[ SOURCE: http://www.secureroot.com/security/advisories/9749181046.html ] -----BEGIN PGP SIGNED MESSAGE----- ============================================================================= FreeBSD-SA-00:69 Security Advisory FreeBSD, Inc. Topic: telnetd allows remote system resource consumption [REVISED] Category: core Module: telnetd Announced: 2000-11-14 Revised: 2000-11-20 Credits: Jouko Pynnonen Affects: FreeBSD 3.x (all releases), FreeBSD 4.x (all releases prior to 4.2), FreeBSD 3.5.1-STABLE and 4.1.1-STABLE prior to the correction date. Corrected: 2000-11-19 (FreeBSD 4.1.1-STABLE) 2000-11-19 (FreeBSD 3.5.1-STABLE) FreeBSD only: NO 0. Revision History v1.0 2000-11-14 Initial release v1.1 2000-11-20 Corrected patch, pointed out by Christos Zoulas I. Background telnetd is the server for the telnet remote login protocol. II. Problem Description The telnet protocol allows for UNIX environment variables to be passed from the client to the user login session on the server. However, some of these environment variables have special meaning to the telnetd child process itself and may be used to affect its operation. Of particular relevance is the ability for remote users to cause an arbitrary file on the system to be searched for termcap data by passing the TERMCAP environment variable. Although any file on the local system can be read since the telnetd server runs as root, the contents of the file will not be reported in any way to the remote user unless it contains a valid termcap entry, in which case the corresponding termcap sequences will be used to format the output sent to the client. It is believed there is no risk of data disclosure through this vulnerability. However, an attacker who forces the server to search through a large file or to read from a device can cause resources to be spent by the server, including CPU cycles and disk read bandwidth, which can increase the server load and may prevent it from servicing legitimate user requests. Since the vulnerability occurs before the login(1) utility is spawned, it does not require authentication to a valid account on the server in order to exploit. All released versions of FreeBSD prior to the correction date including 4.0, 4.1, 4.1.1 and 3.5.1 are vulnerable to this problem, but it was fixed in the 4.1.1-STABLE branch prior to the release of FreeBSD 4.2-RELEASE. III. Impact Remote users without a valid login account on the server can cause resources such as CPU and disk read bandwidth to be consumed, causing increased server load and possibly denying service to legitimate users. IV. Workaround 1) Disable the telnet service, which is usually run out of inetd: comment out the following lines in /etc/inetd.conf, if present. telnet stream tcp nowait root /usr/libexec/telnetd telnetd telnet stream tcp6 nowait root /usr/libexec/telnetd telnetd 2) Impose access restrictions using TCP wrappers (/etc/hosts.allow), or a network-level packet filter such as ipfw(8) or ipf(8) on the perimeter firewall or the local machine, to limit access to the telnet service to trusted machines. V. Solution One of the following: 1) Upgrade your vulnerable FreeBSD system to 4.1.1-STABLE or 3.5.1-STABLE after the respective correction dates. Note that the original patch was incorrect and caused telnetd to behave incorrectly in certain situations. 2) Apply the patch below and recompile the relevant files: Either save this advisory to a file, or download the patch and detached PGP signature from the following locations, and verify the signature using your PGP utility. ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:69/telnetd.patch.v1.1 ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:69/telnetd.patch.v1.1.asc Execute the following commands as root: # cd /usr/src/libexec/telnetd # patch -p < /path/to/patch_or_advisory # make depend && make all install Updated patch for vulnerable systems: Index: ext.h =================================================================== RCS file: /home/ncvs/src/libexec/telnetd/ext.h,v retrieving revision 1.7 retrieving revision 1.8 diff -u -r1.7 -r1.8 --- ext.h 1999/08/28 00:10:22 1.7 +++ ext.h 2000/11/19 10:01:27 1.8 @@ -87,7 +87,7 @@ #endif extern int pty, net; -extern char *line; +extern char line[16]; extern int SYNCHing; /* we are in TELNET SYNCH mode */ #ifndef P Index: sys_term.c =================================================================== RCS file: /home/ncvs/src/libexec/telnetd/sys_term.c,v retrieving revision 1.24 retrieving revision 1.26 diff -u -r1.24 -r1.26 --- sys_term.c 1999/08/28 00:10:24 1.24 +++ sys_term.c 2000/11/19 10:01:27 1.26 @@ -480,14 +480,10 @@ * * Returns the file descriptor of the opened pty. */ -#ifndef __GNUC__ -char *line = "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"; -#else -static char Xline[] = "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"; -char *line = Xline; -#endif #ifdef CRAY -char *myline = "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"; +char myline[16]; +#else +char line[16]; #endif /* CRAY */ int @@ -1799,6 +1795,13 @@ strncmp(*cpp, "_RLD_", 5) && strncmp(*cpp, "LIBPATH=", 8) && #endif + strncmp(*cpp, "LOCALDOMAIN=", 12) && + strncmp(*cpp, "RES_OPTIONS=", 12) && + strncmp(*cpp, "TERMINFO=", 9) && + strncmp(*cpp, "TERMINFO_DIRS=", 14) && + strncmp(*cpp, "TERMPATH=", 9) && + strncmp(*cpp, "TERMCAP=/", 9) && + strncmp(*cpp, "ENV=", 4) && strncmp(*cpp, "IFS=", 4)) *cpp2++ = *cpp; } Index: telnetd.c =================================================================== RCS file: /home/ncvs/src/libexec/telnetd/telnetd.c,v retrieving revision 1.22 retrieving revision 1.24 diff -u -r1.22 -r1.24 --- telnetd.c 2000/01/25 14:52:00 1.22 +++ telnetd.c 2000/11/19 10:01:27 1.24 @@ -805,13 +805,12 @@ #else for (;;) { char *lp; - extern char *line, *getpty(); if ((lp = getpty()) == NULL) fatal(net, "Out of ptys"); if ((pty = open(lp, 2)) >= 0) { - strcpy(line,lp); + strlcpy(line,lp,sizeof(line)); line[5] = 't'; break; } @@ -1115,7 +1114,7 @@ IM = Getstr("im", &cp); IF = Getstr("if", &cp); if (HN && *HN) - (void) strcpy(host_name, HN); + (void) strlcpy(host_name, HN, sizeof(host_name)); if (IF && (if_fd = open(IF, O_RDONLY, 000)) != -1) IM = 0; if (IM == 0) Index: utility.c =================================================================== RCS file: /home/ncvs/src/libexec/telnetd/utility.c,v retrieving revision 1.13 retrieving revision 1.14 diff -u -r1.13 -r1.14 --- utility.c 1999/08/28 00:10:25 1.13 +++ utility.c 2000/10/31 05:29:54 1.14 @@ -330,7 +330,7 @@ { char buf[BUFSIZ]; - (void) sprintf(buf, "telnetd: %s.\r\n", msg); + (void) snprintf(buf, sizeof(buf), "telnetd: %s.\r\n", msg); (void) write(f, buf, (int)strlen(buf)); sleep(1); /*XXX*/ exit(1); @@ -343,7 +343,7 @@ { char buf[BUFSIZ], *strerror(); - (void) sprintf(buf, "%s: %s", msg, strerror(errno)); + (void) snprintf(buf, sizeof(buf), "%s: %s", msg, strerror(errno)); fatal(f, buf); } -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.4 (FreeBSD) Comment: For info see http://www.gnupg.org iQCVAwUBOhmZhlUuHi5z0oilAQECjQP/RJyFP/msuoNj1ebyeE4PjXHFV99FoVIY jeBCjheFN+9kVR2ZqGxzhF8Ds1jsHI2oURhjNwRkf+OGNzCfDKEseTa0/Aa59XG5 68O9DKP2CEZnNra3N5uWCBX7ozGI1iCfJkBstSXBhdpyeumOjhfkEF1cwvJldyWl YMIWv/MwRWs= =wuWd -----END PGP SIGNATURE-----