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                        OpenBSD Security Advisory



                            December 18, 2000



             Single-byte buffer overflow vulnerability in ftpd



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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.



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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.



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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.



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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).



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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



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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.



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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);

 }