diff --git a/react-app/src/shared/utils/externalHref.test.ts b/react-app/src/shared/utils/externalHref.test.ts
index d60146d6d..4ca666efa 100644
--- a/react-app/src/shared/utils/externalHref.test.ts
+++ b/react-app/src/shared/utils/externalHref.test.ts
@@ -6,6 +6,7 @@ describe('externalHref', () => {
     it('returns http(s) URLs unchanged (Angular binds item.url as-is)', () => {
         expect(externalHref('https://example.com/a?b=1')).toBe('https://example.com/a?b=1');
         expect(externalHref('http://example.com')).toBe('http://example.com');
+        expect(externalHref('httpS://example.com/a')).toBe('httpS://example.com/a');
     });
 
     it.each([
@@ -13,6 +14,8 @@ describe('externalHref', () => {
         ['javascript: URL', 'javascript:alert(1)'],
         ['data: URL', 'data:text/html,hi'],
         ['"http" prefix that is not a URL', 'httpfoo'],
+        ['scheme-only https URL', 'https:example.com'],
+        ['scheme-only http URL with one slash', 'http:/example.com'],
         ['empty string', ''],
         ['undefined', undefined],
     ])('returns null for %s', (_, url) => {
diff --git a/react-app/src/shared/utils/externalHref.ts b/react-app/src/shared/utils/externalHref.ts
index f08101d0e..384865e9c 100644
--- a/react-app/src/shared/utils/externalHref.ts
+++ b/react-app/src/shared/utils/externalHref.ts
@@ -1,11 +1,15 @@
 import { isSafeHref } from './sanitizeHtml';
 
 /**
- * Angular `hasUrl` (`url.indexOf('http') === 0`), tightened: only a parseable, safe http(s) URL renders as an
- * external link. Everything else (HN-relative `item?id=N` for Ask/Show/text posts) links to `/item/:id`.
+ * Angular `hasUrl` (`url.indexOf('http') === 0`), tightened: only a parseable, safe, absolute `http(s)://` URL renders
+ * as an external link. Everything else (HN-relative `item?id=N` for Ask/Show/text posts) links to `/item/:id`.
+ * Scheme-only forms like `https:example.com` are rejected: `new URL` accepts them, but as an href the browser resolves
+ * them relative to the page.
  */
+const ABSOLUTE_HTTP = /^http[sS]?:\/\//;
+
 export function externalHref(url: string | null | undefined): string | null {
-    if (typeof url !== 'string' || url.indexOf('http') !== 0 || !isSafeHref(url)) return null;
+    if (typeof url !== 'string' || !ABSOLUTE_HTTP.test(url) || !isSafeHref(url)) return null;
     try {
         const parsed = new URL(url);
         return parsed.protocol === 'http:' || parsed.protocol === 'https:' ? url : null;