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Re: wu-ftpd fb_realpath() off-by-one bug
From: Przemyslaw Frasunek <venglin () freebsd lublin pl>
Date: Mon, 04 Aug 2003 09:55:16 +0200

Użytkownik Janusz Niewiadomski napisał:
This bug may be non-exploitable if size of the buffer is greater than
MAXPATHLEN characters. This may occur for example if wu-ftpd is compiled
with some versions of Linux kernel where PATH_MAX (and MAXPATHLEN accordingly) is defined to be exactly 4095 characters. In such cases, the buffer is padded with an extra byte because of variable alignment which is a result of code optimization.

Actually, this bug is (probably) also non-exploitable when wu-ftpd is compiled using the gcc 3.x, which aligns stack variables in a different way:

(gdb) b fb_realpath
Breakpoint 1 at 0x8063c72: file realpath.c, line 103.
(gdb) cont
(gdb) x/bx &resolved[4096]
0xbfffc770:     0x00
(gdb) awatch *0xbfffc770
Hardware access (read/write) watchpoint 2: *3221210992
(gdb) cont
Hardware access (read/write) watchpoint 2: *3221210992

Value = 0
0x400d81d9 in strcat () from /lib/libc.so.6

In my example (wu-ftpd 2.6.2 compiled on Debian with gcc 3.3.1), the address of NULL-overflowed byte is 0xbfffc770 and the saved %ebp is located at 0xbfffc788:

(gdb) info frame 2
Stack frame at 0xbfffc788:
 eip = 0x8063ae4 in wu_realpath (realpath.c:60); saved eip 0x8053b35
 called by frame at 0xbfffe7d8, caller of frame at 0xbfffb748
 source language c.
 Arglist at 0xbfffc788, args: path=0x808cef0 'A' <repeats 200 times>...,
    resolved_path=0xbfffc7a0 "\001\001", chroot_path=0x8082e60 ""
 Locals at 0xbfffc788, Previous frame's sp in esp
 Saved registers:
  ebx at 0xbfffc784, ebp at 0xbfffc788, eip at 0xbfffc78c

I have tested the generic RedHat 8.0 (which provides wu-ftpd-2.6.2-5 compiled with gcc 3.x) and the behaviour was exactly the same.

Wu-ftpd suppiled with Debian Woody also seems to be non-exploitable -- it's compiled on kernel 2.2 with PATH_MAX 4095.

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