diff --git a/frontend/src/lib/components/instanceSettings.ts b/frontend/src/lib/components/instanceSettings.ts
index 3c60ac990d..d0528ec640 100644
--- a/frontend/src/lib/components/instanceSettings.ts
+++ b/frontend/src/lib/components/instanceSettings.ts
@@ -1,4 +1,5 @@
 import type { ButtonType } from './common/button/model'
+import { allowedOriginsError, parseAllowedOriginsSetting } from './triggers/http/utils'
 import { z } from 'zod'
 import { writable } from 'svelte/store'
 
@@ -270,6 +271,12 @@ export const settings: Record<string, Setting[]> = {
 			fieldType: 'text',
 			placeholder: 'https://app.example.com, https://admin.example.com',
 			storage: 'setting',
+			error:
+				'Each origin must be visible ASCII with no comma, and there can be at most 100 of them. null is not allowed, since every sandboxed iframe sends it.',
+			// The same check the API applies, so a value it would refuse cannot be
+			// saved here and then silently drop to no restriction at the next boot.
+			isValid: (value: string | undefined) =>
+				allowedOriginsError(parseAllowedOriginsSetting(value)) === undefined,
 			ee_only: '',
 			hideInQuickSetup: true
 		},
diff --git a/frontend/src/lib/components/triggers/http/utils.ts b/frontend/src/lib/components/triggers/http/utils.ts
index 5fba39afb3..2c94abd333 100644
--- a/frontend/src/lib/components/triggers/http/utils.ts
+++ b/frontend/src/lib/components/triggers/http/utils.ts
@@ -66,12 +66,22 @@ export function allowedOriginsError(allowed_origins: string[] | undefined): stri
 export function allowedOriginWarning(origin: string): string | undefined {
 	if (origin === '*' || allowedOriginRejection(origin) !== undefined) return undefined
 	const separator = origin.indexOf('://')
-	if (separator < 0) return `'${origin}' has no scheme, such as https://`
+	if (separator <= 0) return `'${origin}' has no scheme, such as https://`
 	const rest = origin.slice(separator + 3)
 	if (rest === '') return `'${origin}' has no host`
 	if (/[/?#]/.test(rest))
 		return `'${origin}' should be scheme://host[:port], with no path, query or fragment`
 	if (rest.includes('@')) return `'${origin}' should not contain userinfo`
+	// Only the port is checked past this point. The host is left alone on
+	// purpose: browsers send origins this cannot anticipate, `chrome-extension`
+	// and IPv6 literals among them, and a warning that cries wolf on a working
+	// origin is worse than one that stays quiet.
+	const port = rest.startsWith('[')
+		? rest.slice(rest.indexOf(']') + 1).replace(/^:/, '')
+		: rest.split(':')[1]
+	if (rest.startsWith(':')) return `'${origin}' has no host`
+	if (port !== undefined && port !== '' && !(/^[0-9]{1,5}$/.test(port) && Number(port) <= 65535))
+		return `'${origin}' has a port no browser can send`
 	return undefined
 }