diff --git a/backend/__tests__/utils/networkValidation.dns.test.js b/backend/__tests__/utils/networkValidation.dns.test.js
new file mode 100644
index 00000000..a2123203
--- /dev/null
+++ b/backend/__tests__/utils/networkValidation.dns.test.js
@@ -0,0 +1,128 @@
+/**
+ * DNS-resolving SSRF guard (GHSA SSRF cluster: webhook / S3 / rsync / SMTP /
+ * IMAP). The literal isPrivateIP check can't see that a public-looking
+ * hostname resolves to an internal/metadata IP; isHostAllowed resolves the
+ * name and vets every A/AAAA record.
+ */
+jest.mock('dns', () => {
+  const actual = jest.requireActual('dns');
+  return { ...actual, promises: { ...actual.promises, lookup: jest.fn() } };
+});
+const dns = require('dns');
+const {
+  isHostAllowed,
+  validateExternalUrlAsync,
+  classifyHost,
+} = require('../../src/utils/networkValidation');
+
+const lookup = dns.promises.lookup;
+
+describe('classifyHost', () => {
+  beforeEach(() => lookup.mockReset());
+
+  it('distinguishes private, unresolved, ok, and invalid', async () => {
+    lookup.mockResolvedValue([{ address: '10.0.0.5', family: 4 }]);
+    expect(await classifyHost('evil.example')).toBe('private');
+
+    lookup.mockResolvedValue([{ address: '93.184.216.34', family: 4 }]);
+    expect(await classifyHost('example.com')).toBe('ok');
+
+    lookup.mockRejectedValue(new Error('EAI_AGAIN'));
+    expect(await classifyHost('blip.example')).toBe('unresolved');
+
+    lookup.mockResolvedValue([]);
+    expect(await classifyHost('empty.example')).toBe('unresolved');
+
+    expect(await classifyHost('')).toBe('invalid');
+    expect(await classifyHost('10.0.0.1')).toBe('private'); // literal, no lookup
+  });
+});
+
+describe('isHostAllowed', () => {
+  beforeEach(() => lookup.mockReset());
+
+  it('rejects a public hostname that resolves to a private IP', async () => {
+    lookup.mockResolvedValue([{ address: '10.0.0.5', family: 4 }]);
+    expect(await isHostAllowed('evil.example.com')).toBe(false);
+  });
+
+  it('rejects when the hostname resolves to the cloud metadata IP', async () => {
+    lookup.mockResolvedValue([{ address: '169.254.169.254', family: 4 }]);
+    expect(await isHostAllowed('metadata-rebind.example')).toBe(false);
+  });
+
+  it('rejects when ANY resolved address is private (rebinding / mixed records)', async () => {
+    lookup.mockResolvedValue([
+      { address: '93.184.216.34', family: 4 },
+      { address: '169.254.169.254', family: 4 },
+    ]);
+    expect(await isHostAllowed('rebind.example')).toBe(false);
+  });
+
+  it('allows a hostname that resolves only to public IPs', async () => {
+    lookup.mockResolvedValue([{ address: '93.184.216.34', family: 4 }]);
+    expect(await isHostAllowed('example.com')).toBe(true);
+  });
+
+  it('fails closed when resolution errors', async () => {
+    lookup.mockRejectedValue(new Error('ENOTFOUND'));
+    expect(await isHostAllowed('nxdomain.invalid')).toBe(false);
+  });
+
+  it('fails closed on an empty resolution', async () => {
+    lookup.mockResolvedValue([]);
+    expect(await isHostAllowed('empty.example')).toBe(false);
+  });
+
+  it('rejects literal private IPs and blocked names without resolving', async () => {
+    expect(await isHostAllowed('127.0.0.1')).toBe(false);
+    expect(await isHostAllowed('10.0.0.1')).toBe(false);
+    expect(await isHostAllowed('localhost')).toBe(false);
+    expect(await isHostAllowed('metadata.google.internal')).toBe(false);
+    expect(await isHostAllowed('foo.internal')).toBe(false);
+    expect(lookup).not.toHaveBeenCalled();
+  });
+
+  it('allows a public IP literal without resolving', async () => {
+    expect(await isHostAllowed('93.184.216.34')).toBe(true);
+    expect(lookup).not.toHaveBeenCalled();
+  });
+
+  it('rejects empty / non-string input', async () => {
+    expect(await isHostAllowed('')).toBe(false);
+    expect(await isHostAllowed(null)).toBe(false);
+  });
+});
+
+describe('validateExternalUrlAsync', () => {
+  beforeEach(() => lookup.mockReset());
+
+  it('rejects a URL whose host resolves to a private address', async () => {
+    lookup.mockResolvedValue([{ address: '10.1.2.3', family: 4 }]);
+    const r = await validateExternalUrlAsync('https://evil.example/hook');
+    expect(r.valid).toBe(false);
+  });
+
+  it('accepts a URL whose host resolves public', async () => {
+    lookup.mockResolvedValue([{ address: '93.184.216.34', family: 4 }]);
+    expect((await validateExternalUrlAsync('https://example.com/hook')).valid).toBe(true);
+  });
+
+  it('rejects a malformed URL', async () => {
+    expect((await validateExternalUrlAsync('not a url')).valid).toBe(false);
+  });
+
+  it('reports reason=unresolved for a transient lookup failure (retryable)', async () => {
+    lookup.mockRejectedValue(new Error('EAI_AGAIN'));
+    const r = await validateExternalUrlAsync('https://blip.example/hook');
+    expect(r.valid).toBe(false);
+    expect(r.reason).toBe('unresolved');
+  });
+
+  it('reports reason=private for a resolved-private host (permanent)', async () => {
+    lookup.mockResolvedValue([{ address: '169.254.169.254', family: 4 }]);
+    const r = await validateExternalUrlAsync('https://rebind.example/hook');
+    expect(r.valid).toBe(false);
+    expect(r.reason).toBe('private');
+  });
+});
diff --git a/backend/src/routes/adminBackup.js b/backend/src/routes/adminBackup.js
index c60b2d31..1dedbe4e 100644
--- a/backend/src/routes/adminBackup.js
+++ b/backend/src/routes/adminBackup.js
@@ -63,14 +63,33 @@ router.put('/config', adminAuth, requirePermission('backup.create'), async (req,
         }
         break;
       case 's3':
-        if (!updates.backup_s3_endpoint || !updates.backup_s3_bucket || 
+        if (!updates.backup_s3_endpoint || !updates.backup_s3_bucket ||
               !updates.backup_s3_access_key || !updates.backup_s3_secret_key) {
           return res.status(400).json({ error: 'S3 backup requires endpoint, bucket, and credentials' });
         }
         break;
       }
     }
-    
+
+    // SSRF: validate an S3 endpoint whenever one is supplied — NOT only when
+    // the payload also flips backup_destination_type to 's3'. The PUT
+    // persists every backup_* field independently, so with S3 already
+    // selected a caller could PATCH just backup_s3_endpoint to a
+    // private-resolving host; the management ops (manifest, bucket/file
+    // browse, cleanup, test-upload) then connect without going through
+    // testConnection. Prod-only; dev points at localhost MinIO deliberately.
+    if (process.env.NODE_ENV === 'production'
+        && updates.backup_s3_endpoint && updates.backup_s3_endpoint !== '••••••••') {
+      const rawEndpoint = updates.backup_s3_endpoint;
+      const withProto = /^https?:\/\//.test(rawEndpoint) ? rawEndpoint : `https://${rawEndpoint}`;
+      let epHost = null;
+      try { epHost = new URL(withProto).hostname; } catch { epHost = null; }
+      const { isHostAllowed } = require('../utils/networkValidation');
+      if (!epHost || !(await isHostAllowed(epHost))) {
+        return res.status(400).json({ error: 'S3 endpoint resolves to a private or internal network address' });
+      }
+    }
+
     // Update settings
     for (const [key, value] of Object.entries(updates)) {
       // An unchanged secret round-trips as the GET mask sentinel — keep the
@@ -364,9 +383,11 @@ router.post('/test-connection', adminAuth, requirePermission('backup.create'), a
         break;
       }
 
-      // SSRF protection: block connections to private/internal addresses
-      const { isPrivateIP } = require('../utils/networkValidation');
-      if (isPrivateIP(host)) {
+      // SSRF protection: resolve the host and block any private/internal
+      // address. ssh does its own DNS at connect time, so a literal-only
+      // check let a hostname resolving to an internal IP through (#GHSA-4jh8).
+      const { isHostAllowed } = require('../utils/networkValidation');
+      if (!(await isHostAllowed(host))) {
         res.json({ success: false, message: 'Host cannot be a private or internal network address' });
         break;
       }
diff --git a/backend/src/routes/adminEmail.js b/backend/src/routes/adminEmail.js
index 169c6061..f23f1a69 100644
--- a/backend/src/routes/adminEmail.js
+++ b/backend/src/routes/adminEmail.js
@@ -66,9 +66,11 @@ router.post('/config', [
       tls_reject_unauthorized
     } = req.body;
 
-    // Validate SMTP host is not a private/internal address (SSRF protection)
-    const { isPrivateIP } = require('../utils/networkValidation');
-    if (isPrivateIP(smtp_host)) {
+    // Validate SMTP host is not a private/internal address (SSRF protection).
+    // Resolves DNS so a public-looking hostname pointing at an internal IP
+    // is caught, not just literal private addresses (#GHSA-ch64).
+    const { isHostAllowed } = require('../utils/networkValidation');
+    if (!(await isHostAllowed(smtp_host))) {
       return res.status(400).json({ error: 'SMTP host cannot point to a private or internal network address' });
     }
 
@@ -152,8 +154,8 @@ router.post('/incoming-config', [
     const errors = validationResult(req);
     if (!errors.isEmpty()) return res.status(400).json({ errors: errors.array() });
     const { imap_host, imap_port, imap_secure, imap_user, imap_pass, imap_folder } = req.body;
-    const { isPrivateIP } = require('../utils/networkValidation');
-    if (isPrivateIP(imap_host)) {
+    const { isHostAllowed } = require('../utils/networkValidation');
+    if (!(await isHostAllowed(imap_host))) {
       return res.status(400).json({ error: 'IMAP host cannot point to a private or internal network address' });
     }
     const existing = await db('email_configs').first();
@@ -183,8 +185,8 @@ router.post('/incoming-config/folders', adminAuth, requirePermission('email.view
   try {
     const { imap_host, imap_port, imap_secure, imap_user, imap_pass } = req.body || {};
     if (imap_host) {
-      const { isPrivateIP } = require('../utils/networkValidation');
-      if (isPrivateIP(imap_host)) {
+      const { isHostAllowed } = require('../utils/networkValidation');
+      if (!(await isHostAllowed(imap_host))) {
         return res.status(400).json({ error: 'IMAP host cannot point to a private or internal network address' });
       }
     }
@@ -205,8 +207,8 @@ router.post('/incoming-config/test', adminAuth, requirePermission('email.view'),
   try {
     const { imap_host, imap_port, imap_secure, imap_user, imap_pass, imap_folder } = req.body || {};
     if (imap_host) {
-      const { isPrivateIP } = require('../utils/networkValidation');
-      if (isPrivateIP(imap_host)) {
+      const { isHostAllowed } = require('../utils/networkValidation');
+      if (!(await isHostAllowed(imap_host))) {
         return res.status(400).json({ error: 'IMAP host cannot point to a private or internal network address' });
       }
     }
@@ -385,11 +387,11 @@ router.post('/accounts', adminAuth, messagingGate, requirePermission('email.edit
     if (!b.account_key) return res.status(400).json({ error: 'account_key is required' });
     // SSRF guard — mirror /config + /incoming-config: neither the IMAP nor the
     // SMTP host may point at a private/internal address.
-    const { isPrivateIP } = require('../utils/networkValidation');
-    if (b.imap_host && isPrivateIP(b.imap_host)) {
+    const { isHostAllowed } = require('../utils/networkValidation');
+    if (b.imap_host && !(await isHostAllowed(b.imap_host))) {
       return res.status(400).json({ error: 'IMAP host cannot point to a private or internal network address' });
     }
-    if (b.smtp_host && isPrivateIP(b.smtp_host)) {
+    if (b.smtp_host && !(await isHostAllowed(b.smtp_host))) {
       return res.status(400).json({ error: 'SMTP host cannot point to a private or internal network address' });
     }
     const patch = {
@@ -434,8 +436,8 @@ router.post('/accounts', adminAuth, messagingGate, requirePermission('email.edit
 router.post('/accounts/test', adminAuth, messagingGate, requirePermission('email.view'), async (req, res) => {
   try {
     const b = req.body || {};
-    const { isPrivateIP } = require('../utils/networkValidation');
-    if (b.imap_host && isPrivateIP(b.imap_host)) {
+    const { isHostAllowed } = require('../utils/networkValidation');
+    if (b.imap_host && !(await isHostAllowed(b.imap_host))) {
       return res.status(400).json({ error: 'IMAP host cannot point to a private or internal network address' });
     }
     let pass = b.imap_pass;
diff --git a/backend/src/routes/adminWebhooks.js b/backend/src/routes/adminWebhooks.js
index fa1d4fec..2ed920a6 100644
--- a/backend/src/routes/adminWebhooks.js
+++ b/backend/src/routes/adminWebhooks.js
@@ -20,7 +20,7 @@ const { body, query, validationResult } = require('express-validator');
 const { db, logActivity } = require('../database/db');
 const { adminAuth } = require('../middleware/auth');
 const { requirePermission } = require('../middleware/permissions');
-const { validateExternalUrl } = require('../utils/networkValidation');
+const { validateExternalUrlAsync } = require('../utils/networkValidation');
 const webhookService = require('../services/webhookService');
 const logger = require('../utils/logger');
 
@@ -78,9 +78,9 @@ router.post(
   requirePermission('settings.edit'),
   [
     body('name').isString().trim().isLength({ min: 1, max: 100 }),
-    body('url').isString().isLength({ max: 2048 }).custom((url) => {
+    body('url').isString().isLength({ max: 2048 }).custom(async (url) => {
       if (ALLOW_PRIVATE_URLS) return true;
-      const check = validateExternalUrl(url);
+      const check = await validateExternalUrlAsync(url);
       if (!check.valid) throw new Error(check.error);
       return true;
     }),
@@ -160,9 +160,9 @@ router.put(
   requirePermission('settings.edit'),
   [
     body('name').optional().isString().trim().isLength({ min: 1, max: 100 }),
-    body('url').optional().isString().isLength({ max: 2048 }).custom((url) => {
+    body('url').optional().isString().isLength({ max: 2048 }).custom(async (url) => {
       if (ALLOW_PRIVATE_URLS) return true;
-      const check = validateExternalUrl(url);
+      const check = await validateExternalUrlAsync(url);
       if (!check.valid) throw new Error(check.error);
       return true;
     }),
diff --git a/backend/src/services/backupService.js b/backend/src/services/backupService.js
index 88db2666..1731a5ba 100644
--- a/backend/src/services/backupService.js
+++ b/backend/src/services/backupService.js
@@ -845,6 +845,14 @@ function parseRsyncStats(output) {
 
 async function performRsyncBackup(config, files) {
   const { spawnAsync } = require('../utils/safeExec');
+  // SSRF: the /test-connection route validates the host, but a scheduled or
+  // manual /run reaches here directly with the stored host. Resolve-and-vet
+  // it right before ssh/rsync does its own DNS at connect time, so a host
+  // that resolves to an internal address can't be reached (GHSA-4jh8).
+  const { isHostAllowed } = require('../utils/networkValidation');
+  if (!(await isHostAllowed(config.backup_rsync_host))) {
+    throw new Error('rsync host resolves to a private or internal network address');
+  }
   // Anchored excludes for the de-selected What-to-Backup paths; rsync
   // otherwise transfers the whole storage root regardless of the walker's
   // file list (which only feeds manifests and file state).
diff --git a/backend/src/services/storage/s3Storage.js b/backend/src/services/storage/s3Storage.js
index b3fd8766..f7bd0a90 100644
--- a/backend/src/services/storage/s3Storage.js
+++ b/backend/src/services/storage/s3Storage.js
@@ -131,6 +131,19 @@ class S3StorageAdapter extends stream.EventEmitter {
    */
   async testConnection() {
     try {
+      // Resolve-and-vet the custom endpoint before the network round-trip
+      // (the constructor's literal check can't catch a public-looking
+      // hostname that resolves to an internal IP). Prod-only, matching the
+      // constructor gate — dev points at localhost MinIO deliberately.
+      if (process.env.NODE_ENV === 'production' && this.config.endpoint) {
+        const { isHostAllowed } = require('../../utils/networkValidation');
+        const { hostname } = new URL(
+          /^https?:\/\//.test(this.config.endpoint) ? this.config.endpoint : `https://${this.config.endpoint}`
+        );
+        if (!(await isHostAllowed(hostname))) {
+          throw new Error('S3 endpoint resolves to a private or internal network address');
+        }
+      }
       await this.s3Client.send(new HeadBucketCommand({ Bucket: this.bucket }));
       logger.info(`Successfully connected to S3 bucket: ${this.bucket}`);
       return true;
diff --git a/backend/src/services/webhookDeliveryWorker.js b/backend/src/services/webhookDeliveryWorker.js
index 4a82eee3..a619e05b 100644
--- a/backend/src/services/webhookDeliveryWorker.js
+++ b/backend/src/services/webhookDeliveryWorker.js
@@ -2,7 +2,7 @@ const axios = require('axios');
 const { db } = require('../database/db');
 const logger = require('../utils/logger');
 const { signPayload, renderTemplate } = require('./webhookService');
-const { validateExternalUrl } = require('../utils/networkValidation');
+const { validateExternalUrlAsync } = require('../utils/networkValidation');
 
 const POLL_INTERVAL_MS = parseInt(process.env.WEBHOOK_DELIVERY_INTERVAL_MS || '5000', 10);
 const CONCURRENCY = parseInt(process.env.WEBHOOK_DELIVERY_CONCURRENCY || '5', 10);
@@ -91,12 +91,24 @@ async function deliverOne(row) {
     return;
   }
 
-  // Re-validate URL per delivery — DNS-rebinding mitigation. Admin can opt
-  // out via WEBHOOK_ALLOW_PRIVATE_URLS=true for local-receiver dev runs.
+  // Re-validate URL per delivery — DNS-rebinding mitigation. Resolves the
+  // host and vets every A/AAAA record (a public-looking name that now
+  // resolves to an internal IP is rejected). Admin can opt out via
+  // WEBHOOK_ALLOW_PRIVATE_URLS=true for local-receiver dev runs.
   if (!allowPrivateUrls) {
-    const urlCheck = validateExternalUrl(webhook.url);
+    const urlCheck = await validateExternalUrlAsync(webhook.url);
     if (!urlCheck.valid) {
-      await markFailedFinal(row, `URL rejected: ${urlCheck.error}`);
+      // A transient lookup failure ('unresolved' — EAI_AGAIN, resolver
+      // briefly down) must NOT connect: falling through to axios would let
+      // an attacker SERVFAIL this preflight and answer axios's own lookup
+      // with a private/metadata IP, defeating the guard. Schedule the
+      // normal retry/backoff instead — no request is made. A confirmed
+      // policy rejection (resolves-to-private / malformed) is permanent.
+      if (urlCheck.reason === 'unresolved') {
+        await scheduleTransientRetry(row, webhook, 'URL host did not resolve — retrying');
+      } else {
+        await markFailedFinal(row, `URL rejected: ${urlCheck.error}`);
+      }
       return;
     }
   }
@@ -217,6 +229,36 @@ async function markFailedFinal(row, reason) {
   await db('webhooks').where({ id: row.webhook_id }).update({ last_failure_at: new Date() });
 }
 
+// Schedule the normal retry/backoff for a transient failure that must not
+// make a network request (e.g. the SSRF preflight lookup failed). Mirrors
+// the failure branch of the main delivery path: retry until MAX_ATTEMPTS,
+// then give up. No response fields — nothing was sent.
+async function scheduleTransientRetry(row, webhook, errorMsg) {
+  const newAttempt = row.attempt_count + 1;
+  if (newAttempt >= MAX_ATTEMPTS) {
+    await db('webhook_deliveries')
+      .where({ id: row.id })
+      .update({
+        status: 'failed',
+        last_error: errorMsg,
+        attempt_count: newAttempt,
+        completed_at: new Date(),
+        next_retry_at: null,
+      });
+  } else {
+    const backoff = BACKOFF_MS[Math.min(newAttempt - 1, BACKOFF_MS.length - 1)];
+    await db('webhook_deliveries')
+      .where({ id: row.id })
+      .update({
+        status: 'pending',
+        last_error: errorMsg,
+        attempt_count: newAttempt,
+        next_retry_at: new Date(Date.now() + backoff),
+      });
+  }
+  await db('webhooks').where({ id: webhook.id }).update({ last_failure_at: new Date() });
+}
+
 function stringifyBody(data) {
   if (data == null) return null;
   if (typeof data === 'string') return data;
diff --git a/backend/src/utils/networkValidation.js b/backend/src/utils/networkValidation.js
index d9b60d0b..8ad21af7 100644
--- a/backend/src/utils/networkValidation.js
+++ b/backend/src/utils/networkValidation.js
@@ -1,5 +1,6 @@
 const { URL } = require('url');
 const net = require('net');
+const dns = require('dns').promises;
 
 /**
  * Check if a hostname or IP resolves to a private/internal network address.
@@ -162,6 +163,11 @@ function isPrivateIPv6(ip) {
 
 /**
  * Validate a URL string, rejecting private/internal targets.
+ *
+ * NOTE: literal-only. For a hostname (not an IP), this checks the string but
+ * NOT what it resolves to — `evil.example` with an A record of 10.0.0.5
+ * passes. Prefer isHostAllowed / validateExternalUrlAsync at any call site
+ * that then actually connects; kept for synchronous callers and fast checks.
  * @param {string} urlString - URL to validate
  * @returns {{ valid: boolean, error?: string }}
  */
@@ -177,4 +183,65 @@ function validateExternalUrl(urlString) {
   }
 }
 
-module.exports = { isPrivateIP, validateExternalUrl };
+/**
+ * Resolve a hostname and reject if it (or ANY of its A/AAAA records) points
+ * at a private/internal address. Closes the SSRF hole where a public-looking
+ * hostname resolves to an internal IP or the cloud metadata endpoint — the
+ * literal isPrivateIP check alone can't see that. Fails closed on resolution
+ * failure. IP literals are decided by isPrivateIP without a lookup.
+ *
+ * Residual: a determined attacker who controls DNS can still rebind between
+ * this check and the client's own resolution (TOCTOU). Fully closing that
+ * needs pinning the connection to the vetted IP, which the underlying
+ * clients (nodemailer/imap/ssh/aws-sdk) don't cleanly support; these actions
+ * are admin-only, so resolve-and-vet is the proportionate mitigation.
+ *
+ * @param {string} hostname
+ * @returns {Promise<boolean>} true when safe to connect
+ */
+async function classifyHost(hostname) {
+  if (!hostname || typeof hostname !== 'string') return 'invalid';
+  // Literal check first: IP literals, blocked names, .internal/.local/.localhost.
+  if (isPrivateIP(hostname)) return 'private';
+  // An IP literal is fully decided above — no name to resolve.
+  const bare = hostname.replace(/^\[|\]$/g, '');
+  if (net.isIP(bare)) return 'ok';
+  let addresses;
+  try {
+    addresses = await dns.lookup(hostname, { all: true });
+  } catch {
+    return 'unresolved'; // transient/NXDOMAIN — caller decides retry vs reject
+  }
+  if (!addresses.length) return 'unresolved';
+  return addresses.every((a) => !isPrivateIP(a.address)) ? 'ok' : 'private';
+}
+
+async function isHostAllowed(hostname) {
+  // Fail-closed boolean for save/test call sites: anything not clearly 'ok'
+  // (including a transient lookup failure) is rejected.
+  return (await classifyHost(hostname)) === 'ok';
+}
+
+/**
+ * Async, DNS-resolving counterpart to validateExternalUrl. Returns a `reason`
+ * so callers with retry semantics (e.g. the webhook worker) can distinguish a
+ * policy rejection ('private'/'invalid') from a transient lookup failure
+ * ('unresolved') that should be retried rather than permanently failed.
+ * @param {string} urlString
+ * @returns {Promise<{ valid: boolean, error?: string, reason: string }>}
+ */
+async function validateExternalUrlAsync(urlString) {
+  let parsed;
+  try {
+    parsed = new URL(urlString);
+  } catch {
+    return { valid: false, error: 'Invalid URL format', reason: 'invalid' };
+  }
+  const reason = await classifyHost(parsed.hostname);
+  if (reason !== 'ok') {
+    return { valid: false, error: 'URL points to a private or internal network address', reason };
+  }
+  return { valid: true, reason: 'ok' };
+}
+
+module.exports = { isPrivateIP, validateExternalUrl, isHostAllowed, validateExternalUrlAsync, classifyHost };