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diff --git a/security/manager/ssl/tests/unit/test_cert_signatures.js b/security/manager/ssl/tests/unit/test_cert_signatures.js
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+++ b/security/manager/ssl/tests/unit/test_cert_signatures.js
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+// -*- indent-tabs-mode: nil; js-indent-level: 2 -*-
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+"use strict";
+
+// Tests that certificates cannot be tampered with without being detected.
+// Tests a combination of cases: RSA signatures, ECDSA signatures, certificate
+// chains where the intermediate has been tampered with, chains where the
+// end-entity has been tampered, tampering of the signature, and tampering in
+// the rest of the certificate.
+
+do_get_profile(); // must be called before getting nsIX509CertDB
+var certdb = Cc["@mozilla.org/security/x509certdb;1"].getService(
+  Ci.nsIX509CertDB
+);
+
+// Reads a PEM-encoded certificate, modifies the nth byte (0-indexed), and
+// returns the base64-encoded bytes of the certificate. Negative indices may be
+// specified to modify a byte from the end of the certificate.
+function readAndTamperWithNthByte(certificatePath, n) {
+  let pem = readFile(do_get_file(certificatePath, false));
+  let der = atob(pemToBase64(pem));
+  if (n < 0) {
+    // remember, n is negative at this point
+    n = der.length + n;
+  }
+  let replacement = "\x22";
+  if (der.charCodeAt(n) == replacement) {
+    replacement = "\x23";
+  }
+  der = der.substring(0, n) + replacement + der.substring(n + 1);
+  return btoa(der);
+}
+
+// The signature on certificates appears last. This should modify the contents
+// of the signature such that it no longer validates correctly while still
+// resulting in a structurally valid certificate.
+const BYTE_IN_SIGNATURE = -8;
+function addSignatureTamperedCertificate(certificatePath) {
+  let base64 = readAndTamperWithNthByte(certificatePath, BYTE_IN_SIGNATURE);
+  certdb.addCertFromBase64(base64, ",,");
+}
+
+function ensureSignatureVerificationFailure(certificatePath) {
+  let cert = constructCertFromFile(certificatePath);
+  return checkCertErrorGeneric(
+    certdb,
+    cert,
+    SEC_ERROR_BAD_SIGNATURE,
+    certificateUsageSSLServer
+  );
+}
+
+function tamperWithSignatureAndEnsureVerificationFailure(certificatePath) {
+  let base64 = readAndTamperWithNthByte(certificatePath, BYTE_IN_SIGNATURE);
+  let cert = certdb.constructX509FromBase64(base64);
+  return checkCertErrorGeneric(
+    certdb,
+    cert,
+    SEC_ERROR_BAD_SIGNATURE,
+    certificateUsageSSLServer
+  );
+}
+
+// The beginning of a certificate looks like this (in hex, using DER):
+// 30 XX XX XX [the XX encode length - there are probably 3 bytes here]
+//    30 XX XX XX [length again]
+//       A0 03
+//          02 01
+//             02
+//       02 XX [length again - 1 byte as long as we're using pycert]
+//          XX XX ... [serial number - 20 bytes as long as we're using pycert]
+// Since we want to modify the serial number, we need to change something from
+// byte 15 to byte 34 (0-indexed). If it turns out that the two length sections
+// we assumed were 3 bytes are shorter (they can't be longer), modifying
+// something from byte 15 to byte 30 will still get us what we want. Since the
+// serial number is a DER INTEGER and because it must be positive, it's best to
+// skip the first two bytes of the serial number so as to not run into any
+// issues there. Thus byte 17 is a good byte to modify.
+const BYTE_IN_SERIAL_NUMBER = 17;
+function addSerialNumberTamperedCertificate(certificatePath) {
+  let base64 = readAndTamperWithNthByte(certificatePath, BYTE_IN_SERIAL_NUMBER);
+  certdb.addCertFromBase64(base64, ",,");
+}
+
+function tamperWithSerialNumberAndEnsureVerificationFailure(certificatePath) {
+  let base64 = readAndTamperWithNthByte(certificatePath, BYTE_IN_SERIAL_NUMBER);
+  let cert = certdb.constructX509FromBase64(base64);
+  return checkCertErrorGeneric(
+    certdb,
+    cert,
+    SEC_ERROR_BAD_SIGNATURE,
+    certificateUsageSSLServer
+  );
+}
+
+add_task(async function () {
+  addCertFromFile(certdb, "test_cert_signatures/ca-rsa.pem", "CTu,,");
+  addCertFromFile(certdb, "test_cert_signatures/ca-secp384r1.pem", "CTu,,");
+
+  // Tamper with the signatures on intermediate certificates and ensure that
+  // end-entity certificates issued by those intermediates do not validate
+  // successfully.
+  addSignatureTamperedCertificate("test_cert_signatures/int-rsa.pem");
+  addSignatureTamperedCertificate("test_cert_signatures/int-secp384r1.pem");
+  await ensureSignatureVerificationFailure("test_cert_signatures/ee-rsa.pem");
+  await ensureSignatureVerificationFailure(
+    "test_cert_signatures/ee-secp384r1.pem"
+  );
+
+  // Tamper with the signatures on end-entity certificates and ensure that they
+  // do not validate successfully.
+  await tamperWithSignatureAndEnsureVerificationFailure(
+    "test_cert_signatures/ee-rsa-direct.pem"
+  );
+  await tamperWithSignatureAndEnsureVerificationFailure(
+    "test_cert_signatures/ee-secp384r1-direct.pem"
+  );
+
+  // Tamper with the serial numbers of intermediate certificates and ensure
+  // that end-entity certificates issued by those intermediates do not validate
+  // successfully.
+  addSerialNumberTamperedCertificate("test_cert_signatures/int-rsa.pem");
+  addSerialNumberTamperedCertificate("test_cert_signatures/int-secp384r1.pem");
+  await ensureSignatureVerificationFailure("test_cert_signatures/ee-rsa.pem");
+  await ensureSignatureVerificationFailure(
+    "test_cert_signatures/ee-secp384r1.pem"
+  );
+
+  // Tamper with the serial numbers of end-entity certificates and ensure that
+  // they do not validate successfully.
+  await tamperWithSerialNumberAndEnsureVerificationFailure(
+    "test_cert_signatures/ee-rsa-direct.pem"
+  );
+  await tamperWithSerialNumberAndEnsureVerificationFailure(
+    "test_cert_signatures/ee-secp384r1-direct.pem"
+  );
+});