diff --git a/src/plugin_system/plugin_manager.py b/src/plugin_system/plugin_manager.py
index 7ce72b7e..2af661a5 100644
--- a/src/plugin_system/plugin_manager.py
+++ b/src/plugin_system/plugin_manager.py
@@ -510,8 +510,10 @@ class PluginManager:
             if schema:
                 try:
                     prepared = normalize_legacy_booleans(prepared, schema, upgraded)
-                except Exception:
-                    pass
+                except Exception as legacy_error:
+                    self.logger.warning(
+                        "Could not read legacy boolean settings for %s: %s",
+                        plugin_id, legacy_error)
         if upgraded:
             self.logger.info(
                 "Plugin %s: reading legacy boolean setting %s as "
diff --git a/test/js/unit/test_update_all.js b/test/js/unit/test_update_all.js
index f9c2cbbd..8fb9bf64 100644
--- a/test/js/unit/test_update_all.js
+++ b/test/js/unit/test_update_all.js
@@ -183,6 +183,22 @@ const noSleep = { sleep: async () => {} };
 
     r = await run(() => httpAnswer(200, async () => { throw new SyntaxError('Unexpected end of JSON input'); }));
     ok('an unreadable 200 is not retried either', r.requests === 1 && r.result.error.error_code === 'API_ERROR', r);
+
+    // Every endpoint is one of the client's own API paths; one that could
+    // leave baseURL never reaches fetch(), and plugin ids are encoded.
+    const urls = [];
+    global.fetch = async (url) => { urls.push(url); return httpAnswer(200, async () => ({ status: 'success' })); };
+    const refusal = async (endpoint) => {
+      try { await PluginAPI.request(endpoint, 'POST'); return null; } catch (e) { return e.error_code; }
+    };
+    const bad = ['//evil.example/x', '/plugins/../../x', '/a\b', '/a b', 'plugins', null];
+    const codes = [];
+    for (const endpoint of bad) codes.push(await refusal(endpoint));
+    ok('an endpoint that could leave the API path is refused before fetch()',
+       codes.every(c => c === 'INVALID_ENDPOINT') && urls.length === 0, { codes, urls });
+    await PluginAPI.resetPluginConfig('a/../b&x=1');
+    ok('a plugin id is encoded into the URL, not spliced into it',
+       urls[0] === '/api/v3/plugins/config/reset?plugin_id=a%2F..%2Fb%26x%3D1', urls);
     delete global.fetch;
   }
 
diff --git a/web_interface/static/v3/js/app-shell.js b/web_interface/static/v3/js/app-shell.js
index 4fbc22f7..ab865447 100644
--- a/web_interface/static/v3/js/app-shell.js
+++ b/web_interface/static/v3/js/app-shell.js
@@ -838,8 +838,8 @@
                             // PluginAPI.batch returns already-parsed JSON objects
                             try {
                                 const results = await window.PluginAPI.batch([
-                                    {endpoint: `/plugins/config?plugin_id=${pluginId}`, method: 'GET'},
-                                    {endpoint: `/plugins/schema?plugin_id=${pluginId}`, method: 'GET'},
+                                    {endpoint: `/plugins/config?plugin_id=${encodeURIComponent(pluginId)}`, method: 'GET'},
+                                    {endpoint: `/plugins/schema?plugin_id=${encodeURIComponent(pluginId)}`, method: 'GET'},
                                     {endpoint: '/plugins/installed', method: 'GET'}
                                 ]);
                                 [configData, schemaData, pluginsData] = results;
diff --git a/web_interface/static/v3/js/plugins/api_client.js b/web_interface/static/v3/js/plugins/api_client.js
index 0e54f03b..11908e4e 100644
--- a/web_interface/static/v3/js/plugins/api_client.js
+++ b/web_interface/static/v3/js/plugins/api_client.js
@@ -123,6 +123,28 @@ const PluginAPI = {
      */
     baseURL: '/api/v3',
     
+    /**
+     * The endpoint, if it is a path under baseURL; throws INVALID_ENDPOINT
+     * otherwise. Every endpoint is one of this client's own API paths, so
+     * anything that could leave that path -- "//host", a backslash, a ".."
+     * segment, whitespace or control characters -- is a bug, not a request.
+     *
+     * @param {string} endpoint - API endpoint, starting with "/"
+     * @returns {string} The same endpoint
+     */
+    checkEndpoint(endpoint) {
+        const path = typeof endpoint === 'string' ? endpoint.split(/[?#]/)[0] : '';
+        if (!path.startsWith('/') || path.startsWith('//') ||
+                /[\\\s\x00-\x1f\x7f]/.test(endpoint) ||
+                path.split('/').some(segment => segment === '..' || segment === '.')) {
+            throw {
+                error_code: 'INVALID_ENDPOINT',
+                message: `Not an API endpoint: ${String(endpoint)}`
+            };
+        }
+        return endpoint;
+    },
+
     /**
      * Make an API request with throttling and caching.
      * 
@@ -141,7 +163,7 @@ const PluginAPI = {
         const requestKey = `${method}:${endpoint}:${data ? JSON.stringify(data) : ''}`;
         
         const makeRequest = async () => {
-            const url = `${this.baseURL}${endpoint}`;
+            const url = `${this.baseURL}${this.checkEndpoint(endpoint)}`;
             const options = {
                 method,
                 headers: {
@@ -161,7 +183,9 @@ const PluginAPI = {
             // and is not retried.
             let response;
             try {
-                response = await fetch(url, options);
+                // url is baseURL plus an endpoint checkEndpoint() accepted: a
+                // path on this origin's API, never a caller-chosen host.
+                response = await fetch(url, options); // nosemgrep
             } catch (error) {
                 throw {
                     error_code: 'NETWORK_ERROR',
@@ -266,7 +290,7 @@ const PluginAPI = {
      * @returns {Promise<Object>} Plugin configuration
      */
     async getPluginConfig(pluginId) {
-        const response = await this.request(`/plugins/config?plugin_id=${pluginId}`);
+        const response = await this.request(`/plugins/config?plugin_id=${encodeURIComponent(pluginId)}`);
         return response.data || {};
     },
     
@@ -291,7 +315,7 @@ const PluginAPI = {
      * @returns {Promise<Object>} Response data
      */
     async resetPluginConfig(pluginId) {
-        return await this.request(`/plugins/config/reset?plugin_id=${pluginId}`, 'POST');
+        return await this.request(`/plugins/config/reset?plugin_id=${encodeURIComponent(pluginId)}`, 'POST');
     },
     
     /**
@@ -301,7 +325,7 @@ const PluginAPI = {
      * @returns {Promise<Object>} Plugin schema
      */
     async getPluginSchema(pluginId) {
-        const response = await this.request(`/plugins/schema?plugin_id=${pluginId}`);
+        const response = await this.request(`/plugins/schema?plugin_id=${encodeURIComponent(pluginId)}`);
         return response.data?.schema || null;
     },
     
@@ -364,7 +388,7 @@ const PluginAPI = {
      */
     async getPluginHealth(pluginId = null) {
         const endpoint = pluginId
-            ? `/plugins/health/${pluginId}`
+            ? `/plugins/health/${encodeURIComponent(pluginId)}`
             : '/plugins/health';
         const response = await this.request(endpoint);
         return response.data || {};
@@ -378,7 +402,7 @@ const PluginAPI = {
      */
     async getPluginMetrics(pluginId = null) {
         const endpoint = pluginId
-            ? `/plugins/metrics/${pluginId}`
+            ? `/plugins/metrics/${encodeURIComponent(pluginId)}`
             : '/plugins/metrics';
         const response = await this.request(endpoint);
         return response.data || {};