diff --git a/.github/scripts/variables.sh b/.github/scripts/variables.sh
index 80494ba0db..e12953942c 100755
--- a/.github/scripts/variables.sh
+++ b/.github/scripts/variables.sh
@@ -18,7 +18,7 @@ get_docker_md5() {
 }
 
 get_go_code_md5() {
-  find . -type f \( -name "*.go" -o -name go.mod -o -name go.sum -o -name "*.tmpl" -o -name "version.txt" \) -not -path "./site*"  -exec md5sum {} + | LC_ALL=C sort  | md5sum | awk '{ print $1 }'
+  find . -type f \( -name "*.go" -o -name go.mod -o -name go.sum -o -name "*.tmpl" -o -name "version.txt" -o -name "*.js" \) -not -path "./site*"  -exec md5sum {} + | LC_ALL=C sort  | md5sum | awk '{ print $1 }'
 }
 
 get_tests_md5() {
diff --git a/internal/configs/oidc/openid_connect.js b/internal/configs/oidc/openid_connect.js
index 1f99ea28d7..eccabca2ba 100644
--- a/internal/configs/oidc/openid_connect.js
+++ b/internal/configs/oidc/openid_connect.js
@@ -3,8 +3,6 @@
  *
  * Copyright (C) 2020 Nginx, Inc.
  */
-var newSession = false; // Used by oidcAuth() and validateIdToken()
-
 export default {auth, codeExchange, validateIdToken, logout};
 
 function retryOriginalRequest(r) {
@@ -32,8 +30,6 @@ function auth(r, afterSyncCheck) {
     }
 
     if (!r.variables.refresh_token || r.variables.refresh_token == "-") {
-        newSession = true;
-
         // Check we have all necessary configuration variables (referenced only by njs)
         var oidcConfigurables = ["authz_endpoint", "scopes", "hmac_key", "cookie_flags"];
         var missingConfig = [];
@@ -54,7 +50,7 @@ function auth(r, afterSyncCheck) {
 
     // Pass the refresh token to the /_refresh location so that it can be
     // proxied to the IdP in exchange for a new id_token
-    r.subrequest("/_refresh", "token=" + r.variables.refresh_token,
+    r.subrequest("/_refresh", generateTokenRequestParams(r, "refresh_token"),
         function(reply) {
             if (reply.status != 200) {
                 // Refresh request failed, log the reason
@@ -142,7 +138,7 @@ function codeExchange(r) {
 
     // Pass the authorization code to the /_token location so that it can be
     // proxied to the IdP in exchange for a JWT
-    r.subrequest("/_token",idpClientAuth(r), function(reply) {
+    r.subrequest("/_token", generateTokenRequestParams(r, "authorization_code"), function(reply) {
             if (reply.status == 504) {
                 r.error("OIDC timeout connecting to IdP when sending authorization code");
                 r.return(504);
@@ -200,7 +196,7 @@ function codeExchange(r) {
 
                         r.headersOut["Set-Cookie"] = "auth_token=" + r.variables.request_id + "; " + r.variables.oidc_cookie_flags;
                         r.return(302, r.variables.redirect_base + decodeURIComponent(r.variables.cookie_auth_redir));
-                    }
+                   }
                 );
             } catch (e) {
                 r.error("OIDC authorization code sent but token response is not JSON. " + reply.responseText);
@@ -241,10 +237,9 @@ function validateIdToken(r) {
         validToken = false;
     }
 
-    // If we receive a nonce in the ID Token then we will use the auth_nonce cookies
-    // to check that the JWT can be validated as being directly related to the
-    // original request by this client. This mitigates against token replay attacks.
-    if (newSession) {
+    // According to OIDC Core 1.0 Section 2:
+    // "If present in the ID Token, Clients MUST verify that the nonce Claim Value is equal to the value of the nonce parameter sent in the Authentication Request."
+    if (r.variables.jwt_claim_nonce) {
         var client_nonce_hash = "";
         if (r.variables.cookie_auth_nonce) {
             var c = require('crypto');
@@ -255,6 +250,9 @@ function validateIdToken(r) {
             r.error("OIDC ID Token validation error: nonce from token (" + r.variables.jwt_claim_nonce + ") does not match client (" + client_nonce_hash + ")");
             validToken = false;
         }
+    } else if (!r.variables.refresh_token || r.variables.refresh_token == "-") {
+        r.error("OIDC ID Token validation error: missing nonce claim in ID Token during initial authentication.");
+        validToken = false;
     }
 
     if (validToken) {
@@ -297,7 +295,7 @@ function logout(r) {
 
         // Construct logout arguments for RP-initiated logout
         var logoutArgs = "?post_logout_redirect_uri=" + encodeURIComponent(logoutRedirectUrl) +
-            "&id_token_hint=" + encodeURIComponent(r.variables.session_jwt);
+                         "&id_token_hint=" + encodeURIComponent(r.variables.session_jwt);
         performLogout(r.variables.oidc_end_session_endpoint + logoutArgs);
     } else {
         // Fallback to traditional logout approach
@@ -337,12 +335,38 @@ function getAuthZArgs(r) {
     return authZArgs;
 }
 
-function idpClientAuth(r) {
-    // If PKCE is enabled we have to use the code_verifier
-    if ( r.variables.oidc_pkce_enable == 1 ) {
-        r.variables.pkce_id = r.variables.arg_state;
-        return "code=" + r.variables.arg_code + "&code_verifier=" + r.variables.pkce_code_verifier;
-    } else {
-        return "code=" + r.variables.arg_code + "&client_secret=" + r.variables.oidc_client_secret;
+function generateTokenRequestParams(r, grant_type) {
+    var body = "grant_type=" + grant_type + "&client_id=" + r.variables.oidc_client;
+
+    switch(grant_type) {
+        case "authorization_code":
+            body += "&code=" + r.variables.arg_code + "&redirect_uri=" + r.variables.redirect_base + r.variables.redir_location;
+            if (r.variables.oidc_pkce_enable == 1) {
+                r.variables.pkce_id = r.variables.arg_state;
+                body += "&code_verifier=" + r.variables.pkce_code_verifier;
+            }
+            break;
+        case "refresh_token":
+            body += "&refresh_token=" + r.variables.refresh_token;
+            break;
+        default:
+            r.error("Unsupported grant type: " + grant_type);
+            return;
     }
+
+    var options = {
+        body: body,
+        method: "POST"
+    };
+
+    if (r.variables.oidc_pkce_enable != 1) {
+        if (r.variables.oidc_client_auth_method === "client_secret_basic") {
+            let auth_basic = "Basic " + Buffer.from(r.variables.oidc_client + ":" + r.variables.oidc_client_secret).toString('base64');
+            options.args = "secret_basic=" + auth_basic;
+        } else {
+            options.body += "&client_secret=" + r.variables.oidc_client_secret;
+        }
+    }
+
+    return options;
 }