diff --git a/.gitea/workflows/lint.yml b/.gitea/workflows/lint.yml
index 73f40bf..692a426 100644
--- a/.gitea/workflows/lint.yml
+++ b/.gitea/workflows/lint.yml
@@ -35,10 +35,27 @@ jobs:
       - name: Run ESLint
         run: npx eslint assets/js/
 
+  requirements:
+    name: PHP requirements (version + extensions)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Install PHP with required extensions
+        run: |
+          apt-get update -qq
+          # Install the extensions declared in config/requirements.php so the
+          # check verifies they are actually installable + loadable, and so this
+          # build fails if a required extension can't be provided.
+          apt-get install -y -qq php-cli php-ldap php-mysql php-curl php-mbstring
+
+      - name: Verify runtime requirements
+        run: php scripts/check_requirements.php
+
   deploy:
     name: Deploy
     runs-on: ubuntu-latest
-    needs: [php-lint, js-lint]
+    needs: [php-lint, js-lint, requirements]
     if: github.event_name == 'push' && (github.ref == 'refs/heads/main' || github.ref == 'refs/heads/development')
     permissions:
       contents: write
@@ -77,7 +94,7 @@ jobs:
   notify-failure:
     name: Notify on failure
     runs-on: ubuntu-latest
-    needs: [php-lint, js-lint]
+    needs: [php-lint, js-lint, requirements]
     if: failure() && github.event_name == 'push'
     steps:
       - name: Send Matrix alert
diff --git a/api/health.php b/api/health.php
index 60a8677..6f712f5 100644
--- a/api/health.php
+++ b/api/health.php
@@ -95,6 +95,40 @@ if (is_dir($rateLimitDir) && is_writable($rateLimitDir)) {
     ];
 }
 
+// Check 5: Required PHP extensions (catches e.g. a PHP upgrade silently
+// dropping php-ldap, which breaks avatars with no other visible error).
+$requirements = require dirname(__DIR__) . '/config/requirements.php';
+$missingExt = array_values(array_filter(
+    $requirements['required_extensions'],
+    fn($ext) => !extension_loaded($ext)
+));
+if (empty($missingExt)) {
+    $checks['php_extensions'] = [
+        'status' => 'ok',
+        'message' => 'All required extensions loaded'
+    ];
+} else {
+    $checks['php_extensions'] = [
+        'status' => 'error',
+        'message' => 'Missing extensions: ' . implode(', ', $missingExt)
+    ];
+    $healthy = false;
+}
+
+// Check 6: PHP version meets the declared minimum
+if (version_compare(PHP_VERSION, $requirements['min_php_version'], '>=')) {
+    $checks['php_version'] = [
+        'status' => 'ok',
+        'message' => PHP_VERSION
+    ];
+} else {
+    $checks['php_version'] = [
+        'status' => 'error',
+        'message' => sprintf('PHP %s < required %s', PHP_VERSION, $requirements['min_php_version'])
+    ];
+    $healthy = false;
+}
+
 // Calculate response time
 $responseTime = round((microtime(true) - $startTime) * 1000, 2);
 
diff --git a/config/config.php b/config/config.php
index e084fed..e25b5a9 100644
--- a/config/config.php
+++ b/config/config.php
@@ -60,6 +60,16 @@ $GLOBALS['config'] = [
     'DB_PASS' => $envVars['DB_PASS'] ?? '',
     'DB_NAME' => $envVars['DB_NAME'] ?? 'tinkertickets',
 
+    // Trusted reverse proxies. Authelia forward-auth (Remote-* headers) is only
+    // honored when REMOTE_ADDR is in this allowlist, so the spoofable identity
+    // headers can't be set by anything that reaches PHP directly. Comma-separated
+    // IPs in .env (e.g. TRUSTED_PROXIES=10.10.10.27). Empty = enforcement OFF
+    // (backward compatible — relies solely on network topology).
+    'TRUSTED_PROXIES' => array_values(array_filter(array_map(
+        'trim',
+        explode(',', (string)($envVars['TRUSTED_PROXIES'] ?? ''))
+    ), fn($ip) => $ip !== '')),
+
     // URL settings
     'BASE_URL' => '', // Empty since we're serving from document root
     'ASSETS_URL' => '/assets', // Assets URL
diff --git a/config/requirements.php b/config/requirements.php
new file mode 100644
index 0000000..d44c6ba
--- /dev/null
+++ b/config/requirements.php
@@ -0,0 +1,28 @@
+<?php
+
+/**
+ * Runtime requirements — single source of truth.
+ *
+ * Consumed by:
+ *   - scripts/check_requirements.php  (CI: fails the build if unmet)
+ *   - api/health.php                  (production: surfaces drift to monitoring)
+ *
+ * This exists because a PHP upgrade once silently dropped the ldap extension,
+ * which broke avatars with no visible error. Keep this list in sync with the
+ * extensions the code actually relies on.
+ */
+
+return [
+    // Minimum supported PHP version (production runs 8.4).
+    'min_php_version' => '8.2',
+
+    // Extensions the application requires to function.
+    'required_extensions' => [
+        'ldap',     // api/user_avatar.php — lldap avatar lookups
+        'mysqli',   // helpers/Database.php — all data access
+        'curl',     // helpers/NotificationHelper.php, SynapseHelper.php — Matrix
+        'mbstring', // multibyte string handling
+        'fileinfo', // api/upload_attachment.php — MIME validation
+        'json',     // request/response encoding (bundled, but assert anyway)
+    ],
+];
diff --git a/middleware/AuthMiddleware.php b/middleware/AuthMiddleware.php
index 79e1dbb..3201e29 100644
--- a/middleware/AuthMiddleware.php
+++ b/middleware/AuthMiddleware.php
@@ -96,6 +96,12 @@ class AuthMiddleware
             }
         }
 
+        // Only honor Authelia forward-auth headers from a trusted reverse proxy.
+        // Without this, anything that can reach PHP directly could spoof
+        // Remote-User / Remote-Groups and log in (as admin). No valid session
+        // exists at this point, so we are about to trust request headers.
+        $this->enforceTrustedProxy();
+
         // Read Authelia forward auth headers
         $username = $this->getHeader('HTTP_REMOTE_USER');
         $displayName = $this->getHeader('HTTP_REMOTE_NAME');
@@ -136,6 +142,33 @@ class AuthMiddleware
         return $user;
     }
 
+    /**
+     * Reject forward-auth headers that did not arrive via a trusted proxy.
+     *
+     * If TRUSTED_PROXIES is configured and the connecting REMOTE_ADDR is not in
+     * the allowlist, the Remote-* headers cannot be trusted, so we refuse rather
+     * than honor a potentially spoofed identity. Empty allowlist = disabled.
+     */
+    private function enforceTrustedProxy(): void
+    {
+        $trusted = $GLOBALS['config']['TRUSTED_PROXIES'] ?? [];
+        if (empty($trusted)) {
+            return; // Enforcement disabled (no allowlist configured)
+        }
+
+        $remoteAddr = $_SERVER['REMOTE_ADDR'] ?? '';
+        if (!in_array($remoteAddr, $trusted, true)) {
+            $this->logSecurityEvent('untrusted_proxy', [
+                'reason' => 'Remote-* auth headers from non-allowlisted source',
+                'remote_addr' => $remoteAddr ?: 'unknown'
+            ]);
+            header('HTTP/1.1 403 Forbidden');
+            header('Content-Type: text/plain; charset=utf-8');
+            echo 'Forbidden: authentication headers must arrive via a trusted proxy.';
+            exit;
+        }
+    }
+
     /**
      * Get header value from server variables
      *
diff --git a/scripts/check_requirements.php b/scripts/check_requirements.php
new file mode 100644
index 0000000..3e60ffb
--- /dev/null
+++ b/scripts/check_requirements.php
@@ -0,0 +1,44 @@
+#!/usr/bin/env php
+<?php
+
+/**
+ * Verify the running PHP environment meets the declared runtime requirements.
+ *
+ * Reads config/requirements.php and checks the PHP version and that every
+ * required extension is loaded. Exits non-zero (failing CI) on any miss.
+ *
+ * Usage: php scripts/check_requirements.php
+ */
+
+$req = require __DIR__ . '/../config/requirements.php';
+
+$errors = [];
+
+// PHP version
+$minPhp = $req['min_php_version'];
+if (version_compare(PHP_VERSION, $minPhp, '<')) {
+    $errors[] = sprintf('PHP %s is below the required minimum %s', PHP_VERSION, $minPhp);
+}
+
+// Required extensions
+foreach ($req['required_extensions'] as $ext) {
+    if (!extension_loaded($ext)) {
+        $errors[] = sprintf('Missing required PHP extension: %s', $ext);
+    }
+}
+
+if (!empty($errors)) {
+    fwrite(STDERR, "Requirement check FAILED:\n");
+    foreach ($errors as $err) {
+        fwrite(STDERR, '  - ' . $err . "\n");
+    }
+    exit(1);
+}
+
+printf(
+    "Requirement check passed: PHP %s (>= %s); extensions: %s\n",
+    PHP_VERSION,
+    $minPhp,
+    implode(', ', $req['required_extensions'])
+);
+exit(0);