The GIF decoding function gdImageCreateFromGifCtx in gd_gif_in.c in the GD Graphics Library (aka libgd), as used in PHP before 5.6.31 and 7.x before 7.1.7, does not zero colorMap arrays before use. A specially crafted GIF image could use the uninitialized tables to read ~700 bytes from the top of the stack, potentially disclosing sensitive information.
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