diff --git a/lib/web/cookies/index.js b/lib/web/cookies/index.js
index be99f6f3dd2..8ebff2b1a4c 100644
--- a/lib/web/cookies/index.js
+++ b/lib/web/cookies/index.js
@@ -32,8 +32,10 @@ function getCookies (headers) {
 
   const cookie = headers.get('cookie')
 
+  // A null prototype keeps a cookie named `__proto__` from hitting the
+  // Object.prototype setter, which would silently drop it.
   /** @type {Record<string, string>} */
-  const out = {}
+  const out = { __proto__: null }
 
   if (!cookie) {
     return out
diff --git a/lib/web/cookies/util.js b/lib/web/cookies/util.js
index ca408e153ab..d4b0bb76cfe 100644
--- a/lib/web/cookies/util.js
+++ b/lib/web/cookies/util.js
@@ -315,8 +315,27 @@ function stringify (cookie) {
     out.push(`Path=${cookie.path}`)
   }
 
-  if (cookie.expires && cookie.expires.toString() !== 'Invalid Date') {
-    out.push(`Expires=${toIMFDate(cookie.expires)}`)
+  // A numeric 0 is the Unix epoch, not an absent value -- the same reason the
+  // Max-Age check above tests the type rather than truthiness.
+  //
+  // expires is Date | number, so the "Invalid Date" check only guards the Date
+  // form. A number that is not a valid time value (NaN, +/-Infinity, or outside
+  // the Date range) would otherwise reach toIMFDate, which splices undefined and
+  // NaN into the header value instead of failing.
+  if (cookie.expires != null) {
+    const expiresMs = cookie.expires instanceof Date
+      ? cookie.expires.getTime()
+      : cookie.expires
+
+    // The largest time value a Date can represent, in milliseconds, i.e.
+    // 100,000,000 days either side of the epoch (ECMA-262 Date Time Values).
+    // Anything within it is safe for toIMFDate to format; anything beyond it
+    // produces an invalid Date and must be ignored instead.
+    const maxTimeValue = 8640000000000000
+
+    if (typeof expiresMs === 'number' && Number.isFinite(expiresMs) && Math.abs(expiresMs) <= maxTimeValue) {
+      out.push(`Expires=${toIMFDate(cookie.expires)}`)
+    }
   }
 
   if (cookie.sameSite) {
diff --git a/test/cookie/cookies.js b/test/cookie/cookies.js
index 32752a5f7db..429a04f0c35 100644
--- a/test/cookie/cookies.js
+++ b/test/cookie/cookies.js
@@ -59,6 +59,24 @@ test('Cookie parser', () => {
   })
 })
 
+test('Cookie parser keeps a cookie named __proto__', () => {
+  const headers = new Headers()
+  headers.set('Cookie', '__proto__=foo; bar=baz')
+
+  const cookies = getCookies(headers)
+
+  assert.deepStrictEqual(Object.keys(cookies).sort(), ['__proto__', 'bar'])
+  assert.strictEqual(
+    Object.getOwnPropertyDescriptor(cookies, '__proto__').value,
+    'foo'
+  )
+  assert.strictEqual(cookies.bar, 'baz')
+
+  for (const value of Object.values(cookies)) {
+    assert.strictEqual(typeof value, 'string')
+  }
+})
+
 test('Cookie Name Validation', () => {
   const tokens = [
     '"id"',
@@ -379,6 +397,68 @@ test('Cookie Set', () => {
     'Space=Cat; Expires=Fri, 07 Jan 1983 15:32:00 GMT'
   )
 
+  // A numeric 0 is the Unix epoch, the canonical "expire now" value, not an
+  // absent option.
+  headers = new Headers()
+  setCookie(headers, {
+    name: 'Space',
+    value: 'Cat',
+    expires: 0
+  })
+  assert.equal(
+    headers.get('Set-Cookie'),
+    'Space=Cat; Expires=Thu, 01 Jan 1970 00:00:00 GMT'
+  )
+
+  headers = new Headers()
+  setCookie(headers, {
+    name: 'Space',
+    value: 'Cat',
+    expires: null
+  })
+  assert.equal(headers.get('Set-Cookie'), 'Space=Cat')
+
+  // A numeric expires outside the range representable by Date must not be
+  // serialized. Before this was guarded, the numeric form skipped the
+  // "Invalid Date" check and toIMFDate spliced undefined/NaN into the header.
+  for (const expires of [8640000000000001, 1e16, Number.MAX_SAFE_INTEGER]) {
+    headers = new Headers()
+    setCookie(headers, { name: 'Space', value: 'Cat', expires })
+    assert.equal(headers.get('Set-Cookie'), 'Space=Cat')
+  }
+
+  // The maximum representable time value is still serialized.
+  headers = new Headers()
+  setCookie(headers, {
+    name: 'Space',
+    value: 'Cat',
+    expires: 8640000000000000
+  })
+  assert.equal(
+    headers.get('Set-Cookie'),
+    'Space=Cat; Expires=Sat, 13 Sep 275760 00:00:00 GMT'
+  )
+
+  // A non-finite number is coerced to 0 by the WebIDL converter before it
+  // reaches serialization, so it still serializes as the epoch.
+  for (const expires of [Infinity, -Infinity, NaN]) {
+    headers = new Headers()
+    setCookie(headers, { name: 'Space', value: 'Cat', expires })
+    assert.equal(
+      headers.get('Set-Cookie'),
+      'Space=Cat; Expires=Thu, 01 Jan 1970 00:00:00 GMT'
+    )
+  }
+
+  // An invalid Date is ignored, consistent with the out-of-range numeric cases.
+  headers = new Headers()
+  setCookie(headers, {
+    name: 'Space',
+    value: 'Cat',
+    expires: new Date(NaN)
+  })
+  assert.equal(headers.get('Set-Cookie'), 'Space=Cat')
+
   headers = new Headers()
   setCookie(headers, { name: '__Secure-Kitty', value: 'Meow' })
   assert.equal(headers.get('Set-Cookie'), '__Secure-Kitty=Meow; Secure')