diff --git a/url/url_canon_path.cc b/url/url_canon_path.cc
index ee1cd962..6e5bfb1a 100644
--- a/url/url_canon_path.cc
+++ b/url/url_canon_path.cc
@@ -162,6 +162,76 @@
   output->set_length(i + 1);
 }
 
+// Looks for problematic nested escape sequences and escapes the output as
+// needed to ensure they can't be misinterpreted.
+//
+// Our concern is that in input escape sequence that's invalid because it
+// contains nested escape sequences might look valid once those are unescaped.
+// For example, "%%300" is not a valid escape sequence, but after unescaping the
+// inner "%30" this becomes "%00" which is valid.  Leaving this in the output
+// string can result in callers re-canonicalizing the string and unescaping this
+// sequence, thus resulting in something fundamentally different than the
+// original input here.  This can cause a variety of problems.
+//
+// This function is called after we've just unescaped a sequence that's within
+// two output characters of a previous '%' that we know didn't begin a valid
+// escape sequence in the input string.  We look for whether the output is going
+// to turn into a valid escape sequence, and if so, convert the initial '%' into
+// an escaped "%25" so the output can't be misinterpreted.
+//
+// |spec| is the input string we're canonicalizing.
+// |next_input_index| is the index of the next unprocessed character in |spec|.
+// |input_len| is the length of |spec|.
+// |last_invalid_percent_index| is the index in |output| of a previously-seen
+// '%' character.  The caller knows this '%' character isn't followed by a valid
+// escape sequence in the input string.
+// |output| is the canonicalized output thus far.  The caller guarantees this
+// ends with a '%' followed by one or two characters, and the '%' is the one
+// pointed to by |last_invalid_percent_index|.  The last character in the string
+// was just unescaped.
+template<typename CHAR>
+void CheckForNestedEscapes(const CHAR* spec,
+                           int next_input_index,
+                           int input_len,
+                           int last_invalid_percent_index,
+                           CanonOutput* output) {
+  const int length = output->length();
+  const char last_unescaped_char = output->at(length - 1);
+
+  // If |output| currently looks like "%c", we need to try appending the next
+  // input character to see if this will result in a problematic escape
+  // sequence.  Note that this won't trigger on the first nested escape of a
+  // two-escape sequence like "%%30%30" -- we'll allow the conversion to
+  // "%0%30" -- but the second nested escape will be caught by this function
+  // when it's called again in that case.
+  const bool append_next_char = last_invalid_percent_index == length - 2;
+  if (append_next_char) {
+    // If the input doesn't contain a 7-bit character next, this case won't be a
+    // problem.
+    if ((next_input_index == input_len) || (spec[next_input_index] >= 0x80))
+      return;
+    output->push_back(static_cast<char>(spec[next_input_index]));
+  }
+
+  // Now output ends like "%cc".  Try to unescape this.
+  int begin = last_invalid_percent_index;
+  unsigned char temp;
+  if (DecodeEscaped(output->data(), &begin, output->length(), &temp)) {
+    // New escape sequence found.  Overwrite the characters following the '%'
+    // with "25", and push_back() the one or two characters that were following
+    // the '%' when we were called.
+    if (!append_next_char)
+      output->push_back(output->at(last_invalid_percent_index + 1));
+    output->set(last_invalid_percent_index + 1, '2');
+    output->set(last_invalid_percent_index + 2, '5');
+    output->push_back(last_unescaped_char);
+  } else if (append_next_char) {
+    // Not a valid escape sequence, but we still need to undo appending the next
+    // source character so the caller can process it normally.
+    output->set_length(length);
+  }
+}
+
 // Appends the given path to the output. It assumes that if the input path
 // starts with a slash, it should be copied to the output. If no path has
 // already been appended to the output (the case when not resolving
@@ -182,10 +252,15 @@
                    CanonOutput* output) {
   int end = path.end();
 
+  // We use this variable to minimize the amount of work done when unescaping --
+  // we'll only call CheckForNestedEscapes() when this points at one of the last
+  // couple of characters in |output|.
+  int last_invalid_percent_index = INT_MIN;
+
   bool success = true;
   for (int i = path.begin; i < end; i++) {
     UCHAR uch = static_cast<UCHAR>(spec[i]);
-    if (sizeof(CHAR) > sizeof(char) && uch >= 0x80) {
+    if (sizeof(CHAR) > 1 && uch >= 0x80) {
       // We only need to test wide input for having non-ASCII characters. For
       // narrow input, we'll always just use the lookup table. We don't try to
       // do anything tricky with decoding/validating UTF-8. This function will
@@ -245,33 +320,40 @@
           unsigned char unescaped_value;
           if (DecodeEscaped(spec, &i, end, &unescaped_value)) {
             // Valid escape sequence, see if we keep, reject, or unescape it.
+            // Note that at this point DecodeEscape() will have advanced |i| to
+            // the last character of the escape sequence.
             char unescaped_flags = kPathCharLookup[unescaped_value];
 
             if (unescaped_flags & UNESCAPE) {
-              // This escaped value shouldn't be escaped, copy it.
+              // This escaped value shouldn't be escaped.  Try to copy it.
               output->push_back(unescaped_value);
-            } else if (unescaped_flags & INVALID_BIT) {
-              // Invalid escaped character, copy it and remember the error.
-              output->push_back('%');
-              output->push_back(static_cast<char>(spec[i - 1]));
-              output->push_back(static_cast<char>(spec[i]));
-              success = false;
+              // If we just unescaped a value within 2 output characters of the
+              // '%' from a previously-detected invalid escape sequence, we
+              // might have an input string with problematic nested escape
+              // sequences; detect and fix them.
+              if (last_invalid_percent_index >= (output->length() - 3)) {
+                CheckForNestedEscapes(spec, i + 1, end,
+                                      last_invalid_percent_index, output);
+              }
             } else {
-              // Valid escaped character but we should keep it escaped. We
-              // don't want to change the case of any hex letters in case
-              // the server is sensitive to that, so we just copy the two
-              // characters without checking (DecodeEscape will have advanced
-              // to the last character of the pair).
+              // Either this is an invalid escaped character, or it's a valid
+              // escaped character we should keep escaped.  In the first case we
+              // should just copy it exactly and remember the error.  In the
+              // second we also copy exactly in case the server is sensitive to
+              // changing the case of any hex letters.
               output->push_back('%');
               output->push_back(static_cast<char>(spec[i - 1]));
               output->push_back(static_cast<char>(spec[i]));
+              if (unescaped_flags & INVALID_BIT)
+                success = false;
             }
           } else {
-            // Invalid escape sequence. IE7 rejects any URLs with such
-            // sequences, while Firefox, IE6, and Safari all pass it through
-            // unchanged. We are more permissive unlike IE7. I don't think this
-            // can cause significant problems, if it does, we should change
-            // to be more like IE7.
+            // Invalid escape sequence. IE7+ rejects any URLs with such
+            // sequences, while other browsers pass them through unchanged. We
+            // use the permissive behavior.
+            // TODO(brettw): Consider testing IE's strict behavior, which would
+            // allow removing the code to handle nested escapes above.
+            last_invalid_percent_index = output->length();
             output->push_back('%');
           }
 
diff --git a/url/url_canon_unittest.cc b/url/url_canon_unittest.cc
index 0ccd6c93..b382a7c 100644
--- a/url/url_canon_unittest.cc
+++ b/url/url_canon_unittest.cc
@@ -1060,6 +1060,21 @@
     {"/%7Ffp3%3Eju%3Dduvgw%3Dd", L"/%7Ffp3%3Eju%3Dduvgw%3Dd", "/%7Ffp3%3Eju%3Dduvgw%3Dd", Component(0, 24), true},
       // @ should be passed through unchanged (escaped or unescaped).
     {"/@asdf%40", L"/@asdf%40", "/@asdf%40", Component(0, 9), true},
+      // Nested escape sequences should result in escaping the leading '%' if
+      // unescaping would result in a new escape sequence.
+    {"/%A%42", L"/%A%42", "/%25AB", Component(0, 6), true},
+    {"/%%41B", L"/%%41B", "/%25AB", Component(0, 6), true},
+    {"/%%41%42", L"/%%41%42", "/%25AB", Component(0, 6), true},
+      // Make sure truncated "nested" escapes don't result in reading off the
+      // string end.
+    {"/%%41", L"/%%41", "/%A", Component(0, 3), true},
+      // Don't unescape the leading '%' if unescaping doesn't result in a valid
+      // new escape sequence.
+    {"/%%470", L"/%%470", "/%G0", Component(0, 4), true},
+    {"/%%2D%41", L"/%%2D%41", "/%-A", Component(0, 4), true},
+      // Don't erroneously downcast a UTF-16 charater in a way that makes it
+      // look like part of an escape sequence.
+    {NULL, L"/%%41\x0130", "/%A%C4%B0", Component(0, 9), true},
 
     // ----- encoding tests -----
       // Basic conversions