diff --git a/index.js b/index.js
index e4d48ef..d37ee99 100644
--- a/index.js
+++ b/index.js
@@ -5,7 +5,6 @@ const { isPlainObject } = require('is-plain-object');
 const deepmerge = require('deepmerge');
 const parseSrcset = require('parse-srcset');
 const { parse: postcssParse } = require('postcss');
-const url = require('url');
 // Tags that can conceivably represent stand-alone media.
 const mediaTags = [
   'img', 'audio', 'video', 'picture', 'svg',
@@ -305,12 +304,24 @@ function sanitizeHtml(html, options, _recursing) {
             if (name === 'iframe' && a === 'src') {
               let allowed = true;
               try {
+                if (value.startsWith('relative:')) {
+                  // An attempt to exploit our workaround for base URLs being
+                  // mandatory for relative URL validation in the WHATWG
+                  // URL parser, reject it
+                  throw new Error('relative: exploit attempt');
+                }
                 // naughtyHref is in charge of whether protocol relative URLs
-                // are cool. We should just accept them
-                // TODO: Replace deprecated `url.parse`
-                // eslint-disable-next-line node/no-deprecated-api
-                parsed = url.parse(value, false, true);
-                const isRelativeUrl = parsed && parsed.host === null && parsed.protocol === null;
+                // are cool. Here we are concerned just with allowed hostnames and
+                // whether to allow relative URLs.
+                //
+                // Build a placeholder "base URL" against which any reasonable
+                // relative URL may be parsed successfully
+                let base = 'relative://relative-site';
+                for (let i = 0; (i < 100); i++) {
+                  base += `/${i}`;
+                }
+                const parsed = new URL(value, base);
+                const isRelativeUrl = parsed && parsed.hostname === 'relative-site' && parsed.protocol === 'relative:';
                 if (isRelativeUrl) {
                   // default value of allowIframeRelativeUrls is true
                   // unless allowedIframeHostnames or allowedIframeDomains specified
diff --git a/test/test.js b/test/test.js
index d83ffdc..634ada4 100644
--- a/test/test.js
+++ b/test/test.js
@@ -1210,4 +1210,63 @@ describe('sanitizeHtml', function() {
       '<div><div><div><div><div><div>nested text</div></div></div></div></div></div>'
     );
   });
+  it('should not allow simple append attacks on iframe hostname validation', function() {
+    assert.equal(
+      sanitizeHtml('<iframe src=//www.youtube.comissocool>', {
+        allowedTags: [ 'iframe' ],
+        allowedAttributes: {
+          iframe: [ 'src' ]
+        },
+        allowedIframeHostnames: [ 'www.youtube.com' ]
+      }),
+      '<iframe></iframe>'
+    );
+  });
+  it('should not allow IDNA (Internationalized Domain Name) iframe validation bypass attacks', function() {
+    assert.equal(
+      sanitizeHtml('<iframe src=//www.youtube.com%C3%9E.93.184.216.34.nip.io>', {
+        allowedTags: [ 'iframe' ],
+        allowedAttributes: {
+          iframe: [ 'src' ]
+        },
+        allowedIframeHostnames: [ 'www.youtube.com' ]
+      }),
+      '<iframe></iframe>'
+    );
+  });
+  it('should parse path-rooted relative URLs sensibly', function() {
+    assert.equal(
+      sanitizeHtml('<a href="/foo"></a>'),
+      '<a href="/foo"></a>'
+    );
+  });
+  it('should parse bare relative URLs sensibly', function() {
+    assert.equal(
+      sanitizeHtml('<a href="foo"></a>'),
+      '<a href="foo"></a>'
+    );
+  });
+  it('should parse ../ relative URLs sensibly', function() {
+    assert.equal(
+      sanitizeHtml('<a href="../../foo"></a>'),
+      '<a href="../../foo"></a>'
+    );
+  });
+  it('should parse protocol relative URLs sensibly', function() {
+    assert.equal(
+      sanitizeHtml('<a href="//foo.com/foo"></a>'),
+      '<a href="//foo.com/foo"></a>'
+    );
+  });
+  it('should reject attempts to hack our use of a relative: protocol in our test base URL', function() {
+    assert.equal(
+      sanitizeHtml('<iframe src="relative://relative-test/aha">', {
+        allowedTags: [ 'iframe' ],
+        allowedAttributes: {
+          iframe: [ 'src' ]
+        }
+      }),
+      '<iframe></iframe>'
+    );
+  });
 });