diff --git a/lib/realtime.js b/lib/realtime.js
index 35b86266..3903eb6f 100644
--- a/lib/realtime.js
+++ b/lib/realtime.js
@@ -29,6 +29,8 @@ const realtime = {
   maintenance: true
 }
 
+const allowedPermissionValues = ['freely', 'editable', 'limited', 'locked', 'protected', 'private']
+
 function onAuthorizeSuccess (data, accept) {
   accept()
 }
@@ -363,13 +365,25 @@ function connectNextSocket () {
 }
 
 function checkViewPermission (req, note) {
-  if (note.permission === 'private') {
-    if (req.user && req.user.logged_in && req.user.id === note.owner) { return true } else { return false }
-  } else if (note.permission === 'limited' || note.permission === 'protected') {
-    if (req.user && req.user.logged_in) { return true } else { return false }
-  } else {
+  const permission = note.permission
+
+  // Private notes should only be readable to their owner
+  if (permission === 'private') {
+    return req.user && req.user.logged_in && req.user.id === note.owner
+  }
+
+  // Limited and protected notes should be readable by all logged-in users
+  if (permission === 'limited' || permission === 'protected') {
+    return req.user && req.user.logged_in
+  }
+
+  // Freely, Editable and Locked notes should be readable by everyone
+  if (permission === 'freely' || permission === 'editable' || permission === 'locked') {
     return true
   }
+
+  // Invalid permission
+  return false
 }
 
 let isConnectionBusy = false
@@ -636,6 +650,10 @@ function ifMayEdit (socket, callback) {
       // only owner can change
       if (!note.owner || note.owner !== socket.request.user.id) { mayEdit = false }
       break
+    default:
+      // invalid permission
+      mayEdit = false
+      break
   }
   // if user may edit and this is a text operation
   if (socket.origin === 'operation' && mayEdit) {
@@ -778,7 +796,14 @@ function connection (socket) {
       const note = notes[noteId]
       // Only owner can change permission
       if (note.owner && note.owner === socket.request.user.id) {
-        if (permission === 'freely' && !config.allowAnonymous && !config.allowAnonymousEdits) return
+        // If config disallows freely permission, do not set it
+        if (permission === 'freely' && !config.allowAnonymous && !config.allowAnonymousEdits) {
+          return
+        }
+        // Ignore invalid permission values
+        if (!allowedPermissionValues.includes(permission)) {
+          return
+        }
         note.permission = permission
         models.Note.update({
           permission
diff --git a/public/docs/release-notes.md b/public/docs/release-notes.md
index a06bc417..5679c7f5 100644
--- a/public/docs/release-notes.md
+++ b/public/docs/release-notes.md
@@ -2,6 +2,10 @@
 
 ## <i class="fa fa-tag"></i> 1.11.1 <i class="fa fa-calendar-o"></i> 2026-07-24
 
+### Security fixes
+
+- [GHSA-93w7-49m2-cqwg](https://github.com/hedgedoc/hedgedoc/security/advisories/GHSA-93w7-49m2-cqwg) reports possible corruption of permission values due to missing validation. This should not impact permission checks, except when the note owner intentionally set their note permission to something invalid.
+
 ### Enhancements
 
 - Added external link warning setting (`externalLinkWarning` in config.json or `CMD_EXTERNAL_LINK_WARNING`) to disable the external link warning page entirely