PyPI · pillow
Pillow: BMP RLE decoder memory-amplification DoS via unbounded delta escape / absolute mode
BmpRleDecoder can be forced to allocate gigabytes of memory from a kilobyte-sized crafted BMP file via unbounded growth in the delta escape and absolute mode branches.
All versions up to and including current HEAD (v13.0.0.dev0, commit e7a7ea84)
BmpRleDecoder.decode() (BmpImagePlugin.py:327-395) builds a Python bytearray data with dest_length = xsize*ysize, looping while len(data) < dest_length. The loop condition is only checked at the TOP; individual branches append attacker-controlled amounts UNBOUNDED by dest_length:
Because a single delta can push len(data) to ~256 * (row size), a tiny RLE stream combined with a large-but-legal xsize (xsize*ysize just under MAX_IMAGE_PIXELS, ~89M) yields a multi-GB data bytearray (then copied again by bytes(data) and set_as_raw). No heap OOB (bytearray/set_as_raw are bounds-safe), but a severe memory-amplification DoS: the attacker file stays tiny.
Python 3.13, Pillow HEAD e7a7ea84
Attached: pillow_poc_bmp_rle_dos.py
A 1084-byte BMP (BI_RLE8, width=3,000,000, height=1 -> 3M px, passes bomb check) with one delta escape (00 02 00 150) produces ~429 MB data; measured peak RSS 911 MB. Scaling width toward the bomb limit and up=255 produces tens of GB from a ~1 KB file.
Cap growth at dest_length in the delta and absolute branches, e.g. clamp the delta fill to (dest_length - len(data)) and stop appending once len(data) >= dest_length; reject deltas that would move the cursor beyond the image, matching the existing per-row clamp at line 341-343.
Credit: Yazan Balawneh - CyStack Security Team (GitHub: sonicnew)
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