alpine 3.7
format weakness #15


Weakness Breakdown


A format string exploit occurs when the data of an input string is evaluated as a command by the program. This class of attacks is very similar to buffer overflows since an attacker could execute code, read the stack or cause new behaviors that compromise security. Learn more about format string attacks on OWASP attack index.

Warning code(s):

If format strings can be influenced by an attacker, they can be exploited.

File Name:



The highlighted line of code below is the trigger point of this particular Alpine 3.7 format weakness.

          /* Anything else is a nonempty position.  (Note that special
             constructs like \< are treated as nonempty strings here;
             an "epsilon closure" effectively makes them nullable later.
             Backreferences have to get a real position so we can detect
             transitions on them later.  But they are nullable.  */
          stk->nullable = d->tokens[i] == BACKREF;

          /* This position is in its own firstpos and lastpos.  */
          stk->nfirstpos = stk->nlastpos = 1;

          --firstpos, --lastpos;
          firstpos->index = lastpos->index = i;
          firstpos->constraint = lastpos->constraint = NO_CONSTRAINT;

#ifdef DEBUG
      /* ... balance the above nonsyntactic #ifdef goo...  */
      fprintf (stderr, "node %zu:", i);
      prtok (d->tokens[i]);
      putc ('\n', stderr);
      fprintf (stderr,
               stk[-1].nullable ? " nullable: yes\n" : " nullable: no\n");
      fprintf (stderr, " firstpos:");
      for (size_t j = stk[-1].nfirstpos; j-- > 0;)
          fprintf (stderr, " %zu:", firstpos[j].index);
          prtok (d->tokens[firstpos[j].index]);
      fprintf (stderr, "\n lastpos:");
      for (size_t j = stk[-1].nlastpos; j-- > 0;)
          fprintf (stderr, " %zu:", lastpos[j].index);
          prtok (d->tokens[lastpos[j].index]);
      putc ('\n', stderr);

#ifdef DEBUG
  for (size_t i = 0; i < d->tindex; ++i)
    if (d->tokens[i] < NOTCHAR || d->tokens[i] == BACKREF
        || d->tokens[i] == ANYCHAR || d->tokens[i] == MBCSET
        || d->tokens[i] >= CSET)
        fprintf (stderr, "follows(%zu:", i);
        prtok (d->tokens[i]); 

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