The GDI+ library can handle bitmaps originating from untrusted sources through a variety of attack vectors, like EMF files, which may embed bitmaps in records such as EMR_PLGBLT, EMR_BITBLT, EMR_STRETCHBLT, EMR_STRETCHDIBITS etc. The GDI+ implementation supports bitmaps compressed with the BI_RLE8 (8-bit Run-Length Encoding) compression algorithm, and performs the actual decompression in the gdiplus!DecodeCompressedRLEBitmap function. The buffer allocated to store the decompressed pixels is not cleared during or directly after the HeapAlloc() call, which causes it contain heap metadata and leftover data of previous allocations. The RLE compression algorithm makes it possible to skip some (in an extreme case: all) bytes in the output buffer; this could be achieved by using escape codes such as "End of line", "End of bitmap" or "Delta". If we start the compressed stream with the "End of bitmap" marker, the entirety of the memory region will remain uninitialized, which will in turn lead to displaying junk bytes as pixels. In the context of GDI+ clients which make it possible to read the rendered pixels back and send them to an attacker or use as part of a larger exploit chain, the bug could result in disclosure of sensitive data or defeat of exploit mitigations such as ASLR.
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