diff --git a/package.json b/package.json
index f4945ee55748..70f9940bb0c9 100644
--- a/package.json
+++ b/package.json
@@ -193,6 +193,7 @@
     "image-diff": "1.6.0",
     "intern": "3.2.3",
     "istanbul-instrumenter-loader": "0.1.3",
+    "jsdom": "9.8.3",
     "karma": "1.2.0",
     "karma-chrome-launcher": "0.2.0",
     "karma-coverage": "0.5.1",
diff --git a/src/ui/__tests__/fixtures/test_app/index.js b/src/ui/__tests__/fixtures/test_app/index.js
new file mode 100644
index 000000000000..4f5115faff15
--- /dev/null
+++ b/src/ui/__tests__/fixtures/test_app/index.js
@@ -0,0 +1,13 @@
+module.exports = kibana => new kibana.Plugin({
+  uiExports: {
+    app: {
+      name: 'test_app',
+      main: 'plugins/test_app/index.js',
+      injectVars() {
+        return {
+          from_test_app: true
+        };
+      }
+    }
+  }
+});
diff --git a/src/ui/__tests__/fixtures/test_app/package.json b/src/ui/__tests__/fixtures/test_app/package.json
new file mode 100644
index 000000000000..3aeb029e4f4c
--- /dev/null
+++ b/src/ui/__tests__/fixtures/test_app/package.json
@@ -0,0 +1,4 @@
+{
+  "name": "test_app",
+  "version": "kibana"
+}
diff --git a/src/ui/__tests__/fixtures/test_app/public/index.js b/src/ui/__tests__/fixtures/test_app/public/index.js
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/src/ui/__tests__/ui_exports_replace_injected_vars.js b/src/ui/__tests__/ui_exports_replace_injected_vars.js
new file mode 100644
index 000000000000..5be6d5476448
--- /dev/null
+++ b/src/ui/__tests__/ui_exports_replace_injected_vars.js
@@ -0,0 +1,125 @@
+import { resolve } from 'path';
+
+import { delay } from 'bluebird';
+import expect from 'expect.js';
+import sinon from 'sinon';
+import jsdom from 'jsdom';
+import { noop } from 'lodash';
+
+import KbnServer from '../../server/kbn_server';
+
+const getInjectedVarsFromResponse = (resp) => {
+  const document = jsdom.jsdom(resp.payload);
+  const data = document.querySelector('kbn-initial-state').getAttribute('data');
+  return JSON.parse(data).vars;
+};
+
+const injectReplacer = (kbnServer, replacer) => {
+  // normally the replacer would be defined in a plugin's uiExports,
+  // but that requires stubbing out an entire plugin directory for
+  // each test, so we fake it and jam the replacer into uiExports
+  kbnServer.uiExports.injectedVarsReplacers.push(replacer);
+};
+
+describe('UiExports', function () {
+  describe('#replaceInjectedVars', function () {
+    this.slow(2000);
+    this.timeout(10000);
+
+    let kbnServer;
+    beforeEach(async () => {
+      kbnServer = new KbnServer({
+        server: { port: 0 }, // pick a random open port
+        logging: { silent: true }, // no logs
+        optimize: { enabled: false },
+        uiSettings: { enabled: false },
+        plugins: {
+          paths: [resolve(__dirname, './fixtures/test_app')] // inject an app so we can hit /app/{id}
+        },
+      });
+
+      await kbnServer.ready();
+      kbnServer.status.get('ui settings').state = 'green';
+      kbnServer.server.decorate('server', 'uiSettings', () => {
+        return { getDefaults: noop };
+      });
+    });
+
+    afterEach(async () => {
+      await kbnServer.close();
+      kbnServer = null;
+    });
+
+    it('allows sync replacing of injected vars', async () => {
+      injectReplacer(kbnServer, () => ({ a: 1 }));
+
+      const resp = await kbnServer.inject('/app/test_app');
+      const injectedVars = getInjectedVarsFromResponse(resp);
+
+      expect(injectedVars).to.eql({ a: 1 });
+    });
+
+    it('allows async replacing of injected vars', async () => {
+      const asyncThing = () => delay(100).return('world');
+
+      injectReplacer(kbnServer, async () => {
+        return {
+          hello: await asyncThing()
+        };
+      });
+
+      const resp = await kbnServer.inject('/app/test_app');
+      const injectedVars = getInjectedVarsFromResponse(resp);
+
+      expect(injectedVars).to.eql({
+        hello: 'world'
+      });
+    });
+
+    it('passes originalInjectedVars, request, and server to replacer', async () => {
+      const stub = sinon.stub();
+      injectReplacer(kbnServer, () => ({ foo: 'bar' }));
+      injectReplacer(kbnServer, stub);
+
+      await kbnServer.inject('/app/test_app');
+
+      sinon.assert.calledOnce(stub);
+      expect(stub.firstCall.args[0]).to.eql({ foo: 'bar' }); // originalInjectedVars
+      expect(stub.firstCall.args[1]).to.have.property('path', '/app/test_app'); // request
+      expect(stub.firstCall.args[1]).to.have.property('server', kbnServer.server); // request
+      expect(stub.firstCall.args[2]).to.be(kbnServer.server);
+    });
+
+    it('calls the methods sequentially', async () => {
+      injectReplacer(kbnServer, () => ({ name: '' }));
+      injectReplacer(kbnServer, orig => ({ name: orig.name + 's' }));
+      injectReplacer(kbnServer, orig => ({ name: orig.name + 'a' }));
+      injectReplacer(kbnServer, orig => ({ name: orig.name + 'm' }));
+
+      const resp = await kbnServer.inject('/app/test_app');
+      const injectedVars = getInjectedVarsFromResponse(resp);
+
+      expect(injectedVars).to.eql({ name: 'sam' });
+    });
+
+    it('propogates errors thrown in replacers', async () => {
+      injectReplacer(kbnServer, async () => {
+        await delay(100);
+        throw new Error('replacer failed');
+      });
+
+      const resp = await kbnServer.inject('/app/test_app');
+      expect(resp).to.have.property('statusCode', 500);
+    });
+
+    it('starts off with the injected vars for the app merged with the default injected vars', async () => {
+      const stub = sinon.stub();
+      injectReplacer(kbnServer, stub);
+      kbnServer.uiExports.defaultInjectedVars.from_defaults = true;
+
+      const resp = await kbnServer.inject('/app/test_app');
+      sinon.assert.calledOnce(stub);
+      expect(stub.firstCall.args[0]).to.eql({ from_defaults: true, from_test_app: true });
+    });
+  });
+});