--- a/security/pkix/lib/pkixcert.cpp
+++ b/security/pkix/lib/pkixcert.cpp
@@ -121,74 +121,73 @@ BackCert::Init()
   if (rv != Success) {
     return rv;
   }
 
   static const uint8_t CSC = der::CONTEXT_SPECIFIC | der::CONSTRUCTED;
 
   // RFC 5280 says: "These fields MUST only appear if the version is 2 or 3
   // (Section 4.1.2.1). These fields MUST NOT appear if the version is 1."
-  // However, for compatibility reasons, we will parse v1/v2 certificates 
-  // according to the v3 rules, and thus accept these fields. The same
-  // rules also apply to "v4" certificates, since these are typically
-  // actually v3 certificates, but generated by software with an off-by-one
-  // error.
+  if (version != der::Version::v1) {
 
-  // Ignore issuerUniqueID if present.
-  if (tbsCertificate.Peek(CSC | 1)) {
-    rv = der::ExpectTagAndSkipValue(tbsCertificate, CSC | 1);
-    if (rv != Success) {
-      return rv;
+    // Ignore issuerUniqueID if present.
+    if (tbsCertificate.Peek(CSC | 1)) {
+      rv = der::ExpectTagAndSkipValue(tbsCertificate, CSC | 1);
+      if (rv != Success) {
+        return rv;
+      }
     }
-  }
 
-  // Ignore subjectUniqueID if present.
-  if (tbsCertificate.Peek(CSC | 2)) {
-    rv = der::ExpectTagAndSkipValue(tbsCertificate, CSC | 2);
-    if (rv != Success) {
-      return rv;
+    // Ignore subjectUniqueID if present.
+    if (tbsCertificate.Peek(CSC | 2)) {
+      rv = der::ExpectTagAndSkipValue(tbsCertificate, CSC | 2);
+      if (rv != Success) {
+        return rv;
+      }
     }
   }
 
-  // RFC 5280 only defines the use of extensions for v3 certificates.
-  // However, for compatibility reasons, we treat v1, v2, v3, and v4 in
-  // the same way, allowing and parsing extensions in all cases.
-  rv = der::OptionalExtensions(tbsCertificate, CSC | 3,
-                               bind(&BackCert::RememberExtension, this, _1,
-                                    _2, _3, _4));
-  if (rv != Success) {
-    return rv;
-  }
-
-  // The Netscape Certificate Type extension is an obsolete
-  // Netscape-proprietary mechanism that we ignore in favor of the standard
-  // extensions. However, some CAs have issued certificates with the Netscape
-  // Cert Type extension marked critical. Thus, for compatibility reasons, we
-  // "understand" this extension by ignoring it when it is not critical, and
-  // by ensuring that the equivalent standardized extensions are present when
-  // it is marked critical, based on the assumption that the information in
-  // the Netscape Cert Type extension is consistent with the information in
-  // the standard extensions.
-  //
-  // Here is a mapping between the Netscape Cert Type extension and the
-  // standard extensions:
-  //
-  // Netscape Cert Type  |  BasicConstraints.cA  |  Extended Key Usage
-  // --------------------+-----------------------+----------------------
-  // SSL Server          |  false                |  id_kp_serverAuth
-  // SSL Client          |  false                |  id_kp_clientAuth
-  // S/MIME Client       |  false                |  id_kp_emailProtection
-  // Object Signing      |  false                |  id_kp_codeSigning
-  // SSL Server CA       |  true                 |  id_pk_serverAuth
-  // SSL Client CA       |  true                 |  id_kp_clientAuth
-  // S/MIME CA           |  true                 |  id_kp_emailProtection
-  // Object Signing CA   |  true                 |  id_kp_codeSigning
-  if (criticalNetscapeCertificateType.GetLength() > 0 &&
-      (basicConstraints.GetLength() == 0 || extKeyUsage.GetLength() == 0)) {
-    return Result::ERROR_UNKNOWN_CRITICAL_EXTENSION;
+  // Extensions were added in v3, so only accept extensions in v3 certificates.
+  // v4 certificates are not defined but there are some certificates issued
+  // with v4 that expect v3 decoding. For compatibility reasons we handle them
+  // as v3 certificates.
+  if (version == der::Version::v3 || version == der::Version::v4) {
+    rv = der::OptionalExtensions(tbsCertificate, CSC | 3,
+                                 bind(&BackCert::RememberExtension, this, _1,
+                                      _2, _3, _4));
+    if (rv != Success) {
+      return rv;
+    }
+    // The Netscape Certificate Type extension is an obsolete
+    // Netscape-proprietary mechanism that we ignore in favor of the standard
+    // extensions. However, some CAs have issued certificates with the Netscape
+    // Cert Type extension marked critical. Thus, for compatibility reasons, we
+    // "understand" this extension by ignoring it when it is not critical, and
+    // by ensuring that the equivalent standardized extensions are present when
+    // it is marked critical, based on the assumption that the information in
+    // the Netscape Cert Type extension is consistent with the information in
+    // the standard extensions.
+    //
+    // Here is a mapping between the Netscape Cert Type extension and the
+    // standard extensions:
+    //
+    // Netscape Cert Type  |  BasicConstraints.cA  |  Extended Key Usage
+    // --------------------+-----------------------+----------------------
+    // SSL Server          |  false                |  id_kp_serverAuth
+    // SSL Client          |  false                |  id_kp_clientAuth
+    // S/MIME Client       |  false                |  id_kp_emailProtection
+    // Object Signing      |  false                |  id_kp_codeSigning
+    // SSL Server CA       |  true                 |  id_pk_serverAuth
+    // SSL Client CA       |  true                 |  id_kp_clientAuth
+    // S/MIME CA           |  true                 |  id_kp_emailProtection
+    // Object Signing CA   |  true                 |  id_kp_codeSigning
+    if (criticalNetscapeCertificateType.GetLength() > 0 &&
+        (basicConstraints.GetLength() == 0 || extKeyUsage.GetLength() == 0)) {
+      return Result::ERROR_UNKNOWN_CRITICAL_EXTENSION;
+    }
   }
 
   return der::End(tbsCertificate);
 }
 
 // XXX: The second value is of type |const Input&| instead of type |Input| due
 // to limitations in our std::bind polyfill.
 Result