diff --git a/server/src/api/validation/statusPageValidation.ts b/server/src/api/validation/statusPageValidation.ts
index cb2ab4ed9..a650129cd 100644
--- a/server/src/api/validation/statusPageValidation.ts
+++ b/server/src/api/validation/statusPageValidation.ts
@@ -2,6 +2,7 @@ import { z } from "zod";
 import { booleanCoercion, dnsHostnameRegex } from "./shared.js";
 import { StatusPageTypes, StatusPageThemes, StatusPageThemeModes } from "@/domain/status-pages/status-page.type.js";
 import { normalizeStatusPageDomain } from "@/utils/statusPageDomain.js";
+import { cssReferencesExternalResource } from "@/utils/customCss.js";
 
 //****************************************
 // Status Page Validations
@@ -48,7 +49,13 @@ export const createStatusPageBodyValidation = z
 		showUptimePercentage: booleanCoercion,
 		showAdminLoginLink: booleanCoercion.optional(),
 		showInfrastructure: booleanCoercion.optional(),
-		customCSS: z.string().max(100000, "Custom CSS must be at most 100000 characters").optional(),
+		customCSS: z
+			.string()
+			.max(100000, "Custom CSS must be at most 100000 characters")
+			.refine((css) => !cssReferencesExternalResource(css), {
+				message: "Custom CSS cannot reference external URLs or use @import",
+			})
+			.optional(),
 		removeLogo: z.union([z.literal("true"), z.literal("false")]).optional(),
 		theme: z.enum(StatusPageThemes).optional(),
 		themeMode: z.enum(StatusPageThemeModes).optional(),
diff --git a/server/src/utils/customCss.ts b/server/src/utils/customCss.ts
new file mode 100644
index 000000000..0705efe46
--- /dev/null
+++ b/server/src/utils/customCss.ts
@@ -0,0 +1,23 @@
+// Normalize the CSS the way a browser does before matching, so obfuscated forms
+// (inline comments, CSS escapes like "\75 rl(") can't hide an external reference.
+const decodeForScan = (css: string): string =>
+	css.replace(/\/\*[\s\S]*?\*\//g, "").replace(/\\([0-9a-fA-F]{1,6})\s?|\\([\s\S])/g, (_match, hex: string, char: string) => {
+		if (!hex) {
+			return char;
+		}
+		const codePoint = parseInt(hex, 16);
+		return codePoint > 0 && codePoint <= 0x10ffff ? String.fromCodePoint(codePoint) : "�";
+	});
+
+// An external reference is @import, or a resource function (url, image-set,
+// image, cross-fade) whose target starts with an http(s) scheme or is
+// protocol-relative. Anchoring on the start of the target keeps data: and
+// relative URLs allowed, including data: SVG that embeds the SVG namespace URL.
+const EXTERNAL_REFERENCE = /(?:@import\b|(?:url|image-set|-webkit-image-set|image|cross-fade|-webkit-cross-fade)\s*\(\s*['"]?\s*(?:https?:|\/\/))/i;
+
+export const cssReferencesExternalResource = (css?: string | null): boolean => {
+	if (typeof css !== "string" || css === "") {
+		return false;
+	}
+	return EXTERNAL_REFERENCE.test(decodeForScan(css));
+};
diff --git a/server/test/unit/utils/customCss.test.ts b/server/test/unit/utils/customCss.test.ts
new file mode 100644
index 000000000..1efdb8012
--- /dev/null
+++ b/server/test/unit/utils/customCss.test.ts
@@ -0,0 +1,58 @@
+import { describe, expect, it } from "@jest/globals";
+import { cssReferencesExternalResource } from "../../../src/utils/customCss.ts";
+
+describe("cssReferencesExternalResource", () => {
+	it("returns false for empty or non-string input", () => {
+		expect(cssReferencesExternalResource("")).toBe(false);
+		expect(cssReferencesExternalResource(null)).toBe(false);
+		expect(cssReferencesExternalResource(undefined)).toBe(false);
+	});
+
+	it("allows benign CSS", () => {
+		expect(cssReferencesExternalResource("li{outline:3px dashed red}footer{display:none}")).toBe(false);
+	});
+
+	it("allows data: and relative urls", () => {
+		expect(cssReferencesExternalResource("li{background:url(data:image/png;base64,AAAA)}")).toBe(false);
+		expect(cssReferencesExternalResource("li{background:url(/assets/logo.png)}")).toBe(false);
+		expect(cssReferencesExternalResource("li{background:url(logo.png)}")).toBe(false);
+	});
+
+	it("allows data: SVG that embeds the SVG namespace URL", () => {
+		const css = `li{background:url("data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg'></svg>")}`;
+		expect(cssReferencesExternalResource(css)).toBe(false);
+	});
+
+	it("flags @import", () => {
+		expect(cssReferencesExternalResource('@import url("https://evil.example/x.css");')).toBe(true);
+		expect(cssReferencesExternalResource('@import "https://evil.example/x.css";')).toBe(true);
+	});
+
+	it("flags external url()", () => {
+		expect(cssReferencesExternalResource('body{background:url("https://evil.example/x")}')).toBe(true);
+		expect(cssReferencesExternalResource("body{background:url(http://evil.example/x)}")).toBe(true);
+	});
+
+	it("flags protocol-relative url()", () => {
+		expect(cssReferencesExternalResource("body{background:url(//evil.example/x)}")).toBe(true);
+	});
+
+	it("flags external string-form image functions", () => {
+		expect(cssReferencesExternalResource('body{background:image-set("https://evil.example/x" 1x)}')).toBe(true);
+		expect(cssReferencesExternalResource('body{background:-webkit-image-set("//evil.example/x" 1x)}')).toBe(true);
+		expect(cssReferencesExternalResource('body{background-image:image("https://evil.example/x")}')).toBe(true);
+	});
+
+	it("flags url() obfuscated with a trailing parse error", () => {
+		expect(cssReferencesExternalResource("body{background:url(http://evil.example/x) !!!}")).toBe(true);
+	});
+
+	it("flags url() obfuscated with an inline comment", () => {
+		expect(cssReferencesExternalResource('body{background:url(/*c*/ "http://evil.example/x")}')).toBe(true);
+	});
+
+	it("flags url() obfuscated with CSS escapes", () => {
+		expect(cssReferencesExternalResource("body{background:\\75 rl(http://evil.example/x)}")).toBe(true);
+		expect(cssReferencesExternalResource('\\40 import "http://evil.example/x";')).toBe(true);
+	});
+});
diff --git a/server/test/unit/validation/statusPageValidation.test.ts b/server/test/unit/validation/statusPageValidation.test.ts
index bc67efc42..a2d659c34 100644
--- a/server/test/unit/validation/statusPageValidation.test.ts
+++ b/server/test/unit/validation/statusPageValidation.test.ts
@@ -48,4 +48,16 @@ describe("createStatusPageBodyValidation", () => {
 		const result = createStatusPageBodyValidation.safeParse(baseCreateBody({ customCSS: 42 }));
 		expect(result.success).toBe(false);
 	});
+
+	it("rejects customCSS that references an external resource", () => {
+		const result = createStatusPageBodyValidation.safeParse(baseCreateBody({ customCSS: 'body{background:url("https://evil.example/beacon")}' }));
+		expect(result.success).toBe(false);
+		if (!result.success) {
+			expect(result.error.issues).toEqual(
+				expect.arrayContaining([
+					expect.objectContaining({ message: "Custom CSS cannot reference external URLs or use @import", path: ["customCSS"] }),
+				])
+			);
+		}
+	});
 });