Description:
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The corruption actually happens at ext/gd/libgd/gd_gif_in.c:590-594

590             while ((v = LWZReadByte(fd, &sd, FALSE, c, ZeroDataBlockP)) >= 0 ) {
591                     /* This how we recognize which colors are actually used. */
592                     if (im->open[v]) {
593                             im->open[v] = 0;
594                     }

Values read from the data stream are not bounds-checked. An attacker can create values of v (type int) which are larger than the array im->open (256 bytes). If the value of im->open[v] happens to be non-zero, it is overwritten with a zero on line 593. This corrupts the heap. The attacker can't control what value is overwritten (always 0), but they could control which bytes are overwritten. libgd should clamp values of v to be between acceptable values [0,255].

Note that values of v are clamped to 4096, since this is the maximum number of bits per codeword that gd will accept, so it isn't an arbitrary overwrite. It is enough to corrupt the heap and cause a DoS though.

Reproduce code:
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I have a sample .gif file. Email me for it.