diff --git a/src/core/auth/oidc.go b/src/core/auth/oidc.go
index 681ded5..c9c5875 100644
--- a/src/core/auth/oidc.go
+++ b/src/core/auth/oidc.go
@@ -1,6 +1,7 @@
 package auth
 
 import (
+	"crypto/sha256"
 	"encoding/base64"
 	"encoding/json"
 	"fmt"
@@ -38,6 +39,11 @@ func (h *OIDCHandler) LoginRedirect(w http.ResponseWriter, r *http.Request) {
 	state := generateToken(16)
 	nonce := generateToken(16)
 
+	// PKCE: generate code verifier and challenge
+	verifier := generateToken(32)
+	challenge := pkceChallenge(verifier)
+
+	// Store state + verifier in cookies
 	http.SetCookie(w, &http.Cookie{
 		Name:     "oidc_state",
 		Value:    state,
@@ -46,14 +52,23 @@ func (h *OIDCHandler) LoginRedirect(w http.ResponseWriter, r *http.Request) {
 		HttpOnly: true,
 		SameSite: http.SameSiteLaxMode,
 	})
+	http.SetCookie(w, &http.Cookie{
+		Name:     "oidc_verifier",
+		Value:    verifier,
+		Path:     "/",
+		MaxAge:   300,
+		HttpOnly: true,
+		SameSite: http.SameSiteLaxMode,
+	})
 
 	authURL := fmt.Sprintf(
-		"%s/api/oidc/authorize?response_type=code&client_id=%s&redirect_uri=%s&scope=openid+profile+email&state=%s&nonce=%s",
+		"%s/api/oidc/authorize?response_type=code&client_id=%s&redirect_uri=%s&scope=openid+profile+email&state=%s&nonce=%s&code_challenge=%s&code_challenge_method=S256",
 		h.config.IssuerURL,
 		url.QueryEscape(h.config.ClientID),
 		url.QueryEscape(h.config.RedirectURL),
 		state,
 		nonce,
+		challenge,
 	)
 
 	http.Redirect(w, r, authURL, http.StatusFound)
@@ -78,14 +93,16 @@ func (h *OIDCHandler) Callback(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// Clear the state cookie
-	http.SetCookie(w, &http.Cookie{
-		Name:     "oidc_state",
-		Value:    "",
-		Path:     "/",
-		MaxAge:   -1,
-		HttpOnly: true,
-	})
+	// Get PKCE verifier from cookie
+	verifierCookie, _ := r.Cookie("oidc_verifier")
+	verifier := ""
+	if verifierCookie != nil {
+		verifier = verifierCookie.Value
+	}
+
+	// Clear state cookies
+	http.SetCookie(w, &http.Cookie{Name: "oidc_state", Value: "", Path: "/", MaxAge: -1, HttpOnly: true})
+	http.SetCookie(w, &http.Cookie{Name: "oidc_verifier", Value: "", Path: "/", MaxAge: -1, HttpOnly: true})
 
 	// Get code from URL query (GET) or form body (POST)
 	code := r.URL.Query().Get("code")
@@ -97,8 +114,8 @@ func (h *OIDCHandler) Callback(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// Exchange code for tokens
-	username, err := h.exchangeCode(code)
+	// Exchange code for tokens (with PKCE verifier)
+	username, err := h.exchangeCode(code, verifier)
 	if err != nil {
 		http.Error(w, "token exchange failed: "+err.Error(), http.StatusInternalServerError)
 		return
@@ -123,9 +140,8 @@ func (h *OIDCHandler) Callback(w http.ResponseWriter, r *http.Request) {
 	http.Redirect(w, r, "/", http.StatusFound)
 }
 
-// exchangeCode exchanges an OIDC authorization code for an ID token
-// and extracts the username (sub claim) from it.
-func (h *OIDCHandler) exchangeCode(code string) (string, error) {
+// exchangeCode exchanges an OIDC authorization code for an ID token.
+func (h *OIDCHandler) exchangeCode(code, verifier string) (string, error) {
 	tokenURL := h.config.IssuerURL + "/api/oidc/token"
 
 	data := url.Values{
@@ -133,7 +149,7 @@ func (h *OIDCHandler) exchangeCode(code string) (string, error) {
 		"code":          {code},
 		"redirect_uri":  {h.config.RedirectURL},
 		"client_id":     {h.config.ClientID},
-		"client_secret": {h.config.ClientSecret},
+		"code_verifier": {verifier},
 	}
 
 	resp, err := http.PostForm(tokenURL, data)
@@ -158,7 +174,6 @@ func (h *OIDCHandler) exchangeCode(code string) (string, error) {
 		return "", fmt.Errorf("no id_token in response")
 	}
 
-	// Decode JWT payload (without signature verification for now)
 	username, err := decodeJWTSub(tokenResp.IDToken)
 	if err != nil {
 		return "", fmt.Errorf("decode id_token: %w", err)
@@ -193,6 +208,12 @@ func decodeJWTSub(token string) (string, error) {
 	return claims.Sub, nil
 }
 
+// pkceChallenge creates a PKCE S256 challenge from a verifier.
+func pkceChallenge(verifier string) string {
+	h := sha256.Sum256([]byte(verifier))
+	return base64.RawURLEncoding.EncodeToString(h[:])
+}
+
 // AuthGateMiddleware protects routes behind OIDC authentication.
 func (h *OIDCHandler) AuthGateMiddleware(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {