diff --git a/frontend/src/lib/planner/DesktopDayTile.svelte b/frontend/src/lib/planner/DesktopDayTile.svelte
index af709f5..e044324 100644
--- a/frontend/src/lib/planner/DesktopDayTile.svelte
+++ b/frontend/src/lib/planner/DesktopDayTile.svelte
@@ -2,6 +2,7 @@
 	import EmptyDayTile from './EmptyDayTile.svelte';
 	import { formatDayAbbr } from '$lib/planner/week';
 	import type { Recipe, Slot, Suggestion } from '$lib/planner/types';
+	import { sanitizeForCssUrl } from '$lib/planner/DesktopDayTile.utils';
 
 	let {
 		slot,
@@ -67,7 +68,7 @@
 
 	const gradientBackground = $derived((() => {
 		if (!slot.recipe) return 'var(--color-surface)';
-		if (slot.recipe.heroImageUrl) return `url(${slot.recipe.heroImageUrl})`;
+		if (slot.recipe.heroImageUrl) return `url("${sanitizeForCssUrl(slot.recipe.heroImageUrl)}")`;
 		const proteinTag = slot.recipe.tags?.find((t) => t.tagType === 'protein');
 		if (proteinTag?.name) {
 			return `var(--gradient-protein-${toCssKey(proteinTag.name)})`;
diff --git a/frontend/src/lib/planner/DesktopDayTile.test.ts b/frontend/src/lib/planner/DesktopDayTile.test.ts
index ecd549a..7be228e 100644
--- a/frontend/src/lib/planner/DesktopDayTile.test.ts
+++ b/frontend/src/lib/planner/DesktopDayTile.test.ts
@@ -2,6 +2,7 @@ import { describe, it, expect, vi } from 'vitest';
 import { render, screen } from '@testing-library/svelte';
 import { userEvent } from '@testing-library/user-event';
 import DesktopDayTile from './DesktopDayTile.svelte';
+import { sanitizeForCssUrl } from './DesktopDayTile.utils';
 
 const filledSlot = {
 	id: 's1',
@@ -18,6 +19,29 @@ const filledSlot = {
 
 const emptySlot = { id: null, slotDate: '2026-04-14', recipe: null };
 
+describe('sanitizeForCssUrl', () => {
+	it('strips parentheses that could break out of url() context', () => {
+		expect(sanitizeForCssUrl('x);}body{display:none}/*')).not.toContain(')');
+	});
+
+	it('strips single quotes', () => {
+		expect(sanitizeForCssUrl("data:image/png;base64,abc'def")).not.toContain("'");
+	});
+
+	it('strips double quotes', () => {
+		expect(sanitizeForCssUrl('data:image/png;base64,abc"def')).not.toContain('"');
+	});
+
+	it('strips backslashes', () => {
+		expect(sanitizeForCssUrl('data:image/png;base64,abc\\def')).not.toContain('\\');
+	});
+
+	it('preserves a safe data URI unchanged', () => {
+		const safe = 'data:image/png;base64,abc123+/==';
+		expect(sanitizeForCssUrl(safe)).toBe(safe);
+	});
+});
+
 describe('DesktopDayTile — filled slot', () => {
 	describe('front face', () => {
 		it('renders recipe name on front face', () => {
diff --git a/frontend/src/lib/planner/DesktopDayTile.utils.ts b/frontend/src/lib/planner/DesktopDayTile.utils.ts
new file mode 100644
index 0000000..6148bde
--- /dev/null
+++ b/frontend/src/lib/planner/DesktopDayTile.utils.ts
@@ -0,0 +1,8 @@
+/**
+ * Strips characters that could break out of a CSS url() context or inject
+ * CSS into an inline style attribute. Safe data URIs (base64) are unaffected
+ * as they contain only A-Z, a-z, 0-9, +, /, = and the data: prefix.
+ */
+export function sanitizeForCssUrl(url: string): string {
+	return url.replace(/['"()\\]/g, '');
+}