diff --git a/src/lib/__tests__/mdn-header-injection.test.ts b/src/lib/__tests__/mdn-header-injection.test.ts
new file mode 100644
index 00000000..40f8f132
--- /dev/null
+++ b/src/lib/__tests__/mdn-header-injection.test.ts
@@ -0,0 +1,199 @@
+import { describe, it, expect } from 'vitest';
+import { buildMdnMessage } from '../mdn';
+
+// GHSA-w38p-hpqv-g89c: every value that lands in a header of the read receipt
+// originates with the sender of the message being answered, and Stalwart hands
+// it back already RFC 2047-decoded. A bare CRLF smuggled inside an encoded-word
+// used to be interpolated verbatim into the Subject: line, letting the sender
+// append From/Reply-To/Content-Type headers and a body to a message the
+// user's own account files into Sent and submits.
+
+const base = {
+  to: 'sender@other.com',
+  fromEmail: 'me@example.com',
+  originalMessageId: 'orig@other.com',
+};
+
+// The decoded subject from the advisory's proof of concept.
+const INJECTED_SUBJECT =
+  'Invoice\r\nFrom: IT Helpdesk <helpdesk@corp.example>\r\nReply-To: attacker@evil.test\r\n'
+  + 'Content-Type: text/html; charset=utf-8\r\n\r\n<html><body><h1>Forged</h1></body></html><!--';
+
+/** Decode a single RFC 2047 B encoded-word the way the server does before handing it to us. */
+function decodeEncodedWord(word: string): string {
+  const m = word.match(/^=\?UTF-8\?B\?([^?]*)\?=$/i);
+  if (!m) throw new Error(`not an encoded-word: ${word}`);
+  return Buffer.from(m[1], 'base64').toString('utf8');
+}
+
+/** Header block (everything before the first blank line) split into lines. */
+function headerLines(msg: string): string[] {
+  const end = msg.indexOf('\r\n\r\n');
+  return msg.slice(0, end).split('\r\n');
+}
+
+/** Everything after the top-level header block. */
+function body(msg: string): string {
+  return msg.slice(msg.indexOf('\r\n\r\n') + 4);
+}
+
+function countHeader(lines: string[], name: string): number {
+  const re = new RegExp(`^${name}:`, 'i');
+  return lines.filter((l) => re.test(l)).length;
+}
+
+describe('buildMdnMessage - header injection (GHSA-w38p-hpqv-g89c)', () => {
+  it('a CRLF inside the localized subject cannot terminate the Subject: header', () => {
+    const msg = buildMdnMessage({ ...base, subject: `Read: ${INJECTED_SUBJECT}` });
+    const lines = headerLines(msg);
+
+    expect(countHeader(lines, 'Subject')).toBe(1);
+    expect(countHeader(lines, 'From')).toBe(1);
+    expect(countHeader(lines, 'Reply-To')).toBe(0);
+    expect(lines.find((l) => /^From:/.test(l))).toBe('From: me@example.com');
+    // The only Content-Type in the header block is our multipart/report.
+    expect(lines.filter((l) => /^Content-Type:/i.test(l))).toEqual([
+      'Content-Type: multipart/report; report-type=disposition-notification;',
+    ]);
+    // The attacker's HTML never becomes a body part (it stays inert text on the Subject line).
+    expect(body(msg)).not.toContain('<h1>Forged</h1>');
+    // The subject text survives on one line with the breaks collapsed.
+    expect(lines.find((l) => /^Subject:/.test(l))).toBe(
+      'Subject: Read: Invoice From: IT Helpdesk <helpdesk@corp.example>'
+        + ' Reply-To: attacker@evil.test Content-Type: text/html; charset=utf-8'
+        + ' <html><body><h1>Forged</h1></body></html><!--',
+    );
+  });
+
+  it('the default "Read: <originalSubject>" path is protected the same way', () => {
+    const msg = buildMdnMessage({ ...base, originalSubject: INJECTED_SUBJECT });
+    const lines = headerLines(msg);
+    expect(countHeader(lines, 'Subject')).toBe(1);
+    expect(countHeader(lines, 'From')).toBe(1);
+    expect(countHeader(lines, 'Reply-To')).toBe(0);
+    expect(body(msg)).not.toContain('<h1>Forged</h1>');
+  });
+
+  it('an encoded-word subject that decodes to CRLF (the audit repro) stays one header line', () => {
+    const payload = 'Contract update\r\nX-Injected: yes\r\nReply-To: attacker@evil.example';
+    const encoded = `=?UTF-8?B?${Buffer.from(payload, 'utf8').toString('base64')}?=`;
+    const decoded = decodeEncodedWord(encoded);
+    expect(decoded).toBe(payload);
+
+    // Both the localized subject (what the banner passes) and the raw original.
+    for (const msg of [
+      buildMdnMessage({ ...base, subject: `Read: ${decoded}` }),
+      buildMdnMessage({ ...base, originalSubject: decoded }),
+    ]) {
+      expect(msg).not.toMatch(/^X-Injected:/m);
+      expect(msg).not.toMatch(/^Reply-To:/m);
+      expect(headerLines(msg).find((l) => /^Subject:/.test(l))).toBe(
+        'Subject: Read: Contract update X-Injected: yes Reply-To: attacker@evil.example',
+      );
+    }
+  });
+
+  it('bare LF and a blank line cannot start extra headers or a body', () => {
+    const msg = buildMdnMessage({
+      ...base,
+      subject: 'Hello\nBcc: leak@evil.test\n\nForged body',
+      originalSubject: 'Hello\n\nForged body',
+    });
+    const lines = headerLines(msg);
+    expect(countHeader(lines, 'Bcc')).toBe(0);
+    expect(lines.find((l) => /^Subject:/.test(l))).toBe('Subject: Hello Bcc: leak@evil.test Forged body');
+    // Our own header block is intact up to the multipart boundary parameter.
+    expect(lines[lines.length - 1]).toMatch(/^\tboundary="/);
+    expect(body(msg)).not.toContain('Forged body');
+  });
+
+  it('a subject that is non-ASCII *and* carries CRLF is stripped before encoding', () => {
+    const msg = buildMdnMessage({ ...base, subject: 'Über\r\nReply-To: x@evil.test\r\n' });
+    const lines = headerLines(msg);
+    expect(countHeader(lines, 'Reply-To')).toBe(0);
+    const subject = lines.find((l) => /^Subject:/.test(l))!;
+    expect(subject).toMatch(/^Subject: =\?UTF-8\?B\?[A-Za-z0-9+/=]+\?=$/);
+    const decoded = decodeEncodedWord(subject.slice('Subject: '.length));
+    expect(decoded).toBe('Über Reply-To: x@evil.test');
+    expect(decoded).not.toMatch(/[\r\n]/);
+  });
+
+  it('other C0 controls (NUL, lone CR, lone LF, VT, FF) are removed too', () => {
+    const msg = buildMdnMessage({ ...base, subject: 'a\0b\rc\nd\x0be\x0cf\x7fg' });
+    expect(headerLines(msg).find((l) => /^Subject:/.test(l))).toBe('Subject: a b c d e f g');
+  });
+
+  it('From display name cannot inject headers and is quoted when it uses specials', () => {
+    const injected = buildMdnMessage({ ...base, fromName: 'Me\r\nReply-To: x@evil.test' });
+    const lines = headerLines(injected);
+    expect(countHeader(lines, 'Reply-To')).toBe(0);
+    expect(lines.find((l) => /^From:/.test(l))).toBe('From: "Me Reply-To: x@evil.test" <me@example.com>');
+
+    const quoted = buildMdnMessage({ ...base, fromName: 'Rath, Linus' });
+    expect(quoted).toMatch(/^From: "Rath, Linus" <me@example\.com>$/m);
+
+    const plain = buildMdnMessage({ ...base, fromName: 'Linus Rath' });
+    expect(plain).toMatch(/^From: Linus Rath <me@example\.com>$/m);
+  });
+
+  it('address atoms (to, fromEmail, originalRecipient) drop controls and whitespace', () => {
+    const msg = buildMdnMessage({
+      ...base,
+      to: 'sender@other.com\r\nBcc: leak@evil.test',
+      fromEmail: 'me@example.com\r\nX-Injected: 1',
+      originalRecipient: 'alias@example.com\nX-Also: 2',
+    });
+    const lines = headerLines(msg);
+    expect(countHeader(lines, 'Bcc')).toBe(0);
+    expect(msg).not.toMatch(/^X-Injected:/m);
+    expect(msg).not.toMatch(/^X-Also:/m);
+    expect(lines.find((l) => /^To:/.test(l))).toBe('To: sender@other.comBcc:leak@evil.test');
+    expect(lines.find((l) => /^From:/.test(l))).toBe('From: me@example.comX-Injected:1');
+    expect(msg).toContain('Original-Recipient: rfc822;alias@example.comX-Also:2');
+    expect(msg).toContain('Final-Recipient: rfc822;alias@example.comX-Also:2');
+  });
+
+  it('a Disposition-Notification-To with a blank line cannot append a body', () => {
+    const msg = buildMdnMessage({ ...base, to: 'sender@other.com\r\n\r\nForged body' });
+    expect(headerLines(msg).find((l) => /^To:/.test(l))).toBe('To: sender@other.comForgedbody');
+    expect(body(msg)).not.toContain('Forged');
+  });
+
+  it('Message-ID from the original cannot inject into In-Reply-To / Original-Message-ID', () => {
+    const msg = buildMdnMessage({
+      ...base,
+      originalMessageId: ['<orig@other.com>\r\nReply-To: x@evil.test'],
+    });
+    expect(msg).not.toMatch(/^Reply-To:/m);
+    expect(msg).toMatch(/^In-Reply-To: <orig@other\.com>Reply-To:x@evil\.test$/m);
+    expect(msg).toMatch(/^Original-Message-ID: <orig@other\.com>Reply-To:x@evil\.test$/m);
+  });
+
+  it('Reporting-UA cannot inject into the disposition-notification part', () => {
+    const msg = buildMdnMessage({ ...base, reportingUa: 'ua\r\nDisposition: deleted' });
+    expect(msg.match(/^Disposition:/gm)).toHaveLength(1);
+    expect(msg).toMatch(/^Reporting-UA: ua Disposition: deleted$/m);
+  });
+
+  it('never emits a header line containing a bare CR or LF for any input', () => {
+    const nasty = 'x\r\ny\rz\nw\r\n\r\nv';
+    const msg = buildMdnMessage({
+      to: nasty,
+      fromEmail: nasty,
+      fromName: nasty,
+      originalMessageId: nasty,
+      originalSubject: nasty,
+      originalRecipient: nasty,
+      reportingUa: nasty,
+      subject: nasty,
+    });
+    // Every line of the message is terminated by CRLF only; no stray CR/LF inside.
+    for (const line of msg.split('\r\n')) {
+      expect(line).not.toMatch(/[\r\n]/);
+    }
+    const lines = headerLines(msg);
+    expect(countHeader(lines, 'From')).toBe(1);
+    expect(countHeader(lines, 'To')).toBe(1);
+    expect(countHeader(lines, 'Subject')).toBe(1);
+  });
+});
diff --git a/src/lib/mdn.ts b/src/lib/mdn.ts
index 5a08f23b..16befa64 100644
--- a/src/lib/mdn.ts
+++ b/src/lib/mdn.ts
@@ -73,17 +73,50 @@ function base64Body(text: string): string {
   return (utf8ToBase64(text).match(/.{1,76}/g) || []).join('\r\n');
 }
 
-/** RFC 2047 encoded-word for header values that contain non-ASCII characters. */
-export function encodeHeaderWord(value: string): string {
+/**
+ * Remove the characters that must never reach a header we assemble as text:
+ * CR, LF, the remaining C0 controls and DEL. Runs collapse to one space.
+ *
+ * Every value that lands in a header here ultimately comes from the message
+ * we are answering (its Subject, Message-ID, Disposition-Notification-To),
+ * and Stalwart hands those back already RFC 2047-decoded. A sender can
+ * therefore smuggle a bare CRLF past SMTP inside an encoded-word; if it were
+ * interpolated verbatim it would terminate the header and let the sender
+ * append headers and a body of their choosing to a message the user's own
+ * account submits (GHSA-w38p-hpqv-g89c).
+ */
+function stripHeaderControls(value: string): string {
   // eslint-disable-next-line no-control-regex
-  if (!/[^\x00-\x7F]/.test(value)) return value;
-  return `=?UTF-8?B?${utf8ToBase64(value)}?=`;
+  return value.replace(/[\x00-\x1F\x7F]+/g, ' ').trim();
+}
+
+/** Address-like atoms (mailboxes, message-ids) may contain no whitespace at all. */
+function headerAtom(value: string): string {
+  // eslint-disable-next-line no-control-regex
+  return value.replace(/[\x00-\x20\x7F]+/g, '');
+}
+
+/**
+ * Header value safe to place after "Name: ". Control characters are stripped
+ * first; the result is RFC 2047-encoded when it contains non-ASCII.
+ */
+export function encodeHeaderWord(value: string): string {
+  const clean = stripHeaderControls(value);
+  if (!/[^\x20-\x7E]/.test(clean)) return clean;
+  return `=?UTF-8?B?${utf8ToBase64(clean)}?=`;
+}
+
+/** Display-name for a mailbox: encoded-word if non-ASCII, quoted if it uses specials. */
+function displayName(value: string): string {
+  const encoded = encodeHeaderWord(value);
+  if (encoded.startsWith('=?') || !/[()<>[\]:;@\\,."]/.test(encoded)) return encoded;
+  return `"${encoded.replace(/["\\]/g, '\\$&')}"`;
 }
 
 function ensureAngles(messageId: string | string[] | null | undefined): string {
   const raw = Array.isArray(messageId) ? messageId[0] : messageId;
   if (typeof raw !== 'string') return '';
-  const trimmed = raw.trim();
+  const trimmed = headerAtom(raw);
   if (!trimmed) return '';
   return trimmed.startsWith('<') ? trimmed : `<${trimmed}>`;
 }
@@ -98,15 +131,19 @@ function randomToken(): string {
  * by the MIME standard so the bytes import/transmit verbatim.
  */
 export function buildMdnMessage(opts: MdnOptions): string {
-  const finalRecipient = opts.originalRecipient || opts.fromEmail;
-  const domain = (opts.fromEmail.split('@')[1] || 'localhost').trim();
+  // Everything interpolated into a header line goes through headerAtom /
+  // encodeHeaderWord so that no CR/LF (or other control) can split a line.
+  const to = headerAtom(opts.to);
+  const fromEmail = headerAtom(opts.fromEmail);
+  const originalRecipient = opts.originalRecipient ? headerAtom(opts.originalRecipient) : '';
+  const finalRecipient = originalRecipient || fromEmail;
+  const domain = fromEmail.split('@')[1] || 'localhost';
   const messageId = `<mdn.${randomToken()}@${domain}>`;
   const boundary = `----=_MDN_${randomToken()}`;
   const origMsgId = ensureAngles(opts.originalMessageId);
 
-  const fromHeader = opts.fromName
-    ? `${encodeHeaderWord(opts.fromName)} <${opts.fromEmail}>`
-    : opts.fromEmail;
+  const fromName = opts.fromName ? displayName(opts.fromName) : '';
+  const fromHeader = fromName ? `${fromName} <${fromEmail}>` : fromEmail;
 
   const subject = encodeHeaderWord(
     opts.subject ?? `Read: ${opts.originalSubject || ''}`.trim(),
@@ -116,7 +153,7 @@ export function buildMdnMessage(opts: MdnOptions): string {
     ? 'automatic-action/MDN-sent-automatically; displayed'
     : 'manual-action/MDN-sent-manually; displayed';
 
-  const reportingUa = opts.reportingUa || `${domain}; Bulwark Mobile`;
+  const reportingUa = stripHeaderControls(opts.reportingUa || '') || `${domain}; Bulwark Mobile`;
 
   const humanText = opts.humanText ?? [
     `This is a return receipt for the message you sent to ${finalRecipient}.`,
@@ -129,7 +166,7 @@ export function buildMdnMessage(opts: MdnOptions): string {
   const mdnFields = [
     `Reporting-UA: ${reportingUa}`,
     `Final-Recipient: rfc822;${finalRecipient}`,
-    ...(opts.originalRecipient ? [`Original-Recipient: rfc822;${opts.originalRecipient}`] : []),
+    ...(originalRecipient ? [`Original-Recipient: rfc822;${originalRecipient}`] : []),
     ...(origMsgId ? [`Original-Message-ID: ${origMsgId}`] : []),
     `Disposition: ${disposition}`,
   ].join('\r\n');
@@ -137,7 +174,7 @@ export function buildMdnMessage(opts: MdnOptions): string {
   return [
     `Date: ${rfc5322Date()}`,
     `From: ${fromHeader}`,
-    `To: ${opts.to}`,
+    `To: ${to}`,
     `Subject: ${subject}`,
     `Message-ID: ${messageId}`,
     ...(origMsgId ? [`In-Reply-To: ${origMsgId}`] : []),