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Multiple Integer Overflow Vulnerabilities In PuTTY SFTP
------------------------------------------------------------------------
SUMMARY
<http://www.chiark.greenend.org.uk/~sgtatham/putty/> PuTTY is "a free
implementation of Telnet and SSH for Win32 and Unix platforms, along with
an xterm terminal emulator".
Simon Tatham's PuTTY has been found to contain multiple remote integer
overflow vulnerabilities .
DETAILS
Vulnerable Systems:
* PuTTY version 0.56
Successful exploitation of an Integer overflow allows remote attackers to
execute arbitrary code under the privileges of the user running PuTTY. The
client must be directed to connect to a malicious server in order to
trigger the vulnerability. It should be noted that this vulnerability may
affect applications which use PuTTY source code or binaries as a SSH
protocol backend. One such product would be WinSCP3, a popular graphical
sftp/scp application for Windows.
First Integer Overflow:
The PuTTY sftp implementation allows attackers to supply arbitrary values
for the stored length of the string in the packet due to insufficient
validation of user-supplied data passed to a memcpy function.
Vulnerable code:
From sftp.c
static void sftp_pkt_getstring(struct sftp_packet *pkt,
char **p, int *length)
{
*p = NULL;
if (pkt->length - pkt->savedpos < 4)
return;
/* length value is taken from user-supplied data */
*length = GET_32BIT(pkt->data + pkt->savedpos);
pkt->savedpos += 4;
/* this check will be passed if length < 0 */
if (pkt->length - pkt->savedpos < *length)
return;
*p = pkt->data + pkt->savedpos;
pkt->savedpos += *length;
}
The usage of the above function is called in fxp_open_recv function:
struct fxp_handle *fxp_open_recv(struct sftp_packet *pktin,
struct sftp_request *req)
{
...
/* sftp_pkt_getstring call with controlled len value */
sftp_pkt_getstring(pktin, &hstring, &len);
...
handle = snew(struct fxp_handle);
/* heap corruption will occur if len == -1 */
handle->hstring = mkstr(hstring, len);
handle->hlen = len;
sftp_pkt_free(pktin);
return handle;
...
}
If length is passed as -1 in the mkstr function, a malloc(0) will occur
when the snewn() macro is called:
static char *mkstr(char *s, int len)
{
/* malloc(0) if len == -1 */
char *p = snewn(len + 1, char);
/* user controlled heap corruption */
memcpy(p, s, len);
p[len] = '\0';
return p;
}
Finally, when the memcpy function is called, an heap corruption will occur
leading to potential code execution.
Second Integer Overflow:
The second vulnerability specifically exists due to insufficient
validation of user-supplied data passed to a malloc function.
Vulnerable code:
struct fxp_names *fxp_readdir_recv(struct sftp_packet *pktin,
struct sftp_request *req) {
/* 32 bit value from packet */
ret->nnames = sftp_pkt_getuint32(pktin);
/*
* The integer overflow occurs when ret->nnames is referenced
* the snewn macro calls malloc() wrapper
* #define snewn(n, type) ((type *)smalloc((n)*sizeof(type)))
*/
ret->names = snewn(ret->nnames, struct fxp_name);
for (i = 0; i < ret->nnames; i++) {
char *str;
int len;
sftp_pkt_getstring(pktin, &str, &len);
/* pointer to arbitrary data from packet */
ret->names[i].filename = mkstr(str, len);
sftp_pkt_getstring(pktin, &str, &len);
/* pointer to arbitrary data from packet */
ret->names[i].longname = mkstr(str, len);
/* pointer to arbitrary data from packet */
ret->names[i].attrs = sftp_pkt_getattrs(pktin);
}
This function is called from scp_get_sink_action() in scp.c and
sftp_cmd_ls() in sftp.c and can lead to remote code execution via heap
corruption.
Sample debugger output of heap corruption:
psftp> ls
Listing directory /home/test
Program received signal SIGSEGV, Segmentation fault.
0x4009173c in memcpy () from /lib/libc.so.6
(gdb) bt
#0 0x4009173c in memcpy () from /lib/libc.so.6
#1 0x0805675f in mkstr (s=0x4e20 <Address 0x4e20 out of bounds>, len=0)
#2 0x0805748e in fxp_readdir_recv (pktin=0x809bc10, req=0x4e20)
#3 0x0804f7b8 in sftp_cmd_ls (cmd=0x4e20) at ../psftp.c:251
#4 0x08051955 in do_sftp (mode=0, modeflags=0, batchfile=0x0)
#5 0x080525f8 in psftp_main (argc=4, argv=0xbffff494)
#6 0x08080500 in main (argc=20000, argv=0x4e20)
(gdb) up 2
#2 0x0805748e in fxp_readdir_recv (pktin=0x809bc10, req=0x4e20)
952 ret->names[i].filename = mkstr(str, len);
(gdb) x/8x *(int)pktin
0x80acc58: 0x01000068 0x66666600 0x00000067 0x42424208
0x80acc68: 0x42424242 0x00000042 0x44444408 0x44444444
(gdb) print (struct sftp_packet)pktin
$2 = {data = 0x809bc10 "X \n\bYF", length = 134885120,
maxlen = -1073744968, savedpos = 134551097, type = 134885088}
Vendor Response:
Vendor advisories for these issues are available at:
<http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/vuln-sftp-string.html>
http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/vuln-sftp-stringhtml
<http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/vuln-sftp-readdir.html>
http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/vuln-sftp-readdir.html
Disclosure Timeline:
02/18/2005 - Initial vendor notification
02/19/2005 - Initial vendor response
02/21/2005 - Public disclosure
ADDITIONAL INFORMATION
The information has been provided by
<mailto:idlabs-advisories@idefense.com> idlabs-advisories@idefense.com.
The original article can be found at:
<http://www.idefense.com/application/poi/display?id=201>
http://www.idefense.com/application/poi/display?id=201
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