fix(google): HTML-escape the emoji in the reaction email's html fallback

The reaction email's text/html fallback interpolated the emoji directly
into markup. A real emoji never contains HTML metacharacters, but the
value is ultimately a user-supplied reaction string (only custom-emoji
refs are pre-filtered, not arbitrary content), so escape it to keep the
outbound email's HTML body free of markup injection. The text/plain and
JSON reaction parts were already safe (base64 / JSON-encoded).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: KrisBraun

connectors/google/src/mail/gmail-api.test.ts

@@ -664,6 +664,23 @@ describe("buildReactionMessage", () => {
     // still show something.
     expect(decodeMimePart(raw, "text/plain")).toBe("💖");
   });
+
+  it("HTML-escapes the emoji in the html fallback (no markup injection)", () => {
+    const raw = decodeRawMessage(
+      buildReactionMessage({
+        to: ["alice@example.com"],
+        cc: [],
+        from: "me@example.com",
+        subject: "Hi",
+        emoji: "<img src=x onerror=alert(1)>",
+        messageId: "<orig@mail.gmail.com>",
+        references: "",
+      })
+    );
+    const html = decodeMimePart(raw, "text/html");
+    expect(html).not.toContain("<img src=x");
+    expect(html).toContain("&lt;img src=x onerror=alert(1)&gt;");
+  });
 });
 
 describe("isSendableGmailReaction", () => {

connectors/google/src/mail/gmail-api.ts

@@ -1883,6 +1883,16 @@ export function isSendableGmailReaction(emoji: string): boolean {
   return true;
 }
 
+/** Escapes the five HTML metacharacters so a value is safe inside HTML text. */
+function escapeHtml(value: string): string {
+  return value
+    .replace(/&/g, "&")
+    .replace(/</g, "&lt;")
+    .replace(/>/g, "&gt;")
+    .replace(/"/g, "&quot;")
+    .replace(/'/g, "&#39;");
+}
+
 /**
  * Builds a Gmail emoji-reaction email for the message identified by
  * `messageId` (its RFC Message-ID). Per
@@ -1928,7 +1938,12 @@ export function buildReactionMessage(options: {
   const altBoundary = mimeBoundary("alt");
   const enc = new TextEncoder();
   const reactionJson = JSON.stringify({ version: 1, emoji });
-  const htmlFallback = `<!DOCTYPE html>\r\n<html><body>\r\n<p>${emoji}</p>\r\n</body></html>`;
+  // HTML-escape the emoji before interpolating it into the html fallback part.
+  // A real emoji never contains HTML metacharacters, but `emoji` is ultimately a
+  // user-supplied reaction string (only custom-emoji refs are pre-filtered, not
+  // arbitrary content), so escape defensively to keep the outbound email's HTML
+  // body injection-free.
+  const htmlFallback = `<!DOCTYPE html>\r\n<html><body>\r\n<p>${escapeHtml(emoji)}</p>\r\n</body></html>`;
 
   const altBlock = [
     `Content-Type: multipart/alternative; boundary="${altBoundary}"`,