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TUCoPS :: Linux :: Apps N-Z :: lnx5316.htm

Qpopper buffer overflow condition



30th Apr 2002 [SBWID-5316]
COMMAND

	Qpopper buffer overflow condition

SYSTEMS AFFECTED

	Qpopper 4.0.3 and 4.0.4. default install.

PROBLEM

	In Marcel Fodor [http://mantra.freeweb.hu] security bug report :
	

	In :
	

	pop_bull.c

	-----------

	int

	CopyOneBull ( POP *p, long bnum, char *name )

	{

	    FILE              *bull;

	    char                buffer [ MAXMSGLINELEN ];

	    BOOL            in_header            = TRUE;

	    BOOL            first_line              = TRUE;

	    int                    nchar; 

	    int                    msg_num;

	    int                    msg_vis_num          = 0;

	    int                    msg_ends_in_nl       = 0;

	    char                bullName [ 256 ];

	    MsgInfoList    *mp;               /* Pointer to message info list */

	.

	.

	.

	    sprintf ( bullName, \"%s/%s\", p->bulldir, name );

	------------

	

	The bullNmae buffer is 256 bytes long, but in the user\'s  configuration
	file you can  define  it  up  to  MAXLINELEN-1-sizeof(\"set  bulldir=\")
	~1010 bytes.
	

	~/.qpopper-options

	--------------

	set bulldir=AAAAAAAAAAA.....AAAAAAAAAAAAAAA // ( bulldir must exist)

	--------------

	

	Since we have only 1010+256 bytes(p->bulldir+name) it is NOT  enough  to
	overflow the funcion`s stack and overwrite EIP.
	

	As you see the following registers can be overwritten:
	

	    BOOL         in_header            = TRUE; 

	    BOOL         first_line              = TRUE; 

	    int                nchar; 

	    int                msg_num;

	    int                msg_vis_num       = 0;

	    int                msg_ends_in_nl   = 0;

	    char             bullName [ 256 ];

	---------------------

	    MsgInfoList   *mp;               /* Pointer to message info list */

	

	

	There are two valuable registers:
	

	    1. msg_num

	    2. msg_vis_num

	

	As the code runs on, server  wants  to  find  memory  location  for  the
	actual message. Here it is:
	

	   mp = p->mlp + msg_num - 1;

	

	As we have control over msg_num, we  can  walk  forward  in  memory,  by
	steps of 0x40 bytes (sizeof(*mp)). On my  system  giving  the  value  of
	0x2dfe423 to msg_num, mp will point to 0xbfffbe6c Now code runs on,  and
	fill up properties.
	

	 

	    ++mp;

	    mp->number          = msg_num;

	    mp->visible_num    = ++msg_vis_num;               <---- we have this

	    mp->lines               = 0;

	    mp->body_lines     = 0;

	    mp->offset             = ftell(p->drop);

	    mp->del_flag         = FALSE;

	    mp->hide_flag       = FALSE;

	    mp->retr_flag        = FALSE;

	

	So, the main thing is the 0x40 bytes steps. For this, the  risk  of  the
	bug is low.

SOLUTION

	 Workaround  :

	 ==========

	

	Servers, not processing user\'s configuration file  (~/.qpopper-options)
	are insensible to this bug. ( qpopper -u)
	

	 Patch :

	 =====

	

	None yet ?


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