diff --git a/service/internal/auth/otoauth2/restapi_auth_oauth2.go b/service/internal/auth/otoauth2/restapi_auth_oauth2.go
index 5266124..a84e1f6 100644
--- a/service/internal/auth/otoauth2/restapi_auth_oauth2.go
+++ b/service/internal/auth/otoauth2/restapi_auth_oauth2.go
@@ -11,6 +11,7 @@ import (
 	"io"
 	"net/http"
 	"os"
+	"sync"
 	"time"
 
 	authTypes "github.com/OliveTin/OliveTin/internal/auth/authpublic"
@@ -21,6 +22,7 @@ import (
 
 type OAuth2Handler struct {
 	cfg                 *config.Config
+	mu                  sync.RWMutex
 	registeredStates    map[string]*oauth2State
 	registeredProviders map[string]*oauth2.Config
 }
@@ -144,11 +146,13 @@ func (h *OAuth2Handler) HandleOAuthLogin(w http.ResponseWriter, r *http.Request)
 		return
 	}
 
+	h.mu.Lock()
 	h.registeredStates[state] = &oauth2State{
 		providerConfig: provider,
 		providerName:   providerName,
 		Username:       "",
 	}
+	h.mu.Unlock()
 
 	h.setOAuthCallbackCookie(w, r, "olivetin-sid-oauth", state)
 
@@ -177,7 +181,9 @@ func (h *OAuth2Handler) checkOAuthCallbackCookie(w http.ResponseWriter, r *http.
 		return nil, state, false
 	}
 
+	h.mu.RLock()
 	registeredState, ok := h.registeredStates[state]
+	h.mu.RUnlock()
 	if !ok {
 		log.Errorf("State not found in server: %v", state)
 		http.Error(w, "State not found in server", http.StatusBadRequest)
@@ -287,8 +293,10 @@ func (h *OAuth2Handler) HandleOAuthCallback(w http.ResponseWriter, r *http.Reque
 	userInfoClient := h.createUserInfoClient(ctx, registeredState.providerConfig, tok, clientSettings)
 	userinfo := getUserInfo(h.cfg, userInfoClient, providerConfig)
 
+	h.mu.Lock()
 	h.registeredStates[state].Username = userinfo.Username
 	h.registeredStates[state].Usergroup = h.computeUsergroup(userinfo, providerConfig)
+	h.mu.Unlock()
 
 	http.Redirect(w, r, "/", http.StatusFound)
 }
@@ -366,34 +374,36 @@ func getDataField(data map[string]any, field string) string {
 	return stringVal
 }
 
+func (h *OAuth2Handler) lookupOAuth2UserByState(state string) (*authTypes.AuthenticatedUser, bool) {
+	h.mu.RLock()
+	serverState, found := h.registeredStates[state]
+	if !found {
+		h.mu.RUnlock()
+		return nil, false
+	}
+	user := &authTypes.AuthenticatedUser{
+		Username:      serverState.Username,
+		UsergroupLine: serverState.Usergroup,
+		Provider:      "oauth2",
+		SID:           state,
+	}
+	h.mu.RUnlock()
+	return user, true
+}
+
 func (h *OAuth2Handler) CheckUserFromOAuth2Cookie(context *authTypes.AuthCheckingContext) *authTypes.AuthenticatedUser {
 	cookie, err := context.Request.Cookie("olivetin-sid-oauth")
-
-	user := &authTypes.AuthenticatedUser{}
-
-	if err != nil {
+	if err != nil || cookie.Value == "" {
 		return nil
 	}
 
-	if cookie.Value == "" {
-		return nil
-	}
-
-	serverState, found := h.registeredStates[cookie.Value]
-
+	user, found := h.lookupOAuth2UserByState(cookie.Value)
 	if !found {
 		log.WithFields(log.Fields{
 			"sid":      cookie.Value,
 			"provider": "oauth2",
 		}).Warnf("Stale session")
-
 		return nil
 	}
-
-	user.Username = serverState.Username
-	user.UsergroupLine = serverState.Usergroup
-	user.Provider = "oauth2"
-	user.SID = cookie.Value
-
 	return user
 }