cases where providers (such as Azure), use the "none"
+	// algorithm.
+	//
+	// This option can only be enabled safely when the ID Token is received directly
+	// from the provider after the token exchange.
+	//
+	// This option MUST NOT be used when receiving an ID Token from sources other
+	// than the token endpoint.
+	InsecureSkipSignatureCheck bool
 }
 
 // Verifier returns an IDTokenVerifier that uses the provider's key set to verify JWTs.
@@ -196,11 +207,6 @@ func resolveDistributedClaim(ctx context.Context, verifier *IDTokenVerifier, src
 //    token, err := verifier.Verify(ctx, rawIDToken)
 //
 func (v *IDTokenVerifier) Verify(ctx context.Context, rawIDToken string) (*IDToken, error) {
-	jws, err := jose.ParseSigned(rawIDToken)
-	if err != nil {
-		return nil, fmt.Errorf("oidc: malformed jwt: %v", err)
-	}
-
 	// Throw out tokens with invalid claims before trying to verify the token. This lets
 	// us do cheap checks before possibly re-syncing keys.
 	payload, err := parseJWT(rawIDToken)
@@ -288,6 +294,15 @@ func (v *IDTokenVerifier) Verify(ctx context.Context, rawIDToken string) (*IDTok
 		}
 	}
 
+	if v.config.InsecureSkipSignatureCheck {
+		return t, nil
+	}
+
+	jws, err := jose.ParseSigned(rawIDToken)
+	if err != nil {
+		return nil, fmt.Errorf("oidc: malformed jwt: %v", err)
+	}
+
 	switch len(jws.Signatures) {
 	case 0:
 		return nil, fmt.Errorf("oidc: id token not signed")

oidc/verify_test.go

@@ -3,6 +3,7 @@ package oidc
 import (
 	"context"
 	"crypto"
+	"encoding/base64"
 	"errors"
 	"io"
 	"net/http"
@@ -122,6 +123,24 @@ func TestVerify(t *testing.T) {
 			},
 			signKey: newRSAKey(t),
 		},
+		{
+			name:    "unsigned token",
+			idToken: `{"iss":"https://foo"}`,
+			config: Config{
+				SkipClientIDCheck: true,
+				SkipExpiryCheck:   true,
+			},
+			wantErr: true,
+		},
+		{
+			name:    "unsigned token InsecureSkipSignatureCheck",
+			idToken: `{"iss":"https://foo"}`,
+			config: Config{
+				SkipClientIDCheck:          true,
+				SkipExpiryCheck:            true,
+				InsecureSkipSignatureCheck: true,
+			},
+		},
 	}
 	for _, test := range tests {
 		t.Run(test.name, test.run)
@@ -537,7 +556,14 @@ type verificationTest struct {
 }
 
 func (v verificationTest) runGetToken(t *testing.T) (*IDToken, error) {
-	token := v.signKey.sign(t, []byte(v.idToken))
+	var token string
+	if v.signKey != nil {
+		token = v.signKey.sign(t, []byte(v.idToken))
+	} else {
+		token = base64.RawURLEncoding.EncodeToString([]byte(`{alg: "none"}`))
+		token += "."
+		token += base64.RawURLEncoding.EncodeToString([]byte(v.idToken))
+	}
 
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
@@ -547,10 +573,10 @@ func (v verificationTest) runGetToken(t *testing.T) (*IDToken, error) {
 		issuer = v.issuer
 	}
 	var ks KeySet
-	if v.verificationKey == nil {
-		ks = &StaticKeySet{PublicKeys: []crypto.PublicKey{v.signKey.pub}}
-	} else {
+	if v.verificationKey != nil {
 		ks = &StaticKeySet{PublicKeys: []crypto.PublicKey{v.verificationKey.pub}}
+	} else if v.signKey != nil {
+		ks = &StaticKeySet{PublicKeys: []crypto.PublicKey{v.signKey.pub}}
 	}
 	verifier := NewVerifier(issuer, ks, &v.config)