Tuesday Mar 10, 2009

The Overhaul of Java UTF-8 Charset

The UTF-8 charset implementation (in all JDK/JRE releases from Sun) has been updated recently to reject non-shortest-form UTF-8 byte sequences, since the old implementation might be leveraged in security attacks. Since then I have been asked many times about what this "non-shortest-form" issue is and what the possible impact might be, so here are the answers.

The first question usually goes "what is the non-shortest-form issue"? The detailed and official answer can be found at Unicode Corrigendum #1: UTF-8 Shortest Form. Put it in simple words, the problem is that the Unicode characters could be represented in more than one way (form) in the "UTF-8 encoding" that many people think/believe it is. When be asked what the UTF-8 encoding looks like, the easy/simple/brief explain would be the bit pattern showed below


# Bits Bit pattern
1 7 0xxxxxxx
2 11 110xxxxx 10xxxxxx
3 16 1110xxxx 10xxxxxx 10xxxxxx
4 21 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx

It's close, but it's actually WRONG, based on the latest definition/spec of UTF-8. The pattern above has a loophole that you can actually have more than one form to represent a Unicode character. For example, for ASCII characters from u+0000 to u+007f, the UTF-8 encoding form maintains transparency for all of them, so they keep their ASCII code values of 0x00..0x7f (in one-byte form) in UTF-8. However if based on the above pattern, These characters can also be represented in 2-bytes form as [c0, 80]..[c1, bf], the "non-shortest-form". The code below shows all of these non-shortest-2-bytes-form for these ASCII characters, if you run code against the "OLD" version of JDK/JRE.


byte[] bb = new byte[2];
for (int b1 = 0xc0; b1 < 0xc2; b1++) {
for (int b2 = 0x80; b2 < 0xc0; b2++) {
bb[0] = (byte)b1;
bb[1] = (byte)b2;
String cstr = new String(bb, "UTF8");
char c = cstr.toCharArray()[0];
System.out.printf("[%02x, %02x] -> U+%04x [%s]%n",
b1, b2, c & 0xffff, (c>=0x20)?cstr:"ctrl");
}
}

The output would be


...
[c0, a0] -> U+0020 [ ]
[c0, a1] -> U+0021 [!]
...
[c0, b6] -> U+0036 [6]
[c0, b7] -> U+0037 [7]
[c0, b8] -> U+0038 [8]
[c0, b9] -> U+0039 [9]
...
[c1, 80] -> U+0040 [@]
[c1, 81] -> U+0041 [A]
[c1, 82] -> U+0042 [B]
[c1, 83] -> U+0043 [C]
[c1, 84] -> U+0044 [D]
...

so for a string like "ABC" you would have two forms of UTF-8 sequences

"0x41 0x42 0x43" and "0xc1 0x81 0xc1 0x82 0xc1 0x83"

The Unicode Corrigendum #1: UTF-8 Shortest Form specifies explicitly that

"The definition of each UTF specifies the illegal code unit sequences in that UTF. For example, the definition of UTF-8 (D36) specifies that code unit sequences such as [C0, AF] are ILLEGAL."

Our old implementation accepts those non-shortest-form (while never generates them when encoding). The new UTF_8 charset now rejects the non-shortest-form byte sequences for all BMP characters, only the "legal byte sequences" listed below are accepted.


/\* Legal UTF-8 Byte Sequences
\*
\* # Code Points Bits Bit/Byte pattern
\* 1 7 0xxxxxxx
\* U+0000..U+007F 00..7F
\* 2 11 110xxxxx 10xxxxxx
\* U+0080..U+07FF C2..DF 80..BF
\* 3 16 1110xxxx 10xxxxxx 10xxxxxx
\* U+0800..U+0FFF E0 A0..BF 80..BF
\* U+1000..U+FFFF E1..EF 80..BF 80..BF
\* 4 21 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
\* U+10000..U+3FFFF F0 90..BF 80..BF 80..BF
\* U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
\* U+100000..U10FFFF F4 80..8F 80..BF 80..BF
\*/

The next question would be "What would be the issue/problem if we keep using the old version of JDK/JRE?".

First, I'm not a lawyer...oops, I meant I'm not a security expert:-) so my word does not count. So we consulted with our security experts' opinion. Their conclusion is "it is NOT a security vulnerability in Java SE per se, but it may be leveraged to attack systems running software that relies on the UTF-8 charset to reject these non-shortest form of UTF-8 sequences".

A simple scenario that might give you an idea about what the above "may be leveraged to attack..." really means is

(1)A Java application would like to filter the incoming UTF-8 input stream to reject certain key words, for example "ABC"

(2)Instead of decoding the input UTF-8 byte sequences into Java char representation and then filter out the keyword string "ABC" at Java "char" level, for example,


String utfStr = new String(bytes, "UTF-8");
if ("ABC".equals(strUTF)) { ... }

The application might choose to filter the raw UTF-8 byte sequences "0x41 0x42 0x43" (only) directly against the UTF-8 byte input stream and then rely on (assume) the Java UTF-8 charset to reject any other non-shortest-form of the target keyword, if there is any.

(3)The consequence is the non-shortest form input "0xc1 0x81 0xc1 0x82 0xc1 0x83" will penetrate the filter and trigger a possible security vulnerability, if the underlying JDK/JRE runtime is an OLD version.


So the recommendation is to update to the latest JDK/JRE releases to avoid the potential risk.

Wait, there is also a big bonus for the updating. The UTF-8 charset implementation has not been updated/touched for years, given the fact that the UTF-8 encoding is so widely used (the default encoding for the XML and more and more websites use UTF-8 as their page encoding), we have been taking the "defensive" position of "don't change it if it works" the past years. So Martin and I decided to take this as an opportunity to give it a speed boost as well. The data below is taken from one of my benchmark (this is NOT an official benchmark, provided to give a rough idea of the performance boost) run data which compares the decoding/encoding operations of new implementation and old implementation under -server vm. The new implementation is much faster, especially when de/encoding the single bytes (those ASCIIs). The new decoding and encoding are faster under -client vm as well, but the gap is not as big as in -server vm,I wanted to show you the best:-)


Method Millis Millis(OLD)
Decoding 1b UTF-8 : 1786 12689
Decoding 2b UTF-8 : 21061 30769
Decoding 3b UTF-8 : 23412 44256
Decoding 4b UTF-8 : 30732 35909
Decoding 1b (direct)UTF-8 : 16015 22352
Decoding 2b (direct)UTF-8 : 63813 82686
Decoding 3b (direct)UTF-8 : 89999 111579
Decoding 4b (direct)UTF-8 : 73126 60366
Encoding 1b UTF-8 : 2528 12713
Encoding 2b UTF-8 : 14372 33246
Encoding 3b UTF-8 : 25734 26000
Encoding 4b UTF-8 : 23293 31629
Encoding 1b (direct)UTF-8 : 18776 19883
Encoding 2b (direct)UTF-8 : 50309 59327
Encoding 3b (direct)UTF-8 : 77006 74286
Encoding 4b (direct)UTF-8 : 61626 66517

The new UTF-8 charset implementation has been integrated in
JDK7, JDK6-open, JDK6-u11 and later, JDK5.0u17 and 1.4.2_19.

And if you are interested in what the change looks like, you can take a peek on the webrev of new UTF_8.java for OpenJDK7.

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