From proff Thu Sep 12 16:35:11 1996 Received: (proff@localhost) by suburbia.net (8.7.4/Proff-950810) id QAA17583 for best-of-security; Thu, 12 Sep 1996 16:35:10 +1000 Received: from brimstone.netspace.org ([128.148.157.143]) by suburbia.net (8.7.4/Proff-950810) with ESMTP id QAA17421; Thu, 12 Sep 1996 16:24:50 +1000 Received: from netspace.org ([128.148.157.6]) by brimstone.netspace.org with ESMTP id <23930-19893>; Thu, 12 Sep 1996 01:59:37 -0500 Received: from netspace.org (netspace [128.148.157.6]) by netspace.org (8.7/8.6.12) with SMTP id CAA14723; Thu, 12 Sep 1996 02:01:49 -0400 Received: from NETSPACE.ORG by NETSPACE.ORG (LISTSERV-TCP/IP release 1.8b) with spool id 383009 for BUGTRAQ@NETSPACE.ORG; Thu, 12 Sep 1996 01:50:29 +2000 Received: from netspace.org (netspace [128.148.157.6]) by netspace.org (8.7/8.6.12) with SMTP id BAA13589 for ; Thu, 12 Sep 1996 01:49:40 -0400 Approved-By: ALEPH1@UNDERGROUND.ORG Received: from l0pht.com (l0pht.com [199.201.145.1]) by netspace.org (8.7/8.6.12) with ESMTP id NAA31950 for ; Wed, 11 Sep 1996 13:21:14 -0400 Received: from localhost (mudge@localhost) by l0pht.com (8.7.5/8.6.9) with SMTP id MAA18280; Wed, 11 Sep 1996 12:54:24 -0400 (EDT) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Approved-By: What we're dealing with here is a blatant disrespect of the law! Message-ID: Date: Wed, 11 Sep 1996 12:54:23 -0400 Reply-To: "What we're dealing with here is a blatant disrespect of the law!" Sender: proff From: "What we're dealing with here is a blatant disrespect of the law!" Subject: Sendmail 8.7.5 vulnerability X-To: best-of-security@suburbia.net To: Multiple recipients of list BUGTRAQ L0pht Security Advisory Application: Sendmail 8.7.5 Platforms: All Severity: any local user can gain root priveledges. Author: mudge@l0pht.com Scenario: Due to a problem with the code in sendmail a buffer overflow condition exists that allows a user to overwrite the information in a saved stack frame. When the function returns, the saved frame is popped off of the stack and user code can be executed. An exploit script will be made public upon the actual release of Sendmail 8.8 which fixes this particular exploitable code segment. Example: > id uid=621(mudge) gid=200(users) > ./sploit.sh 3883 chfn: rebuilding the database... chfn: done using arg of [0x-------- (hex) + 3883(dec)] # id uid=621(mudge) euid=0(root) gid=200(users) # ./up # id uid=0(root) gid=200(users) If a user is able to alter his/her gecos field then that user can exploit a coding flaw in sendmail to elevate their effective UID to 0. Various operating systems ship with chfn(1) which enables users to change their gecos field. Some of the operating systems that ship with this program are NetBSD, FreeBSD, BSDI, OpenBSD, and Linux. It has not been extensively researched as to what others come out of the box with this functionality. Even if your operating system does not ship with this functionality, it has been witnessed that many service providers offering shell accounts add these, or equivalent utils, in order to minimize their administrative tasks and to facilitate user functionality. No matter, the flaw is a coding problem in sendmail and not the fact that these other programs exist. The actual problem in the code is quite apparent. Inside recipient.c we find the following: char nbuf[MAXNAME + 1]; ... buildfname(pw->pw_gecos, pw->pw_name, nbuf); The problem is that nbuf[MAXNAME + 1] is a fixed length buffer and as we will soon see, buildfname() does not honor this. from util.c: void buildfname(gecos, login, buf) register char *gecos; char *login; char *buf; { register char *p; register char *bp = buf; int l; ... /* now fill in buf */ for (p = gecos; *p != '\0' && *p != ',' && *p != ';' && *p != '%'; p++) { if (*p == '&') { (void) strcpy(bp, login); *bp = toupper(*bp); while (*bp != '\0') bp++; } else *bp++ = *p; } *bp = '\0'; } Here we see that buildfname() happily copies whatever size we can hand it into nbuf[MAXNAME +1]. The function is even nice enough to append a null to the string in case we wanted to put our machine opcodes and operands inside the gecos field. Though this is one way of doing it, we opted for another method that enabled us more freedom with the various methods of altering ones gecos field. Solution: This particular problem has been fixed in Sendmail 8.8 beta. A temporary fix is to remove the ability for users on a local system to change their gecos (commonly referred to as 'real-name') field. mudge@l0pht.com