diff --git a/pom.xml b/pom.xml
index fba2dad..6b79bb1 100644
--- a/pom.xml
+++ b/pom.xml
@@ -59,6 +59,11 @@
       <artifactId>commons-lang3</artifactId>
       <version>3.4</version>
     </dependency>
+    <dependency>
+      <groupId>commons-codec</groupId>
+      <artifactId>commons-codec</artifactId>
+      <version>1.16.0</version>
+    </dependency>
   </dependencies>
   <build>
     <plugins>
diff --git a/src/main/java/com/ysoft/geecon/OAuthResource.java b/src/main/java/com/ysoft/geecon/OAuthResource.java
index 812de81..6695cd5 100644
--- a/src/main/java/com/ysoft/geecon/OAuthResource.java
+++ b/src/main/java/com/ysoft/geecon/OAuthResource.java
@@ -103,6 +103,9 @@ public class OAuthResource {
         validateClient(params);
         var session = sessionsRepo.redeemAuthorizationCode(params.getCode())
                 .orElseThrow(() -> new OAuthException("Invalid code"));
+        if (!session.validateCodeChallenge(params.getCodeVerifier())) {
+            throw new OAuthException("Invalid code verifier");
+        }
 
         String idToken = null;
 
diff --git a/src/main/java/com/ysoft/geecon/dto/AuthParams.java b/src/main/java/com/ysoft/geecon/dto/AuthParams.java
index bcc9f74..3c1c630 100644
--- a/src/main/java/com/ysoft/geecon/dto/AuthParams.java
+++ b/src/main/java/com/ysoft/geecon/dto/AuthParams.java
@@ -24,6 +24,12 @@ public class AuthParams {
     String scope;
     @RestQuery("state")
     String state;
+    @RestQuery("code_challenge_method")
+    String codeChallengeMethod;
+    @RestQuery("code_challenge")
+    String codeChallenge;
+    @RestQuery("nonce")
+    String nonce;
 
     public String getLoginHint() {
         return loginHint;
@@ -76,4 +82,28 @@ public class AuthParams {
     public void setState(String state) {
         this.state = state;
     }
+
+    public String getCodeChallengeMethod() {
+        return codeChallengeMethod;
+    }
+
+    public void setCodeChallengeMethod(String codeChallengeMethod) {
+        this.codeChallengeMethod = codeChallengeMethod;
+    }
+
+    public String getCodeChallenge() {
+        return codeChallenge;
+    }
+
+    public void setCodeChallenge(String codeChallenge) {
+        this.codeChallenge = codeChallenge;
+    }
+
+    public String getNonce() {
+        return nonce;
+    }
+
+    public void setNonce(String nonce) {
+        this.nonce = nonce;
+    }
 }
diff --git a/src/main/java/com/ysoft/geecon/dto/AuthorizationSession.java b/src/main/java/com/ysoft/geecon/dto/AuthorizationSession.java
index 5b55f85..d9a28fa 100644
--- a/src/main/java/com/ysoft/geecon/dto/AuthorizationSession.java
+++ b/src/main/java/com/ysoft/geecon/dto/AuthorizationSession.java
@@ -36,4 +36,15 @@ public record AuthorizationSession(AuthParams params,
     public String scope() {
         return acceptedScopes == null ? null : String.join(" ", acceptedScopes);
     }
+
+    public boolean validateCodeChallenge(String codeVerifier) {
+        if (params.codeChallengeMethod == null) {
+            return true;
+        }
+        if (codeVerifier == null) {
+            return false;
+        }
+        return Pkce.validate(params.codeChallengeMethod, params.codeChallenge, codeVerifier);
+    }
+
 }
diff --git a/src/main/java/com/ysoft/geecon/dto/Pkce.java b/src/main/java/com/ysoft/geecon/dto/Pkce.java
new file mode 100644
index 0000000..a1946d1
--- /dev/null
+++ b/src/main/java/com/ysoft/geecon/dto/Pkce.java
@@ -0,0 +1,18 @@
+package com.ysoft.geecon.dto;
+
+import org.apache.commons.codec.binary.Base64;
+import org.apache.commons.codec.digest.DigestUtils;
+
+public class Pkce {
+    static boolean validate(String challengeMethod, String codeChallenge, String codeVerifier) {
+        return switch (challengeMethod) {
+            case "plain" -> codeVerifier.equals(codeChallenge);
+            case "S256" -> codeChallenge.equals(s256(codeVerifier));
+            default -> false;
+        };
+    }
+
+    static String s256(String codeVerifier) {
+        return Base64.encodeBase64URLSafeString(DigestUtils.sha256(codeVerifier));
+    }
+}
diff --git a/src/main/java/com/ysoft/geecon/dto/TokenParams.java b/src/main/java/com/ysoft/geecon/dto/TokenParams.java
index a1a2e04..0b5f8b6 100644
--- a/src/main/java/com/ysoft/geecon/dto/TokenParams.java
+++ b/src/main/java/com/ysoft/geecon/dto/TokenParams.java
@@ -1,10 +1,6 @@
 package com.ysoft.geecon.dto;
 
 import jakarta.ws.rs.FormParam;
-import org.jboss.resteasy.reactive.RestQuery;
-
-import java.util.Arrays;
-import java.util.List;
 
 public class TokenParams {
 
@@ -23,6 +19,9 @@ public class TokenParams {
     @FormParam("code")
     private String code;
 
+    @FormParam("code_verifier")
+    private String codeVerifier;
+
     public String getGrantType() {
         return grantType;
     }
@@ -62,4 +61,12 @@ public class TokenParams {
     public void setCode(String code) {
         this.code = code;
     }
+
+    public String getCodeVerifier() {
+        return codeVerifier;
+    }
+
+    public void setCodeVerifier(String codeVerifier) {
+        this.codeVerifier = codeVerifier;
+    }
 }
diff --git a/src/main/java/com/ysoft/geecon/error/OAuthException.java b/src/main/java/com/ysoft/geecon/error/OAuthException.java
index 7e17808..7e5779d 100644
--- a/src/main/java/com/ysoft/geecon/error/OAuthException.java
+++ b/src/main/java/com/ysoft/geecon/error/OAuthException.java
@@ -1,6 +1,7 @@
 package com.ysoft.geecon.error;
 
 public class OAuthException extends RuntimeException {
+    // https://www.ietf.org/archive/id/draft-ietf-oauth-v2-1-09.html#name-error-response-2
     public OAuthException(String message) {
         super(message);
     }
diff --git a/src/main/resources/application.properties b/src/main/resources/application.properties
index e69de29..d6c772b 100644
--- a/src/main/resources/application.properties
+++ b/src/main/resources/application.properties
@@ -0,0 +1,2 @@
+quarkus.http.cors=true
+quarkus.http.cors.origins=/.*/
\ No newline at end of file
diff --git a/src/test/java/com/ysoft/geecon/dto/PkceTest.java b/src/test/java/com/ysoft/geecon/dto/PkceTest.java
new file mode 100644
index 0000000..f6bed2e
--- /dev/null
+++ b/src/test/java/com/ysoft/geecon/dto/PkceTest.java
@@ -0,0 +1,13 @@
+package com.ysoft.geecon.dto;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+class PkceTest {
+
+    @Test
+    void validate() {
+        assertTrue(Pkce.validate("S256", "W2ap_IuB0HJkMIkuxaXV2l8_Gx5mVmMStG_HQrAnQxA", "7AmAEXcl2Km9LQMwtUhif7GQ97HZy9RT72KZBwmxBRI"));
+    }
+}
\ No newline at end of file