CVE-2026-96778

Pillow is a Python imaging library. From 9.4.0 until 13.0.0, ImagingJpegEncode() in src/libImaging/JpegEncode.c can call jpeg_write_marker() for a JPEG COM comment when an ICC profile has exactly filled the 65536-byte encoder buffer. libjpeg writes one byte before checking the exhausted free_in_buffer counter, which then wraps and permits the attacker-controlled comment and subsequent compressed image data to overwrite adjacent heap memory. Re-saving an untrusted image as JPEG while preserving its ICC profile and forwarded comment can crash the process and may enable code execution in suitable runtime and heap layouts. This issue is fixed in version 13.0.0.

Pillow is a Python imaging library. From 9.4.0 until 13.0.0, ImagingJpegEncode() in src/libImaging/JpegEncode.c can call jpeg_write_marker() for a JPEG COM comment when an ICC profile has exactly filled the 65536-byte encoder buffer. libjpeg writes one byte before checking the exhausted free_in_buffer counter, which then wraps and permits the attacker-controlled comment and subsequent compressed image data to overwrite adjacent heap memory. Re-saving an untrusted image as JPEG while preserving its ICC profile and forwarded comment can crash the process and may enable code execution in suitable runtime and heap layouts. This issue is fixed in version 13.0.0.

============================================================================
Heap buffer overflow writing JPEG comment after ICC profile fills buffer
Location: src/libImaging/JpegEncode.c:330  in ImagingJpegEncode()
Commit:   a707c6b21c313880d0771e54965d142dd42358b6
============================================================================

A heap buffer overflow in Pillow's JPEG encoder lets anyone who can supply an image write up to 64 KB of bytes they choose, followed by the rest of the compressed image, past the end of a 64 KB heap buffer. The target is any application that re-saves the image as JPEG while keeping its colour profile. The reliable result is that the Python process is killed outright rather than raising an exception. Because the overwritten memory sits directly in front of function pointers that the encoder calls next, this could allow code execution, though the attached reproducer demonstrates only the crash.

The bug is in ImagingJpegEncode() in src/libImaging/JpegEncode.c. State 4 (line 330) calls jpeg_write_marker() for the COM comment marker without checking that the output buffer has room. Meanwhile, JpegImagePlugin._save() sizes that buffer from the EXIF and ICC/XMP data only and reserves nothing for the comment. When a 65498-byte ICC profile fills the 65536-byte buffer exactly, libjpeg's emit_byte() stores one byte past the end and then decrements free_in_buffer from 0. The size_t counter wraps, so the marker, the comment and the entire compressed image are written out of bounds.

Triggering it takes only a valid JPEG (or PNG) carrying that profile and any comment, passed through Image.open() and then im.save(..., "JPEG", icc_profile=im.info.get("icc_profile")). That is the documented way to preserve colour. The comment is forwarded from im.info automatically, and no non-default setting is involved.

The attached reproducer re-saves such a file on the stock build, and the process dies with SIGSEGV. An identical file without the comment saves normally. An ASan build of Pillow will not flag this, because the stray writes happen inside the system libjpeg-turbo. Valgrind does report it, as an invalid write immediately after the 65569-byte allocation.

The unguarded call was introduced in e50ae85ea406 ("Use jpeg_write_marker to write comment", 2022-12-05) and first shipped in 9.4.0.

Details

ImagingJpegEncode() (src/libImaging/JpegEncode.c) is a resumable state machine that fills a caller-supplied output buffer of bytes bytes and returns when it is full. State 4 writes the JPEG COM marker with jpeg_write_marker() without first checking that the buffer has any room left. When the preceding state 3 — the verbatim copy of the "extra" APP segments (ICC profile / XMP) — ends exactly at the last byte of the buffer, jpeg_write_marker() is entered with free_in_buffer == 0. libjpeg's emit_byte() stores the byte first and tests --free_in_buffer == 0 afterwards, so the size_t counter wraps to 2^64-1 instead of triggering the "buffer full" path, and the 4-byte marker header, the whole comment, and then (state 5) the DQT/SOF/DHT/SOS headers and the entire entropy-coded image are written linearly past the end of the 65536-byte heap buffer allocated in src/encode.c.

A JPEG laid out as

FFD8                                   SOI
FFE2 <len> "ICC_PROFILE\0" 01 01 <65498 bytes>     -> im.info["icc_profile"]
FFFE <len> <comment, any length 1..65533>           -> im.info["comment"]
<any baseline RGB JPEG body>

opened with Image.open() and re-saved with the usual colour-profile-preserving call

im.save(out, "JPEG", icc_profile=im.info.get("icc_profile"))

kills the Python process on the stock build at this commit: glibc aborts with realloc(): invalid next size / corrupted size vs. prev_size, or the process takes SIGSEGV inside jpeg_write_marker/jpeg_write_scanlines. Stopping at JpegEncode.c:330 in gdb shows `next_output_byte - buf ==…

Pillow is a Python imaging library. From 9.4.0 until 13.0.0, ImagingJpegEncode() in src/libImaging/JpegEncode.c can call jpeg_write_marker() for a JPEG COM comment when an ICC profile has exactly filled the 65536-byte encoder buffer. libjpeg writes one byte before checking the exhausted free_in_buffer counter, which then wraps and permits the attacker-controlled comment and subsequent compressed image data to overwrite adjacent heap memory. Re-saving an untrusted image as JPEG while preserving its ICC profile and forwarded comment can crash the process and may enable code execution in suitable runtime and heap layouts. This issue is fixed in version 13.0.0.

Pillow is a Python imaging library. From 9.4.0 until 13.0.0, ImagingJpegEncode() in src/libImaging/JpegEncode.c can call jpeg_write_marker() for a JPEG COM comment when an ICC profile has exactly filled the 65536-byte encoder buffer. libjpeg writes one byte before checking the exhausted free_in_buffer counter, which then wraps and permits the attacker-controlled comment and subsequent compressed image data to overwrite adjacent heap memory. Re-saving an untrusted image as JPEG while preserving its ICC profile and forwarded comment can crash the process and may enable code execution in suitable runtime and heap layouts. This issue is fixed in version 13.0.0.

Pillow is a Python imaging library. From 9.4.0 until 13.0.0, ImagingJpegEncode() in src/libImaging/JpegEncode.c can call jpeg_write_marker() for a JPEG COM comment when an ICC profile has exactly filled the 65536-byte encoder buffer. libjpeg writes one byte before checking the exhausted free_in_buffer counter, which then wraps and permits the attacker-controlled comment and subsequent compressed image data to overwrite adjacent heap memory. Re-saving an untrusted image as JPEG while preserving its ICC profile and forwarded comment can crash the process and may enable code execution in suitable runtime and heap layouts. This issue is fixed in version 13.0.0.