fix(core): validate CDN-supplied apiHost before building upload URL

devbox.lock

@@ -199,7 +199,7 @@
     },
     "nodejs@22": {
       "last_modified": "2026-03-27T11:17:38Z",
-      "plugin_version": "0.0.4",
+      "plugin_version": "0.0.2",
       "resolved": "github:NixOS/nixpkgs/832efc09b4caf6b4569fbf9dc01bec3082a00611#nodejs_22",
       "source": "devbox-search",
       "version": "22.22.2",

packages/core/src/__tests__/util.test.ts

@@ -1,5 +1,11 @@
 import { UserTraits } from '../types';
-import { chunk, allSettled, deepCompare, getURL } from '../util';
+import {
+  chunk,
+  allSettled,
+  deepCompare,
+  getURL,
+  validateApiHost,
+} from '../util';
 
 describe('#chunk', () => {
   it('handles empty array', () => {
@@ -210,3 +216,39 @@ describe('getURL function', () => {
     );
   });
 });
+
+describe('validateApiHost', () => {
+  it('accepts a bare hostname', () => {
+    expect(validateApiHost('api.segment.io')).toBe(true);
+  });
+
+  it('accepts hostname with path (normal Segment format)', () => {
+    expect(validateApiHost('api.segment.io/v1')).toBe(true);
+    expect(validateApiHost('events.eu1.segmentapis.com')).toBe(true);
+  });
+
+  it('accepts hostname with port', () => {
+    expect(validateApiHost('api.segment.io:443/v1')).toBe(true);
+  });
+
+  it('rejects values with a scheme', () => {
+    expect(validateApiHost('https://api.segment.io/v1')).toBe(false);
+    expect(validateApiHost('http://api.segment.io/v1')).toBe(false);
+  });
+
+  it('rejects values with credentials', () => {
+    expect(validateApiHost('user:pass@api.segment.io')).toBe(false);
+  });
+
+  it('rejects values with a query string', () => {
+    expect(validateApiHost('attacker.com/collect?x=')).toBe(false);
+  });
+
+  it('rejects values with a fragment', () => {
+    expect(validateApiHost('attacker.com/path#fragment')).toBe(false);
+  });
+
+  it('rejects empty string', () => {
+    expect(validateApiHost('')).toBe(false);
+  });
+});

packages/core/src/plugins/SegmentDestination.ts

@@ -10,7 +10,7 @@ import {
   SegmentEvent,
   UpdateType,
 } from '../types';
-import { chunk, createPromise, getURL } from '../util';
+import { chunk, createPromise, getURL, validateApiHost } from '../util';
 import { uploadEvents } from '../api';
 import type { SegmentClient } from '../analytics';
 import { DestinationMetadataEnrichment } from './DestinationMetadataEnrichment';
@@ -446,8 +446,13 @@ export class SegmentDestination extends DestinationPlugin {
       segmentSettings?.apiHost !== undefined &&
       segmentSettings?.apiHost !== null
     ) {
-      //assign the api host from segment settings (domain/v1)
-      this.apiHost = `https://${segmentSettings.apiHost}/b`;
+      if (validateApiHost(segmentSettings.apiHost)) {
+        this.apiHost = `https://${segmentSettings.apiHost}/b`;
+      } else {
+        console.error(
+          `[Segment] Invalid apiHost "${segmentSettings.apiHost}" received from settings — ignoring, using default endpoint.`
+        );
+      }
     }
 
     // Read httpConfig: prefer integration-level settings from CDN, fall back to

packages/core/src/plugins/__tests__/SegmentDestination.test.ts

@@ -375,6 +375,28 @@ describe('SegmentDestination', () => {
       });
     });
 
+    it('ignores a tampered apiHost containing a scheme or query string and falls back to default', async () => {
+      const events = [{ messageId: 'msg-1' }] as SegmentEvent[];
+
+      for (const badHost of [
+        'https://attacker.com/collect',
+        'http://attacker.com',
+        'attacker.com/collect?x=',
+        'user:pass@attacker.com',
+      ]) {
+        const { plugin, sendEventsSpy } = createTestWith({
+          events,
+          settings: { apiKey: '', apiHost: badHost },
+        });
+        jest.spyOn(console, 'error').mockImplementation(jest.fn());
+        await plugin.flush();
+        expect(sendEventsSpy).toHaveBeenCalledWith(
+          expect.objectContaining({ url: defaultApiHost })
+        );
+        sendEventsSpy.mockClear();
+      }
+    });
+
     it.each([
       [false, false], // No proxy, No segment endpoint
       [false, true], // No proxy, Yes segment endpoint

packages/core/src/util.ts

@@ -258,6 +258,16 @@ export const createPromise = <T>(
   };
 };
 
+// Accepts a bare host[/path] value (e.g. "api.segment.io/v1") as supplied by
+// the settings CDN. Rejects anything that contains a scheme, credentials,
+// query string, or fragment — all of which could redirect uploads to an
+// attacker-controlled endpoint when interpolated into `https://${apiHost}/b`.
+export function validateApiHost(apiHost: string): boolean {
+  return /^[a-zA-Z0-9][a-zA-Z0-9._-]*(:\d{2,5})?(\/[a-zA-Z0-9._\-/]*)?$/.test(
+    apiHost
+  );
+}
+
 export function getURL(host: string, path: string, allowInsecure = false) {
   if (!host.startsWith('https://') && !host.startsWith('http://')) {
     host = 'https://' + host;