---------------------------------------------------------------------------- OpenBSD Security Advisory December 18, 2000 Single-byte buffer overflow vulnerability in ftpd ---------------------------------------------------------------------------- SYNOPSIS A relatively obscure one-byte buffer overflow bug present in ftpd(8) turns out to be a serious problem, yielding remote users root access under certain conditions. For a system to be vulnerable, ftpd must have been explicitly enabled by the administrator (OpenBSD ships with it OFF by default) and the attacker must have write access to at least one directory. Therefore, anonymous read-only FTP servers are safe (we recommend applying the patch regardless, of course). Non-anonymous FTP administrators should seriously consider using a more secure transport like SSH. A fix for this problem was committed on December 4th. OpenBSD developers became aware of a publicly available exploit on December 17th. ---------------------------------------------------------------------------- AFFECTED SYSTEMS This vulnerability affects OpenBSD versions through 2.8. FreeBSD is reportedly not vulnerable. NetBSD is vulnerable to the same bug and a patch was applied to their tree on December 14th. ---------------------------------------------------------------------------- TECHNICAL DETAILS The offending code is as follows: char npath[MAXPATHLEN]; int i; for (i = 0; *name != '\0' && i < sizeof(npath) - 1; i++, name++) { npath[i] = *name; if (*name == '"') npath[++i] = '"'; } npath[i] = '\0'; In , MAXPATHLEN is defined to be 1024 bytes. The for() construct here correctly bounds variable `i' to be < 1023, such that when the loop has ended, no byte past npath[1023] may be written with '\0'. However, since `i' is also incremented in the nested statements here, it can become as large as 1024, and npath[1024] is past the end of the allocated buffer space. ---------------------------------------------------------------------------- RESOLUTION OpenBSD does not ship with ftpd enabled by default. If you are using it, disable it until you are fixed by editing /etc/inetd.conf and restarting the inetd(8) daemon. Then, apply the fix below to your OpenBSD 2.8 source tree. The patch is also available at http://www.openbsd.org/errata.html (005). ---------------------------------------------------------------------------- REFERENCES The original bug report, http://www.geocrawler.com/lists/3/OpenBSD/254/75/4767480/ Security and errata, http://www.openbsd.org/security.html http://www.openbsd.org/errata.html Olaf Kirch discusses one-byte overruns in a post to BUGTRAQ in 1998 with subject "The poisoned NUL byte", http://www.securityfocus.com/archive/1/10884 ---------------------------------------------------------------------------- CREDITS This vulnerability was first reported to OpenBSD Kristian Vlaardingerbroek through the bugs@openbsd.org mailing list. Kristian acknowledged in a later post that it was Ronald (a.k.a. Scrippie) who originally found the bug. The fix provided in the original bug report is incorrect. Credit goes to Jun-ichiro itojun Hagino for applying a safe fix to the OpenBSD 2.8-current tree. This patch has also been applied to the stable branch. ---------------------------------------------------------------------------- OPENBSD 2.8 PATCH Apply by doing: cd /usr/src patch -p0 < 005_ftpd.patch And then rebuild and install ftpd: cd libexec/ftpd make obj make depend make make install Index: libexec/ftpd/ftpd.c =================================================================== RCS file: /cvs/src/libexec/ftpd/ftpd.c,v retrieving revision 1.79 diff -u -r1.79 ftpd.c --- libexec/ftpd/ftpd.c 2000/09/15 07:13:45 1.79 +++ libexec/ftpd/ftpd.c 2000/12/05 17:06:29 @@ -1959,15 +1959,21 @@ replydirname(name, message) const char *name, *message; { + char *p, *ep; char npath[MAXPATHLEN]; - int i; - for (i = 0; *name != '\0' && i < sizeof(npath) - 1; i++, name++) { - npath[i] = *name; - if (*name == '"') - npath[++i] = '"'; + p = npath; + ep = &npath[sizeof(npath) - 1]; + while (*name) { + if (*name == '"' && ep - p >= 2) { + *p++ = *name++; + *p++ = '"'; + } else if (ep - p >= 1) + *p++ = *name++; + else + break; } - npath[i] = '\0'; + *p = '\0'; reply(257, "\"%s\" %s", npath, message); }