-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 - ------ TESO Security Advisory 06/10/2001 Multiple vendor Telnet Daemon vulnerability Summary =================== Within most of the current telnet daemons in use today there exist a buffer overflow in the telnet option handling. Under certain circumstances it may be possible to exploit it to gain root priviledges remotely. Systems Affected =================== System | vulnerable | exploitable * ----------------------------------------+--------------+------------------ BSDI 4.x default | yes | yes FreeBSD [2345].x default | yes | yes IRIX 6.5 | yes | no Linux netkit-telnetd < 0.14 | yes | ? Linux netkit-telnetd >= 0.14 | no | NetBSD 1.x default | yes | yes OpenBSD 2.x | yes | ? OpenBSD current | no | Solaris 2.x sparc | yes | ? | yes | ? ----------------------------------------+--------------+------------------ * = From our analysis and conclusions, which may not be correct or we may have overseen things. Do not rely on this. Details about the systems can be found below. Impact =================== Through sending a specially formed option string to the remote telnet daemon a remote attacker might be able to overwrite sensitive information on the static memory pages. If done properly this may result in arbitrary code getting executed on the remote machine under the priviledges the telnet daemon runs on, usually root. Explanation =================== Within every BSD derived telnet daemon under UNIX the telnet options are processed by the 'telrcv' function. This function parses the options according to the telnet protocol and its internal state. During this parsing the results which should be send back to the client are stored within the 'netobuf' buffer. This is done without any bounds checking, since it is assumed that the reply data is smaller than the buffer size (which is BUFSIZ bytes, usually). However, using a combination of options, especially the 'AYT' Are You There option, it is possible to append data to the buffer, usually nine bytes long. To trigger this response, two bytes in the input buffer are necessary. Since this input buffer is BUFSIZ bytes long, you can exceed the output buffer by as much as (BUFSIZ / 2) * 9) - BUFSIZ bytes. For the common case that BUFSIZ is defined to be 1024, this results in a buffer overflow by up to 3584 bytes. On systems where BUFSIZ is defined to be 4096, this is an even greater value (14336). Due to the limited set of characters an attacker is able to write outside of the buffer it is difficult - if not impossible on some systems - to exploit this buffer overflow. Another hurdle for a possible attacker may be the lack of interesting information to modify after the buffer. This buffer overflow should be considered serious nevertheless, since experience has shown that even complicated vulnerabilities can be exploited by skilled attackers, BIND TSIG and SSH deattack come to mind. We have constructed a working exploit for any version of BSDI, NetBSD and FreeBSD. Exploitation on Solaris sparc may be possible but if it is, it is very difficult involving lots of arcane tricks. OpenBSD is not as easily exploitable as the other BSD's, because they do compile with other options by default, changing memory layout. Solution =================== The vendors have been notified of the problem at the same time as the general public, vendor patches for your telnet daemon that fix the bug will show up soon. Sometimes a fix might not be trivial and require a lot of changes to the source code, due to the insecure nature the 'nfrontp' pointer is handled. The best long term solution is to disable the telnet daemon at all, since there are good and free replacements. Acknowledgements =================== The bug has been discovered by scut. The tests and further analysis were done by smiler, lorian, zip and scut. Contact Information =================== The TESO crew can be reached by mailing to teso@team-teso.net Our web page is at http://www.team-teso.net/ References =================== [1] TESO http://www.team-teso.net/ Disclaimer =================== This advisory does not claim to be complete or to be usable for any purpose. Especially information on the vulnerable systems may be inaccurate or wrong. Possibly supplied exploit code is not to be used for malicious purposes, but for educational purposes only. This advisory is free for open distribution in unmodified form. Articles that are based on information from this advisory should include link [1]. Exploit =================== Not this time. Not here. - ------ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.6 (GNU/Linux) Comment: For info see http://www.gnupg.org iD8DBQE7VfBscZZ+BjKdwjcRAsTcAJ9esSlkS7BGkYM1Yulaz3zINqxpmgCeM885 3thubMQc+6S4RpHasL0qz0Y= =VT7y -----END PGP SIGNATURE-----