An out-of-bounds write vulnerability was reported that occurs when processing bitmap images due to crt_row being inc/dec-remented (depending on whether the image is topdown) while processing delta escapes without any boundary checking.

Vulnerable code (pngxrbmp.c):

| 210 static size_t
| 211 bmp_read_rows(png_bytepp begin_row, png_bytepp end_row, size_t row_size,
| 212 unsigned int compression, FILE *stream)
| 213 {
| ...
| 272 crt_row = begin_row;
| 273 for ( ; ; )
| 274 {
| 275 ch = getc(stream); b1 = (unsigned int)ch;
| 276 ch = getc(stream); b2 = (unsigned int)ch;
| 277 if (ch == EOF)
| 278 break;
| 279 if (b1 == 0) /* escape */
| 280 {
| ...
| 307 else if (b2 == 2) /* delta */
| 308 {
| 309 ch = getc(stream); b1 = (unsigned int)ch; /* horiz. offset */
| 310 ch = getc(stream); b2 = (unsigned int)ch; /* vert. offset */
| ...
| 314 if (b2 > (size_t)((end_row - crt_row) * inc))
| 315 b2 = (unsigned int)((end_row - crt_row) * inc);
| 316 for ( ; b2 > 0; --b2)
| 317 {
| ...
| 319 crt_row += inc;
| ...
| 322 }
| ...
| 324 }
| 325 else /* b2 >= 3 bytes in absolute mode */
| 326 {
| 327 len = (b2 <= endn - crtn) ? b2 : (unsigned int)(endn - crtn);
| 328 if (bmp_fread_fn(*crt_row, crtn, len, stream) != len)
| 329 break;
| 330 crtn += len;
| 331 }
| 332 }
| ...
| 352 }

A delta escape moving crt_row beyond its allocated chunk followed by fread() in absolute mode would, with GNU libc combined with a !topdown image, result in a write to an address based on the specified bitmap height.

Upstream report : https://sourceforge.net/p/optipng/bugs/59/
Patch is merged with CVE-2016-3981, presented as a set of 2 patches :
  -> https://sourceforge.net/p/optipng/bugs/56/#5168
  -> https://sourceforge.net/p/optipng/bugs/56/#d6e6