GHSA-24rc-8hj2-v98c

Pillow: BMP RLE decoder memory-amplification DoS via unbounded delta escape / absolute mode

Description

## Summary 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. ## Affected versions All versions up to and including current HEAD (v13.0.0.dev0, commit e7a7ea84) ## Root cause 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: - delta escape (line 370): data += b"\x00" * (right + up * self.state.xsize) with right,up in 0..255 -> up to ~255*xsize bytes in ONE iteration. - absolute mode (line 385): data += bytes_read (up to 255). 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. ## Reproduction 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. ## Suggested fix 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) [pillow_poc_bmp_rle_dos.py](https://github.com/user-attachments/files/30244974/pillow_poc_bmp_rle_dos.py)

Affected packages

References

• https://github.com/python-pillow/Pillow/security/advisories/GHSA-24rc-8hj2-v98c WEB
• https://nvd.nist.gov/vuln/detail/CVE-2026-96638 ADVISORY
• https://github.com/python-pillow/Pillow/pull/10015 WEB
• https://github.com/python-pillow/Pillow/commit/e7794835e323ff2691d9b3a974cade1a44301b9a WEB
• https://github.com/python-pillow/Pillow PACKAGE
• https://github.com/python-pillow/Pillow/releases/tag/13.0.0 WEB

Sources