CVE-2026-112399

============================================================================
APNG background disposal allocates full canvas before decompression bomb check
Location: src/PIL/PngImagePlugin.py:1000  in PngImageFile._seek()
Commit:   a707c6b21c313880d0771e54965d142dd42358b6
============================================================================

A 126-byte animated PNG file makes Pillow's Image.open() allocate and fill memory sized by the image dimensions the file claims, before Pillow's built-in check for oversized images runs. A file claiming 22000×22000 pixels costs about 1.9 GB. Anyone who can get an application to open an image they supply, with default settings and no further calls such as load(), can cause a multi-gigabyte memory spike per file. If the claimed size exceeds the machine's free memory, the operating system kills the process outright.

The bug is in PngImageFile._seek() in src/PIL/PngImagePlugin.py (around line 1000). For any APNG, _open() calls self._seek(0). When the first fcTL chunk requests OP_BACKGROUND disposal, _seek() calls Image.core.fill(self.mode, self.size) with the full IHDR size, which allocates and zero-fills width×height×4 bytes. OP_PREVIOUS takes the same path, because frame 0 rewrites it to OP_BACKGROUND. This all happens inside the plugin factory, so Image.open()'s _decompression_bomb_check(im.size) only runs after the memory has been touched. A 1×1 frame rectangle passes the only checks on the way.

Below available memory, the allocation is freed when DecompressionBombError propagates, so sustained pressure needs repeated or concurrent uploads. A 45000×45000 header gets the process killed by the out-of-memory killer on an 8 GB host. The attached reproducer opens two such files that differ in one byte (dispose_op 0 vs 1). Both end in DecompressionBombError, but peak resident memory is 17 MB for the first and 1864 MB for the second. The eager fill was introduced in 5e4e0fa6ee67 ("Use disposal settings from previous frame", December 2020) and first shipped in 8.1.0. Two candidate patches with a regression test are attached.

Details

For an APNG whose first fcTL chunk requests OP_BACKGROUND disposal, PngImageFile._open() creates and zero-fills a core image of the full IHDR-declared size. This happens inside the plugin factory, i.e. before Image.open() gets to _decompression_bomb_check(im.size), so the check that is meant to reject oversized headers without allocating anything runs only after the allocation has been made and touched.

The following 126-byte file (all chunk lengths and CRCs valid) makes Image.open(io.BytesIO(data)), with default MAX_IMAGE_PIXELS and no load(), reach 1.86 GB resident (ru_maxrss) and then raise DecompressionBombError. The byte-identical file with dispose_op=0 raises the same error at 17 MB.

IHDR  width=22000 height=22000 bit_depth=8 colour_type=6 (RGBA)
acTL  num_frames=2 num_plays=0
fcTL  seq=0 w=1 h=1 x=0 y=0 delay=1/10 dispose_op=1 (OP_BACKGROUND) blend_op=0
IDAT  zlib(00 00 00 00 00)                      # one 1x1 RGBA scanline
IEND

With width=height=45000 (8.1 GB canvas) the same call attempts the full allocation: under a 4 GiB RLIMIT_AS it fails with MemoryError from the allocator instead of DecompressionBombError; without a cap, on an 8 GB host with no swap, the process is OOM-killed.

Code path

Image.open() runs the bomb check only after the plugin's constructor (and therefore _open()) has returned:

# src/PIL/Image.py:3699-3701
                    im = factory(fp, filename)
                    _decompression_bomb_check(im.size)
                    return im

For a plain PNG that ordering is fine: PngImageFile._open() only parses chunks up to the first IDAT and records self._size from IHDR; nothing is allocated from it. For an APNG (acTL seen, so im_n_frames is set) _open() additionally positions on frame 0:

#…

Pillow is a Python imaging library. From 8.1.0 until 13.0.0, Pillow can allocate an APNG disposal background at the full canvas size before applying the decompression-bomb check. A crafted APNG with a small first frame, a very large declared canvas, and background disposal or first-frame OP_PREVIOUS reaches PngImageFile._seek while Image.open() positions frame zero. The full-canvas allocation occurs inside Image.open() without requiring pixel loading or frame iteration. This can exhaust memory and deny service. This issue is fixed in version 13.0.0.

Pillow is a Python imaging library. From 8.1.0 until 13.0.0, Pillow can allocate an APNG disposal background at the full canvas size before applying the decompression-bomb check. A crafted APNG with a small first frame, a very large declared canvas, and background disposal or first-frame OP_PREVIOUS reaches PngImageFile._seek while Image.open() positions frame zero. The full-canvas allocation occurs inside Image.open() without requiring pixel loading or frame iteration. This can exhaust memory and deny service. This issue is fixed in version 13.0.0.

TuxCare rebuilt pillow to address 10 CVEs. Fixed in 11.3.0.post8+tuxcare.

TuxCare rebuilt pillow to address 10 CVEs. Fixed in 11.2.1.post10+tuxcare.

TuxCare rebuilt pillow to address 10 CVEs. Fixed in 10.4.0.post12+tuxcare.

TuxCare rebuilt pillow to address 9 CVEs. Fixed in 9.5.0.post10+tuxcare.

TuxCare rebuilt pillow to address 9 CVEs. Fixed in 9.4.0.post11+tuxcare.

TuxCare rebuilt pillow to address 9 CVEs. Fixed in 8.4.0.post11+tuxcare.

Root has patched CVE-2026-112399 in the pillow package for Root:PyPI. Multiple fixed versions available.

This update for python-Pillow fixes the following issues:

• CVE-2026-96636: decompression-bomb protection bypass via multi-frame seek() in MPO and DCX (bsc#1286802).
• CVE-2026-96637: decompression bomb check bypass in SpiderImagePlugin.seek() leads to unbounded allocation DoS (bsc#1286801).
• CVE-2026-96640: PDF append accepts malformed shared page-tree DAG, which can cause resource exhaustion (bsc#1286798).
• CVE-2026-96641: reachable undefined behavior in native integer operations can lead to denial of service or logic integrity compromise (bsc#1286797).
• CVE-2026-96642: TarIO negative size field causes infinite CPU-spin loop (bsc#1286796).
• CVE-2026-112390: Denial of Service via infinite loop during crafted MIC image parsing (bsc#1286256).
• CVE-2026-112391: Denial of Service via unbounded recursion in PDF parser (bsc#1286273).
• CVE-2026-112392: Denial of Service via zero-size images in rank filters (bsc#1286287).
• CVE-2026-112393: heap buffer overflow and stale-heap disclosure in _tiffWriteProc when saving compressed TIFF data (bsc#1286325).
• CVE-2026-112394: P;2L and P;4L decoders may read 1 to 3 bytes past the passed buffer size (bsc#1286311).
• CVE-2026-112397: out-of-bounds access in _imagingmorph on a zero-width image (bsc#1286314).
• CVE-2026-112398: heap buffer overflow in FPX JPEG decoding can lead to code execution (bsc#1286406).
• CVE-2026-112399: Denial of service via crafted APNG image (bsc#1286368).
• CVE-2026-112400: Denial of Service via memory exhaustion from crafted GIF image (bsc#1286370).
• CVE-2026-112401: Denial of Service via crafted GIF comments (bsc#1286372).
• CVE-2026-112402: Denial of service via double free during image transformation (bsc#1286381).
• CVE-2026-112403: heap buffer overflow in ImagingJpegDecode allows for code execution (bsc#1286405).
• CVE-2026-112405: NULL pointer dereference in Image.merge via an unvalidated band 0 (bsc#1286404).
• CVE-2026-112406: infinite loop in PdfParser.rewrite_pages on page tree Parent cycle (bsc#1286403).
• CVE-2026-112891: quadratic GIF comment copying during save (bsc#1286402).
• CVE-2026-112892: MSP RLE decoder expands a tiny file into unbounded memory (bsc#1286401).
• CVE-2026-112894: Denial of Service via crafted multi-frame image conversion (bsc#1286408).
• CVE-2026-112895: Denial of service via crafted PDF during append operations (bsc#1286411).
• CVE-2026-112896: TIFF encoder ColorMap plane-size mismatch causes a heap out-of-bounds read (bsc#1286400).
• CVE-2026-112897: TiffDecode returns uninitialized memory as pixels for short JPEG strips (bsc#1286398).
• CVE-2026-112899: EPS load(scale) lets Ghostscript render past the decompression bomb limit (bsc#1286489).
• CVE-2026-112900: argument Injection in Ghostscript call can lead to remote code execution (bsc#1286397).
• CVE-2026-112901: ReDoS in XbmImagePlugin can be triggered by 512-byte file (bsc#1286396).
• CVE-2026-112902: memory disclosure and segfault in libtiff encoder via crafted TIFF tags (bsc#1286804).