diff --git a/gateway-applications/src/main/resources/applications/knoxauth/app/login.html b/gateway-applications/src/main/resources/applications/knoxauth/app/login.html
index 8c69ec9c20..6a30f61065 100644
--- a/gateway-applications/src/main/resources/applications/knoxauth/app/login.html
+++ b/gateway-applications/src/main/resources/applications/knoxauth/app/login.html
@@ -36,40 +36,86 @@
         <!-- Knox Theme Loader (Inline & Blocking to prevent FOUC) -->
         <script type="text/javascript">
            (function() {
+               // Theme names are attacker-influenced (URL parameter and localStorage), so
+               // every candidate is validated before it is stored or used to build a URL.
+               // Rejecting quotes, angle brackets, dots and path separators means the only
+               // URL this can ever produce is styles/themes/<name>/theme.css - so the set
+               // of themes actually installed on the server is the effective allowlist,
+               // and an unknown name simply 404s and leaves the base styles in place.
+               var THEME_PATTERN = /^[a-zA-Z0-9_-]{1,64}$/;
+
+               function sanitize(candidate) {
+                   return (candidate && THEME_PATTERN.test(candidate)) ? candidate : null;
+               }
+
                // Load theme based on: deployment lock > URL parameter > localStorage > deployment default
-               var urlParams = new URLSearchParams(window.location.search);
                var theme;
+               // True when the theme about to load is also the value held in localStorage,
+               // which is what makes it eligible for cleanup if the stylesheet fails.
+               var themeIsPersisted = false;
 
                // Check if theme is locked (admin-enforced theme)
                if (typeof KNOX_THEME_LOCKED !== 'undefined' && KNOX_THEME_LOCKED === true) {
                    // Theme is locked, use deployment default only
-                   theme = KNOX_DEFAULT_THEME || 'default';
+                   theme = sanitize(KNOX_DEFAULT_THEME) || 'default';
                } else {
                    // Normal mode: URL parameter > localStorage > deployment default
-                   theme = urlParams.get('theme');
+                   var urlParams = new URLSearchParams(window.location.search);
+                   var requested = sanitize(urlParams.get('theme'));
 
-                   // If theme parameter exists, save to localStorage for persistence
-                   if (theme) {
+                   if (requested) {
+                       // Only a validated theme is persisted for future visits
                        try {
-                           localStorage.setItem('knox-auth-theme', theme);
+                           localStorage.setItem('knox-auth-theme', requested);
+                           themeIsPersisted = true;
                        } catch (e) {
                            // LocalStorage may be disabled, continue without saving
                        }
+                       theme = requested;
                    } else {
-                       // Try to load from localStorage, fallback to deployment default
+                       // Try to load from localStorage, fallback to deployment default.
+                       // Stored values are re-validated so an entry saved before this
+                       // validation existed cannot take effect, and is discarded.
                        try {
-                           theme = localStorage.getItem('knox-auth-theme') || KNOX_DEFAULT_THEME || 'default';
+                           var saved = localStorage.getItem('knox-auth-theme');
+                           theme = sanitize(saved);
+                           if (saved && !theme) {
+                               localStorage.removeItem('knox-auth-theme');
+                           }
+                           themeIsPersisted = theme !== null;
                        } catch (e) {
                            // LocalStorage may be disabled, use deployment default
-                           theme = KNOX_DEFAULT_THEME || 'default';
+                           theme = null;
                        }
+                       theme = theme || sanitize(KNOX_DEFAULT_THEME) || 'default';
                    }
                }
 
-               // Load theme CSS if specified and not default
-               // Use document.write to load synchronously and prevent FOUC
-               if (theme && theme !== 'default') {
-                   document.write('<link rel="stylesheet" type="text/css" href="styles/themes/' + theme + '/theme.css" id="knox-theme">');
+               // Load theme CSS if specified and not default.
+               // Built with DOM APIs rather than document.write so the theme name can
+               // never be interpreted as markup; appending to head during head parsing
+               // is still render-blocking, which is what prevents FOUC.
+               if (theme !== 'default') {
+                   var link = document.createElement('link');
+                   link.rel = 'stylesheet';
+                   link.type = 'text/css';
+                   link.id = 'knox-theme';
+                   // The server is the authority on which themes exist. If the theme held
+                   // in localStorage fails to load, it is not installed here, so forget it
+                   // straight away rather than replaying it on the next visit. A failure of
+                   // an admin-configured theme is left alone - that is a deployment problem
+                   // to fix, not the user's preference to discard.
+                   if (themeIsPersisted) {
+                       link.onerror = function() {
+                           try {
+                               localStorage.removeItem('knox-auth-theme');
+                           } catch (e) {
+                               // LocalStorage may be disabled, nothing to clean up
+                           }
+                       };
+                   }
+                   link.href = 'styles/themes/' + theme + '/theme.css';
+                   document.head.appendChild(link);
                }
            })();
         </script>
diff --git a/gateway-applications/src/main/resources/applications/knoxauth/app/styles/themes/DEPLOYMENT.md b/gateway-applications/src/main/resources/applications/knoxauth/app/styles/themes/DEPLOYMENT.md
index 220c3177a6..e746fdd331 100644
--- a/gateway-applications/src/main/resources/applications/knoxauth/app/styles/themes/DEPLOYMENT.md
+++ b/gateway-applications/src/main/resources/applications/knoxauth/app/styles/themes/DEPLOYMENT.md
@@ -332,7 +332,18 @@ Organizations with branding requirements or compliance needs should use `KNOX_TH
 
 ### Theme Name Validation
 
-The theme loader only loads files from `styles/themes/THEME_NAME/theme.css`. Path traversal attacks (e.g., `?theme=../../etc/passwd`) are prevented by the URL structure.
+The `?theme=` parameter and the saved localStorage preference are attacker-influenceable,
+so the theme loader validates every candidate against `^[a-zA-Z0-9_-]{1,64}$` before it is
+stored or used. This rejects quotes, angle brackets, dots and path separators, which
+blocks both markup injection and path traversal (e.g. `?theme=../../etc/passwd`).
+Validation is applied to values read back from localStorage as well as to the URL
+parameter, so a value saved by an earlier visit cannot bypass it, and the stylesheet
+element is built with DOM APIs rather than string concatenation.
+
+Because the only URL the loader can produce is `styles/themes/THEME_NAME/theme.css`, the
+themes actually installed on the server are the effective allowlist. A name that does not
+match an installed theme fails to load, the base Knox styles remain in effect, and the
+saved preference is discarded.
 
 ### Content Security Policy
 
@@ -340,6 +351,11 @@ If you have strict CSP, ensure it allows:
 - Loading CSS from same origin
 - Loading fonts from Google Fonts (if using modern theme)
 
+A policy can be applied to the knoxauth route with the WebAppSec provider's
+`SecurityHeaderFilter`, which emits arbitrary response headers from its init
+parameters. Note that `login.html` currently uses inline scripts and inline event
+handlers, so a policy for this page needs `'unsafe-inline'` for `script-src`.
+
 Example CSP:
 ```
 Content-Security-Policy: style-src 'self' https://fonts.googleapis.com;
diff --git a/gateway-applications/src/main/resources/applications/knoxauth/app/styles/themes/README.md b/gateway-applications/src/main/resources/applications/knoxauth/app/styles/themes/README.md
index e15aa77214..4bbfd58725 100644
--- a/gateway-applications/src/main/resources/applications/knoxauth/app/styles/themes/README.md
+++ b/gateway-applications/src/main/resources/applications/knoxauth/app/styles/themes/README.md
@@ -441,9 +441,18 @@ Modern CSS features used:
 
 ## Security Considerations
 
-1. **XSS Protection**: Theme names are not executed as code, only used to construct file paths
-2. **Path Traversal**: Theme loader only loads files from `styles/themes/` directory
-3. **Content Security Policy**: Ensure CSP allows loading external fonts if using Google Fonts
+1. **Theme Name Validation**: Theme names arrive from untrusted sources (the `?theme=`
+   URL parameter and the saved localStorage preference), so each candidate must match
+   `^[a-zA-Z0-9_-]{1,64}$` before it is stored or used. Validation is applied on the
+   localStorage read path as well as the URL, and a value that fails is discarded.
+2. **XSS Protection**: The stylesheet element is created with DOM APIs
+   (`document.createElement`) rather than by concatenating markup, so a theme name can
+   never be parsed as HTML.
+3. **Path Traversal**: The validation pattern rejects dots and path separators, so the
+   only URL the loader can produce is `styles/themes/THEME_NAME/theme.css`. A name that
+   does not correspond to an installed theme simply fails to load and the base styles
+   remain in effect - the themes present on the server are the effective allowlist.
+4. **Content Security Policy**: Ensure CSP allows loading external fonts if using Google Fonts
 
 ## Troubleshooting