decouple service accounts from root credentials (#14534)

changing root credentials makes service accounts
in-operable, this PR changes the way sessionToken
is generated for service accounts.

It changes service account behavior to generate
sessionToken claims from its own secret instead
of using global root credential.

Existing credentials will be supported by
falling back to verify using root credential.

fixes #14530

cmd/auth-handler.go

@@ -197,13 +197,7 @@ func mustGetClaimsFromToken(r *http.Request) map[string]interface{} {
 	return claims
 }
 
-// Fetch claims in the security token returned by the client.
-func getClaimsFromToken(token string) (map[string]interface{}, error) {
-	if token == "" {
-		claims := xjwt.NewMapClaims()
-		return claims.Map(), nil
-	}
-
+func getClaimsFromTokenWithSecret(token, secret string) (map[string]interface{}, error) {
 	// JWT token for x-amz-security-token is signed with admin
 	// secret key, temporary credentials become invalid if
 	// server admin credentials change. This is done to ensure
@@ -212,9 +206,15 @@ func getClaimsFromToken(token string) (map[string]interface{}, error) {
 	// hijacking the policies. We need to make sure that this is
 	// based an admin credential such that token cannot be decoded
 	// on the client side and is treated like an opaque value.
-	claims, err := auth.ExtractClaims(token, globalActiveCred.SecretKey)
+	claims, err := auth.ExtractClaims(token, secret)
 	if err != nil {
-		return nil, errAuthentication
+		if subtle.ConstantTimeCompare([]byte(secret), []byte(globalActiveCred.SecretKey)) == 1 {
+			return nil, errAuthentication
+		}
+		claims, err = auth.ExtractClaims(token, globalActiveCred.SecretKey)
+		if err != nil {
+			return nil, errAuthentication
+		}
 	}
 
 	// If OPA is set, return without any further checks.
@@ -241,23 +241,50 @@ func getClaimsFromToken(token string) (map[string]interface{}, error) {
 	return claims.Map(), nil
 }
 
+// Fetch claims in the security token returned by the client.
+func getClaimsFromToken(token string) (map[string]interface{}, error) {
+	return getClaimsFromTokenWithSecret(token, globalActiveCred.SecretKey)
+}
+
 // Fetch claims in the security token returned by the client and validate the token.
 func checkClaimsFromToken(r *http.Request, cred auth.Credentials) (map[string]interface{}, APIErrorCode) {
 	token := getSessionToken(r)
 	if token != "" && cred.AccessKey == "" {
+		// x-amz-security-token is not allowed for anonymous access.
 		return nil, ErrNoAccessKey
 	}
-	if cred.IsServiceAccount() && token == "" {
-		token = cred.SessionToken
-	}
-	if subtle.ConstantTimeCompare([]byte(token), []byte(cred.SessionToken)) != 1 {
+
+	if token == "" && cred.IsTemp() {
+		// Temporary credentials should always have x-amz-security-token
 		return nil, ErrInvalidToken
 	}
-	claims, err := getClaimsFromToken(token)
-	if err != nil {
-		return nil, toAPIErrorCode(r.Context(), err)
+
+	if token != "" && !cred.IsTemp() {
+		// x-amz-security-token should not present for static credentials.
+		return nil, ErrInvalidToken
 	}
-	return claims, ErrNone
+
+	if cred.IsTemp() && subtle.ConstantTimeCompare([]byte(token), []byte(cred.SessionToken)) != 1 {
+		// validate token for temporary credentials only.
+		return nil, ErrInvalidToken
+	}
+
+	secret := globalActiveCred.SecretKey
+	if cred.IsServiceAccount() {
+		token = cred.SessionToken
+		secret = cred.SecretKey
+	}
+
+	if token != "" {
+		claims, err := getClaimsFromTokenWithSecret(token, secret)
+		if err != nil {
+			return nil, toAPIErrorCode(r.Context(), err)
+		}
+		return claims, ErrNone
+	}
+
+	claims := xjwt.NewMapClaims()
+	return claims.Map(), ErrNone
 }
 
 // Check request auth type verifies the incoming http request