fix(security): sanitize third-party RSS HTML before {@html}

The /updates page rendered manga/novel feed fields (content, titles,
series names) from mangaupdates/syosetu/wlnupdates via {@html} with no
sanitization. CSP already blocks script execution, but injected markup
could still phish, redirect, or track. Add sanitizeFeedHtml (DOMPurify
with a small safe allow-list) and apply it on ingest. A behaviour-gate
test plus a check against the live mangaupdates feed confirm legitimate
formatting (entities, <i>/<b>/<a href>) is preserved while <script>,
event handlers, <iframe>/<meta>/<style> and javascript: URLs are removed.

2 files changed, 89 insertions, 0 deletions

diff --git a/src/lib/Utility/sanitizeHtml.test.ts b/src/lib/Utility/sanitizeHtml.test.ts
new file mode 100644
index 00000000..1094635e
--- /dev/null
+++ b/src/lib/Utility/sanitizeHtml.test.ts
@@ -0,0 +1,57 @@
+// @vitest-environment jsdom
+import { describe, expect, it } from "vitest";
+import { sanitizeFeedHtml } from "./sanitizeHtml";
+
+describe("sanitizeFeedHtml", () => {
+	// Behaviour gate: the formatting real feeds use must survive untouched.
+	it("preserves entities, inline formatting and safe links", () => {
+		expect(sanitizeFeedHtml("Fruits & Vegetables")).toBe(
+			"Fruits & Vegetables",
+		);
+		expect(sanitizeFeedHtml("<i>italic</i> and <b>bold</b>")).toBe(
+			"<i>italic</i> and <b>bold</b>",
+		);
+		expect(sanitizeFeedHtml("Vol. 1 <em>Ch.</em> 5")).toBe(
+			"Vol. 1 <em>Ch.</em> 5",
+		);
+		expect(
+			sanitizeFeedHtml('<a href="https://example.com/x">link</a>'),
+		).toContain('href="https://example.com/x"');
+		expect(sanitizeFeedHtml("line<br>break")).toContain("<br");
+	});
+
+	it("returns empty string for nullish input", () => {
+		expect(sanitizeFeedHtml(undefined)).toBe("");
+		expect(sanitizeFeedHtml(null)).toBe("");
+		expect(sanitizeFeedHtml("")).toBe("");
+	});
+
+	// The fix: scripts, handlers, dangerous tags and URLs must be removed.
+	it("strips scripts, event handlers and dangerous tags/urls", () => {
+		const script = sanitizeFeedHtml("<script>alert(1)</script>safe");
+		expect(script).not.toContain("script");
+		expect(script).toContain("safe");
+
+		const onerror = sanitizeFeedHtml("before<img src=x onerror=alert(1)>after");
+		expect(onerror).not.toContain("onerror");
+		expect(onerror).not.toContain("<img");
+		expect(onerror).toContain("before");
+		expect(onerror).toContain("after");
+
+		expect(
+			sanitizeFeedHtml('<a href="javascript:alert(1)">x</a>'),
+		).not.toContain("javascript:");
+		expect(
+			sanitizeFeedHtml('<iframe src="https://evil.example.com"></iframe>'),
+		).not.toContain("iframe");
+		expect(
+			sanitizeFeedHtml(
+				'<meta http-equiv="refresh" content="0;url=https://evil.example.com">',
+			),
+		).not.toContain("meta");
+		expect(sanitizeFeedHtml("<style>body{display:none}</style>")).not.toContain(
+			"style",
+		);
+		expect(sanitizeFeedHtml('<div onclick="steal()">text</div>')).toBe("text");
+	});
+});
diff --git a/src/lib/Utility/sanitizeHtml.ts b/src/lib/Utility/sanitizeHtml.ts
new file mode 100644
index 00000000..3d0229e4
--- /dev/null
+++ b/src/lib/Utility/sanitizeHtml.ts
@@ -0,0 +1,32 @@
+import DOMPurify from "dompurify";
+
+const feedConfig = {
+	ALLOWED_TAGS: [
+		"a",
+		"b",
+		"i",
+		"em",
+		"strong",
+		"u",
+		"s",
+		"br",
+		"p",
+		"span",
+		"small",
+		"sup",
+		"sub",
+		"code",
+	],
+	ALLOWED_ATTR: ["href", "title"],
+	ALLOWED_URI_REGEXP: /^(?:https?|mailto):/i,
+};
+
+/**
+ * Sanitise HTML coming from third-party RSS feeds before it reaches an `{@html}`
+ * sink. Keeps the light formatting these feeds actually use (HTML entities,
+ * `<i>`/`<b>`/`<a href>`) and strips anything that could inject content or
+ * behaviour: `<script>`, event-handler attributes, `<iframe>`/`<meta>`/`<style>`,
+ * `javascript:` URLs, and so on. Browser-only — call it from client code.
+ */
+export const sanitizeFeedHtml = (html: string | undefined | null): string =>
+	html ? DOMPurify.sanitize(html, feedConfig) : "";