-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 ========================================================================== 0xbadc0ded Advisory #03 - 2004/05/06 - DeleGate <= 8.9.2 (SSL-filter) ========================================================================== Reference http://0xbadc0ded.org/advisories/0401.txt PGP-key http://0xbadc0ded.org/advisories/pubkey.asc Application DeleGate <= 8.9.2 (SSL-filter) Discovered By Joel Eriksson Researched By Joel Eriksson Overview DeleGate is a multi-purpose application level gateway which runs on a variety of platforms, such as Unix, Windows, MacOS X and OS/2. DeleGate can be used to mediate communication of various protocols, including but not limited to HTTP, FTP, NTTP, SMTP, POP, Telnet and SOCKS. It has the ability to apply caching and conversion for mediated data, access control from clients and routing towards servers. It is also able to translate between protocols and apply SSL (TLS) to arbitrary protocols. Problem A remotely exploitable buffer overflow vulnerability exists in the SSLway filter that is used when SSL should be applied to a client or server connection. The bug can be triggered using a certificate with field contents large enough to make the subject or issuer name larger than 256 bytes. The vulnerability is caused by this piece of code in filters/sslway.c: static ssl_prcert(ssl,show,outssl,outfd,what) SSL *ssl; char *what; { X509 *peer; char subjb[256],*sb,issrb[256],*is; char *dp,ident[256]; ident[0] = 0; if( peer = SSL_get_peer_certificate(ssl) ){ sb = X509_NAME_oneline(X509_get_subject_name(peer),subjb,1024); is = X509_NAME_oneline(X509_get_issuer_name(peer),issrb,1024); The second argument to X509_NAME_oneline() is the buffer to write to and the third argument is the size of that buffer. In the case above a buffer size of 1024 is specified, but the buffers are only 256 bytes large. This allows us to, for instance, overwrite the saved return address in ssl_prcert()'s stack frame. Exploit X509_NAME_oneline() converts chars below 0x20 or above 0x7e to '\xHH' where HH is the hexadecimal value of the char. This makes the bug pretty hard to exploit on at least x86 Unix variants since we will usually need to write chars outside that range to construct a valid address where we can place shellcode. Creative use of a partially overwritten pointer or address is likely to be possible, but I haven't investigated that thouroughly. For instance, the "peer" or "ssl" pointers may be of use, since at least the SSL struct contains pointers to callback functions, and of course partially overwriting the saved ebp or eip is a possibility. Any pointer that is dereferenced and written or called to can possibly be abused. What I find most ironic about this flaw and other flaws where a restricted set of characters can be written is that the address space randomization feature in kernel patches such as PaX can actually make the flaws easier to exploit. The sendmail prescan() bug comes to mind.. The only protection against standard return-to-lib(c) techniques in PaX is address space randomization, with 16 bits of entropy (e.g. 65536 possibilities). This means bruteforcing the offset can be done in a few minutes on most systems. For bugs that the attacker only has one shot to succeed with, randomization may be considered good enough by some. However, in the case of daemons that handle each connection in a separate process (which is the case here) or for local SUID/SGID vulnerabilities, randomization is no adequate protection. To create an SSL certificate that can be used to trigger this bug, it's easiest to use the OpenSSL command line tool: [je@vudo ~]$ cat>openssl.cnf< [ req ] > distinguished_name = req_dn > prompt = no > [ req_dn ] > CN=bof > 0.OU=AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA > 1.OU=AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA > 2.OU=AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA > 3.OU=AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA EOF [je@vudo ~]$ openssl req -x509 -new -nodes -out bof.pem -keyout bof.pem -config openssl.cnf Generating a 512 bit RSA private key .........++++++++++++ ..........++++++++++++ writing new private key to 'bof.pem' For an example of an actual exploit using the return-to-lib(c) technique to defeat address space randomization as provided by PaX, and possibly also restricted charset such as in X509_NAME_oneline(), take a look at: http://0xbadc0ded.org/exploits/pax-poc.tar.gz Fix Upgrade to DeleGate 8.9.3 or edit filters/sslway.c and change: sb = X509_NAME_oneline(X509_get_subject_name(peer),subjb,1024); is = X509_NAME_oneline(X509_get_issuer_name(peer),issrb,1024); to: sb = X509_NAME_oneline(X509_get_subject_name(peer),subjb,sizeof(subjb)); is = X509_NAME_oneline(X509_get_issuer_name(peer),issrb,sizeof(issrb)); Disclosure Timeline 2004/05/02 Notified the DeleGate team 2004/05/05 DeleGate 8.9.3 was released with fix (without notifying us) 2004/05/06 Public release ========================================================================== The 0xbadc0ded.org team is hosted and sponsored by Bitnux: www.bitnux.com ========================================================================== Bitnux is a company located in Sweden focused on security research and system development. We offer services such as: - Code Reviews - Exploit Development - Reverse Engineering of Code - Security Revisions of Systems and Software - Custom System Development for Unix/Linux/BSD and Windows E-mail : info@bitnux.com Phone : +46-70-228 64 16 Chat : http://bitnux.com/live -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.1 (GNU/Linux) iD8DBQFAmiNuqnq6VG/4jhQRAokjAKDQbc79S5KGkawUYBSAyPnVaF6RKACeJimZ d3DpniP2OMX/HiCEHKuAGaA= =jfZJ -----END PGP SIGNATURE-----