diff --git a/lib/bcoin/script.js b/lib/bcoin/script.js
index 0adc2478..35a8a3c9 100644
--- a/lib/bcoin/script.js
+++ b/lib/bcoin/script.js
@@ -176,6 +176,9 @@ script.execute = function execute(s, stack, tx, index) {
       if (!constants.rhashType[type & 0x7f])
         return false;
 
+      if (!script.isValidSig(sig))
+        return false;
+
       var subscript = input.out.tx.getSubscript(input.out.index);
       var hash = tx.subscriptHash(index, subscript, type);
 
@@ -226,6 +229,9 @@ script.execute = function execute(s, stack, tx, index) {
         var subscript = input.out.tx.getSubscript(input.out.index);
         var hash = tx.subscriptHash(index, subscript, type);
 
+        if (!script.isValidSig(sig))
+          return false;
+
         var res = false;
         for (; !res && j < n; j++)
           res = script.verify(hash, sig.slice(0, -1), keys[j]);
@@ -413,3 +419,98 @@ script.isNullData = function isNullData(s) {
          Array.isArray(s[1]) &&
          s[1].length <= 40;
 };
+
+// https://github.com/bitcoin/bips/blob/master/bip-0066.mediawiki
+/**
+ * A canonical signature exists of: <30> <total len> <02> <len R> <R> <02> <len S> <S> <hashtype>
+ * Where R and S are not negative (their first byte has its highest bit not set), and not
+ * excessively padded (do not start with a 0 byte, unless an otherwise negative number follows,
+ * in which case a single 0 byte is necessary and even required).
+ *
+ * See https://bitcointalk.org/index.php?topic=8392.msg127623#msg127623
+ *
+ * This function is consensus-critical since BIP66.
+ */
+script.isValidSig = function(sig) {
+  // Empty signature. Not strictly DER encoded, but allowed to provide a
+  // compact way to provide an invalid signature for use with CHECK(MULTI)SIG
+  if (sig.length === 0)
+    return true;
+
+  // Format: 0x30 [total-length] 0x02 [R-length] [R] 0x02 [S-length] [S] [sighash]
+  // * total-length: 1-byte length descriptor of everything that follows,
+  //   excluding the sighash byte.
+  // * R-length: 1-byte length descriptor of the R value that follows.
+  // * R: arbitrary-length big-endian encoded R value. It must use the shortest
+  //   possible encoding for a positive integers (which means no null bytes at
+  //   the start, except a single one when the next byte has its highest bit set).
+  // * S-length: 1-byte length descriptor of the S value that follows.
+  // * S: arbitrary-length big-endian encoded S value. The same rules apply.
+  // * sighash: 1-byte value indicating what data is hashed (not part of the DER
+  //   signature)
+
+  // Minimum and maximum size constraints.
+  if (sig.length < 9)
+    return false;
+  if (sig.length > 73)
+    return false;
+
+  // A signature is of type 0x30 (compound).
+  if (sig[0] !== 0x30)
+    return false;
+
+  // Make sure the length covers the entire signature.
+  if (sig[1] !== sig.length - 3)
+    return false;
+
+  // Extract the length of the R element.
+  var lenR = sig[3];
+
+  // Make sure the length of the S element is still inside the signature.
+  if (5 + lenR >= sig.length)
+    return false;
+
+  // Extract the length of the S element.
+  var lenS = sig[5 + lenR];
+
+  // Verify that the length of the signature matches the sum of the length
+  // of the elements.
+  if (lenR + lenS + 7 !== sig.length)
+    return false;
+
+  // Check whether the R element is an integer.
+  if (sig[2] !== 0x02)
+    return false;
+
+  // Zero-length integers are not allowed for R.
+  if (lenR === 0)
+    return false;
+
+  // Negative numbers are not allowed for R.
+  if (sig[4] & 0x80)
+    return false;
+
+  // Null bytes at the start of R are not allowed, unless R would
+  // otherwise be interpreted as a negative number.
+  if (lenR > 1 && (sig[4] === 0x00) && !(sig[5] & 0x80))
+    return false;
+
+  // Check whether the S element is an integer.
+  if (sig[lenR + 4] !== 0x02)
+    return false;
+
+  // Zero-length integers are not allowed for S.
+  if (lenS === 0)
+    return false;
+
+  // Negative numbers are not allowed for S.
+  if (sig[lenR + 6] & 0x80)
+    return false;
+
+  // Null bytes at the start of S are not allowed, unless S would otherwise be
+  // interpreted as a negative number.
+  if (lenS > 1 && (sig[lenR + 6] === 0x00) && !(sig[lenR + 7] & 0x80))
+    return false;
+
+  return true;
+};