GHSA-5xm2-6676-qcp9

Pillow: APNG background disposal allocates full canvas before decompression bomb check

Description

``` ============================================================================ 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: ```python # 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: ```python # src/PIL/PngImagePlugin.py:845-853 if self.png.im_n_frames is not None: … self._seek(0) ``` `_seek(0)` copies `disposal` and `bbox` from the `fcTL` chunk into `self.dispose_op` / `self.dispose_extent` and then prepares the disposal image for the *next* frame eagerly, using the full image size: ```python # src/PIL/PngImagePlugin.py:991-1001 if self._prev_im is None and self.dispose_op == Disposal.OP_PREVIOUS: self.dispose_op = Disposal.OP_BACKGROUND self.dispose = None if self.dispose_op == Disposal.OP_PREVIOUS: … elif self.dispose_op == Disposal.OP_BACKGROUND: self.dispose = Image.core.fill(self.mode, self.size) self.dispose = self._crop(self.dispose, self.dispose_extent) ``` `Image.core.fill` is `_fill()` in `src/_imaging.c:711`, which does `ImagingNewDirty(mode, xsize, ysize)` followed by `ImagingFill()`; `ImagingFill` (`src/libImaging/Fill.c:23`) writes every row, so the whole `width*height*4` bytes become resident rather than merely reserved. `self.size` here is the IHDR size (22000×22000), not the 1×1 frame rectangle. The `_crop` on the next line does call `_decompression_bomb_check`, but on the `fcTL` rectangle (1×1) and only after the fill has completed. Lines 991-992 rewrite `OP_PREVIOUS` to `OP_BACKGROUND` on frame 0, so `dispose_op=2` takes the same path (verified: identical 1.86 GB peak). When the first `fcTL` rectangle is large, the `_crop` copy is a second extent-sized buffer on top of the full-canvas fill, so the transient peak can reach about twice the canvas: a 13000×13000 full-frame `fcTL` (below the error threshold, so `Image.open()` succeeds with only a `DecompressionBombWarning`) peaks at 1.31 GB for a 0.66 GB canvas. The only validation `chunk_fcTL` applies (`src/PIL/PngImagePlugin.py:740-745`) is that the frame rectangle fits inside the IHDR size, which a 1×1 frame always does; IHDR width/height themselves are unconstrained 32-bit values at this point. Control then returns to `_open_core`, `_decompression_bomb_check((22000, 22000))` raises, and the 1.9 GB core image is released when the half-constructed `PngImageFile` is discarded. The eager full-size fill in `_seek()` was introduced in `5e4e0fa6ee67` ("Use disposal settings from previous frame", 2020-12-24) and first shipped in 8.1.0; before it, the `OP_BACKGROUND` fill lived in `load_end()` and could not be reached from `Image.open()`. ## Impact Any application that calls `Image.open()` on untrusted bytes with default settings — the configuration in which the decompression-bomb check is supposed to make that call cheap and safe — can be made to allocate and write an attacker-chosen amount of memory from a ~126-byte PNG upload, before any error is reported and without `load()`, frame iteration, or `LOAD_TRUNCATED_IMAGES`. The amount is set directly by the IHDR width/height (4 bytes per pixel for colour type 6): values below the host's free memory cause a transient multi-GB spike per request (≈15,000,000× amplification at 22000×22000) that is freed when the exception propagates, so concurrent or repeated uploads are needed to sustain pressure; values above it get the worker process OOM-killed outright. PNG is accepted by essentially every upload pipeline, and the file passes `_accept` and chunk CRC checks as a well-formed APNG. This is the same class as the recent bomb-check additions for WMF (load at a different DPI), FITS, GIF and FontFile — an allocation sized from file-declared numbers on a path `_decompression_bomb_check` does not cover — but here the uncovered path is `Image.open()` itself rather than a later `load()`/`seek()`. ## Reproducing `repro` is a single Python script (no build step, no companion files). It builds two PNGs in memory, opens each one with plain `Image.open(io.BytesIO(data))` in a fresh child interpreter using default settings and no `load()`, and has each child report the exception it got and its own peak RSS (`ru_maxrss`). It needs about 2 GB of free RAM and finishes in about a second. The two files are 126 bytes each and differ in exactly one byte, the `dispose_op` of the first `fcTL` chunk: `0` (`OP_NONE`) for the control and `1` (`OP_BACKGROUND`) for the attack. Both declare IHDR 22000×22000 RGBA. That is 484 Mpixel, above the 2×`MAX_IMAGE_PIXELS` error threshold, so `Image.open()` has to reject both with `DecompressionBombError`. Each chunk has a job: - The `acTL` chunk (2 frames) makes `_open()` call `_seek(0)`. - The 1×1 `fcTL` rectangle passes `chunk_fcTL`'s bounds check and `_crop`'s bomb check. - The one-scanline `IDAT` ends header parsing. Expected output on this tree: ``` 126-byte APNG declaring 22000x22000 RGBA; Image.open(BytesIO(data)), defaults control dispose_op=0 OP_NONE -> DecompressionBombError peak RSS 17 MB attack dispose_op=1 OP_BACKGROUND -> DecompressionBombError peak RSS 1864 MB Image.open() touched a full 22000x22000 canvas before the bomb check ran BUG ``` Both files are rejected, but the control is rejected at interpreter size. The attack file only reaches the bomb check after `Image.core.fill("RGBA", (22000, 22000))` has allocated and zero-filled about 1.9 GB. The script prints `BUG` if the attack child's peak RSS is at least 1000 MB and at least 20× the control's, or if the attack child is killed by `SIGKILL` (for example by the OOM killer on a small host). Otherwise it prints `OK`; any other non-zero child exit is treated as a harness failure, printed with the child's output, and also ends in `OK`. With either attached patch applied, both children report about 17–22 MB and the script prints `OK`. The allocation grows with the header, so it can be scaled by changing `SIZE` in the script. The 45000×45000 case (an 8.1 GB canvas) is covered in Details and is not part of the script. ## Suggested fix Do not build the `OP_BACKGROUND` disposal image from the declared size during `_open()`. Two alternative patches are attached; both change only `src/PIL/PngImagePlugin.py`, and both add the same regression test to `Tests/test_file_apng.py` (a 2³¹−1 × 1 APNG built in memory with a 1×1 `OP_BACKGROUND` first frame must make `Image.open()` raise `DecompressionBombError`; on the current tree the fill fails first with `MemoryError`). Attached patch 1 is the preferred fix. **Attached patch 1** moves the fill to its only point of use, the `frame > 0` branch of `_seek()` right after `self.load()` — which can only run once `Image.open()` has returned (bomb check passed) and the canvas itself has been allocated — and, as `GifImagePlugin` already does, fills only the previous frame's dispose extent instead of a full canvas that is then cropped: ```diff --- a/src/PIL/PngImagePlugin.py +++ b/src/PIL/PngImagePlugin.py @@ -931,6 +931,13 @@ class PngImageFile(ImageFile.ImageFile): # ensure previous frame was loaded self.load() + if self.dispose_op == Disposal.OP_BACKGROUND: + # Created here rather than in the previous _seek(), so that + # nothing is allocated from the declared image size before + # the decompression bomb check in Image.open() has run. + # Only dispose the extent of the previous frame. + x0, y0, x1, y1 = self.dispose_extent + self.dispose = Image.core.fill(self.mode, (x1 - x0, y1 - y0)) if self.dispose: self.im.paste(self.dispose, self.dispose_extent) self._prev_im = self.im.copy() @@ -996,9 +1003,6 @@ class PngImageFile(ImageFile.ImageFile): if self._prev_im: self.dispose = self._prev_im.copy() self.dispose = self._crop(self.dispose, self.dispose_extent) - elif self.dispose_op == Disposal.OP_BACKGROUND: - self.dispose = Image.core.fill(self.mode, self.size) - self.dispose = self._crop(self.dispose, self.dispose_extent) ``` At that point `self.dispose_op` and `self.dispose_extent` still hold the previous frame's values (they are reassigned further down in `_seek`), so disposal semantics are unchanged; the `OP_PREVIOUS` copy stays eager because `load_end()` pastes into `_prev_im` in place (`src/PIL/PngImagePlugin.py:1110`). With this patch `Image.open()` allocates nothing from the declared size, including in the `DecompressionBombWarning` range. **Attached patch 2** is a smaller diff that keeps the eager disposal setup in `_seek()` and mirrors `GifImagePlugin._seek()`'s `OP_BACKGROUND` handling exactly: it computes the size of the `fcTL` rectangle (already validated by `chunk_fcTL` to lie within the image), runs `Image._decompression_bomb_check()` on it, and fills an image of only that size instead of filling `self.size` and cropping. It also narrows the `dispose_extent` annotation to `tuple[int, int, int, int]` so the bomb-check call type-checks. The trade-off is that a full-canvas first frame still allocates a bomb-checked image inside `Image.open()` — up to about 2×`MAX_IMAGE_PIXELS`×4 bytes (660 MB for a 13000×13000 full-frame `fcTL`, with `DecompressionBombWarning`) — and a first `fcTL` covering a canvas above the error threshold now raises `DecompressionBombError` from `_seek(0)` rather than from `Image.open()`'s own check. With either patch applied to a clean checkout, the reproducer prints `OK` and all 54 files in `Tests/images/apng/` decode byte-identically frame by frame, including after a rewind. ## The patch (1 of 2) Create the APNG OP_BACKGROUND disposal image when the next frame is sought, sized to the frame extent, not in _open() from the IHDR size Applies to `src/PIL/PngImagePlugin.py` at the commit above. ```diff diff --git a/Tests/test_file_apng.py b/Tests/test_file_apng.py index 15689476c..6e61c8b4b 100644 --- a/Tests/test_file_apng.py +++ b/Tests/test_file_apng.py @@ -161,6 +161,37 @@ def test_apng_dispose_op_background_p_mode() -> None: assert im.size == (128, 64) +def test_apng_dispose_op_background_decompression_bomb() -> None: + # The disposal image must not be created from the declared image size + # before the decompression bomb check in Image.open() has run. + # A canvas this wide cannot be allocated, so if it is attempted, + # a MemoryError is raised instead of a DecompressionBombError. + o16 = PngImagePlugin.o16 + o32 = PngImagePlugin.o32 + + b = BytesIO() + b.write(PngImagePlugin._MAGIC) + PngImagePlugin.putchunk(b, b"IHDR", o32(2**31 - 1), o32(1), b"\x08\x06\x00\x00\x00") + PngImagePlugin.putchunk(b, b"acTL", o32(2), o32(0)) + PngImagePlugin.putchunk( + b, + b"fcTL", + o32(0), # sequence number + o32(1), # width + o32(1), # height + o32(0), # x offset + o32(0), # y offset + o16(1), # delay numerator + o16(10), # delay denominator + bytes([PngImagePlugin.Disposal.OP_BACKGROUND, PngImagePlugin.Blend.OP_SOURCE]), + ) + PngImagePlugin.putchunk(b, b"IDAT") + PngImagePlugin.putchunk(b, b"IEND") + + with pytest.raises(Image.DecompressionBombError): + Image.open(b) + + def test_apng_blend() -> None: with Image.open("Tests/images/apng/blend_op_source_solid.png") as im: assert isinstance(im, PngImagePlugin.PngImageFile) diff --git a/src/PIL/PngImagePlugin.py b/src/PIL/PngImagePlugin.py index 0eb2fd5e4..24fa587c1 100644 --- a/src/PIL/PngImagePlugin.py +++ b/src/PIL/PngImagePlugin.py @@ -931,6 +931,13 @@ class PngImageFile(ImageFile.ImageFile): # ensure previous frame was loaded self.load() + if self.dispose_op == Disposal.OP_BACKGROUND: + # Created here rather than in the previous _seek(), so that + # nothing is allocated from the declared image size before + # the decompression bomb check in Image.open() has run. + # Only dispose the extent of the previous frame. + x0, y0, x1, y1 = self.dispose_extent + self.dispose = Image.core.fill(self.mode, (x1 - x0, y1 - y0)) if self.dispose: self.im.paste(self.dispose, self.dispose_extent) self._prev_im = self.im.copy() @@ -996,9 +1003,6 @@ class PngImageFile(ImageFile.ImageFile): if self._prev_im: self.dispose = self._prev_im.copy() self.dispose = self._crop(self.dispose, self.dispose_extent) - elif self.dispose_op == Disposal.OP_BACKGROUND: - self.dispose = Image.core.fill(self.mode, self.size) - self.dispose = self._crop(self.dispose, self.dispose_extent) def tell(self) -> int: return self.__frame ``` ## The patch (2 of 2) Bomb-check and fill only the fcTL extent for APNG OP_BACKGROUND disposal instead of a full IHDR-sized canvas in PngImageFile._seek() Applies to `src/PIL/PngImagePlugin.py` at the commit above. ```diff diff --git a/Tests/test_file_apng.py b/Tests/test_file_apng.py index 15689476c..6e61c8b4b 100644 --- a/Tests/test_file_apng.py +++ b/Tests/test_file_apng.py @@ -161,6 +161,37 @@ def test_apng_dispose_op_background_p_mode() -> None: assert im.size == (128, 64) +def test_apng_dispose_op_background_decompression_bomb() -> None: + # The disposal image must not be created from the declared image size + # before the decompression bomb check in Image.open() has run. + # A canvas this wide cannot be allocated, so if it is attempted, + # a MemoryError is raised instead of a DecompressionBombError. + o16 = PngImagePlugin.o16 + o32 = PngImagePlugin.o32 + + b = BytesIO() + b.write(PngImagePlugin._MAGIC) + PngImagePlugin.putchunk(b, b"IHDR", o32(2**31 - 1), o32(1), b"\x08\x06\x00\x00\x00") + PngImagePlugin.putchunk(b, b"acTL", o32(2), o32(0)) + PngImagePlugin.putchunk( + b, + b"fcTL", + o32(0), # sequence number + o32(1), # width + o32(1), # height + o32(0), # x offset + o32(0), # y offset + o16(1), # delay numerator + o16(10), # delay denominator + bytes([PngImagePlugin.Disposal.OP_BACKGROUND, PngImagePlugin.Blend.OP_SOURCE]), + ) + PngImagePlugin.putchunk(b, b"IDAT") + PngImagePlugin.putchunk(b, b"IEND") + + with pytest.raises(Image.DecompressionBombError): + Image.open(b) + + def test_apng_blend() -> None: with Image.open("Tests/images/apng/blend_op_source_solid.png") as im: assert isinstance(im, PngImagePlugin.PngImageFile) diff --git a/src/PIL/PngImagePlugin.py b/src/PIL/PngImagePlugin.py index 0eb2fd5e4..0111a17d7 100644 --- a/src/PIL/PngImagePlugin.py +++ b/src/PIL/PngImagePlugin.py @@ -985,7 +985,7 @@ class PngImageFile(ImageFile.ImageFile): msg = "image not found in APNG frame" raise EOFError(msg) if dispose_extent: - self.dispose_extent: tuple[float, float, float, float] = dispose_extent + self.dispose_extent: tuple[int, int, int, int] = dispose_extent # setup frame disposal (actual disposal done when needed in the next _seek()) if self._prev_im is None and self.dispose_op == Disposal.OP_PREVIOUS: @@ -997,8 +997,14 @@ class PngImageFile(ImageFile.ImageFile): self.dispose = self._prev_im.copy() self.dispose = self._crop(self.dispose, self.dispose_extent) elif self.dispose_op == Disposal.OP_BACKGROUND: - self.dispose = Image.core.fill(self.mode, self.size) - self.dispose = self._crop(self.dispose, self.dispose_extent) + # only dispose the extent in this frame, rather than filling an + # image of the declared size before Image.open() has been able to + # run the decompression bomb check + x0, y0, x1, y1 = self.dispose_extent + dispose_size = (x1 - x0, y1 - y0) + + Image._decompression_bomb_check(dispose_size) + self.

Affected packages

References

• https://github.com/python-pillow/Pillow/security/advisories/GHSA-5xm2-6676-qcp9 WEB
• https://nvd.nist.gov/vuln/detail/CVE-2026-112399 ADVISORY
• https://github.com/python-pillow/Pillow/commit/f1acfc54c53502948d0d14121872061df931db48 WEB
• https://github.com/python-pillow/Pillow PACKAGE
• https://github.com/python-pillow/Pillow/releases/tag/13.0.0 WEB

Sources