AUSCERT External Security Bulletin Redistribution
                  ESB-2000.402 -- OpenBSD Security Advisory
              Single-byte buffer overflow vulnerability in ftpd
                              21 December 2000


	AusCERT Security Bulletin Summary

Product:                ftpd
Vendor:                 OpenBSD
Operating System:       OpenBSD
Impact:                 Root Compromise
Access Required:        Remote

- --------------------------BEGIN INCLUDED TEXT--------------------

- ----------------------------------------------------------------------------
                        OpenBSD Security Advisory
                            December 18, 2000

             Single-byte buffer overflow vulnerability in ftpd

- ----------------------------------------------------------------------------


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.

- ----------------------------------------------------------------------------


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.

- ----------------------------------------------------------------------------


The offending code is as follows:

	char npath[MAXPATHLEN];
	int i;

	for (i = 0; *name != '' && i < sizeof(npath) - 1; i++, name++) {
		npath[i] = *name;
		if (*name == '"')
			npath[++i] = '"';
	npath[i] = '';

In <sys/param.h>, 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 ''.
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.

- ----------------------------------------------------------------------------


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).

- ----------------------------------------------------------------------------


The original bug report,


Security and errata,


Olaf Kirch discusses one-byte overruns in a post to BUGTRAQ in 1998 with
subject "The poisoned NUL byte",


- ----------------------------------------------------------------------------


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 <itojun@openbsd.org> for applying a safe fix to
the OpenBSD 2.8-current tree.  This patch has also been applied to the
stable branch.

- ----------------------------------------------------------------------------


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 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 != '' && 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] = '';
+	*p = '';
 	reply(257, ""%s" %s", npath, message);

- --------------------------END INCLUDED TEXT--------------------

This security bulletin is provided as a service to AusCERT's members.  As
AusCERT did not write the document quoted above, AusCERT has had no control
over its content.  The decision to use any or all of this information is
the responsibility of each user or organisation, and should be done so in
accordance with site policies and procedures.

NOTE: This is only the original release of the security bulletin.  It may
not be updated when updates to the original are made.  If downloading at
a later date, it is recommended that the bulletin is retrieved directly
from the original authors to ensure that the information is still current.

Contact information for the authors of the original document is included
in the Security Bulletin above.  If you have any questions or need further
information, please contact them directly.

Previous advisories and external security bulletins can be retrieved from:


If you believe that your system has been compromised, contact AusCERT or
your representative in FIRST (Forum of Incident Response and Security

Internet Email: auscert@auscert.org.au
Facsimile:	(07) 3365 7031
Telephone:	(07) 3365 4417 (International: +61 7 3365 4417)
		AusCERT personnel answer during Queensland business hours
		which are GMT+10:00 (AEST).
		On call after hours for emergencies.

Version: 2.6.3i
Charset: noconv
Comment: ftp://ftp.auscert.org.au/pub/auscert/AUSCERT_PGP.key