Heap overflow in Pillow's bundled OpenJPEG JPEG 2000 decoder

### Summary Every Pillow wheel from 11.1.0 onward bundles a libopenjp2 containing a heap-buffer-overflow **write** that is fixed upstream on master but has never appeared in an OpenJPEG release. See https://osv.dev/vulnerability/OSV-2025-219 Because the wheel bundles its own copy, a patched host OS does not help. It is reachable from `Image.open(fp); im.load()` with default settings. I am reporting it here rather than only upstream because Pillow is where the vulnerable binary actually reaches users. Note the inversion that makes this easy to miss: **OpenJPEG 2.5.2 and earlier are safe.** The bug was introduced in `954c6e3c` (2024-06-25), so keeping the dependency current is what creates the exposure. ### Details ## Affected wheels From `.github/dependencies.json` (`openjpeg`) at each tag: | Pillow | bundled OpenJPEG | status | |---|---|---| | 10.4.0 | 2.5.2 | safe | | 11.0.0 | 2.5.2 | safe | | **11.1.0, 11.2.1, 11.3.0** | **2.5.3** | **affected** | | **12.0.0 … 12.3.0** | **2.5.4** | **affected** | Confirmed on an installed wheel: `PIL.features.version_codec('jpg_2000')` → `2.5.4`, bundled file `pillow.libs/libopenjp2-*.so.2.5.4`. `main` still pins `2.5.4` as of 2026-10-06. ## The defect `src/lib/openjp2/j2k.c`, in `opj_j2k_read_sod`: ```c OPJ_UINT32 l_current_tile_part = l_cstr_index->tile_index[p_j2k->m_current_tile_number].current_tpsno; l_cstr_index->tile_index[...].tp_index[l_current_tile_part].end_header = l_current_pos; l_cstr_index->tile_index[...].tp_index[l_current_tile_part].end_pos = l_current_pos + p_j2k->m_specific_param.m_decoder.m_sot_length + 2; ``` `tp_index` is neither null-checked nor bounds-checked, and `l_current_tile_part` comes directly from the one-byte `TPsot` field of the SOT marker. The correct guard already exists a few lines later in `opj_j2k_add_tlmarker` (j2k.c:8459), writing the **same array at the same index** behind `if (tp_index && l_current_tile_part < nb_tps)`. One writer checked, its sibling not. Reached by giving the codestream a *valid* TLM marker — so `tp_index` is allocated once by `opj_j2k_build_tp_index_from_tlm`, sized to the TLM entry count, and `opj_j2k_read_sot` then skips all resizing — while setting `TNsot = 0` in every SOT, which keeps the TLM "valid" and lets `TPsot` walk past the allocation. ## Patch status Fixed upstream in `91d08b11` ("Fix heap buffer overlfow in opj_j2k_read_sod", 2026-02-10). The newest OpenJPEG tag is **v2.5.4 (2025-09-20)**, and `compare/v2.5.4...91d08b11` reports *ahead 6, behind 0* — the fix is **in no release, 8 months on**. ## Suggested remediation 1. Carry `91d08b11` as a patch in the wheel build until an OpenJPEG release includes it — three lines, identical to a guard already present in the same file. 2. Consider asking OpenJPEG to cut a release; they have had an unreleased memory-corruption fix on master since February 2026, and every distro is shipping the vulnerable code. 3. Your security handbook already tells applications to block EPS at the application layer. A similarly explicit note that `Image.open()` ignores filename/Content-Type and will reach any compiled-in codec unless `formats=` is passed would help downstream apps a lot. ## Not a Pillow bug, but relevant: the extension allowlist most apps rely on does not apply `Image.open()` sniffs magic bytes, so a JPEG 2000 codestream in a file named `avatar.png` declared `Content-Type: image/png` reaches the JP2 decoder anyway. I verified this against a production service documenting PNG/JPEG-only uploads: it accepted and fully decoded a JP2 through exactly that path, with 16 formats reachable from one upload endpoint against 43 registered openers. That service has since fixed it by passing `formats=` and validating on the detected format rather than the client's `Content-Type`. That is the reason a decoder-level issue like this one has much wider reach than downstream maintainers tend to assume. ### Proof of concept ASan build of OpenJPEG 2.5.4 interposed under a **stock Pillow 12.3.0**, running `Image.open(p); im.load()`: ``` ==766332==ERROR: AddressSanitizer: heap-buffer-overflow WRITE of size 8 #0 opj_j2k_read_sod j2k.c:5086 #1 opj_j2k_read_tile_header j2k.c:9912 #2 opj_read_tile_header openjpeg.c:575 #3 j2k_decode_entry src/libImaging/Jpeg2KDecode.c:793 #4 ImagingJpeg2KDecode src/libImaging/Jpeg2KDecode.c:949 #5 _decode src/decode.c:132 located 8 bytes to the right of a 24-byte region allocated by opj_j2k_build_tp_index_from_tlm j2k.c:8939 ``` Frames 3–5 are Pillow's own decoder, not a synthetic harness. No non-default decoder options are involved; strict mode is on, which is Pillow's behaviour since Pillow never calls `opj_decoder_set_strict_mode`. A benign twin image runs the same path cleanly and returns a valid image, so this is the payload and not a parse failure. I have deliberately **not** attached the PoC file to this initial report. I'm happy to attach the PoC, the builder script and the RIP-control harness here on request, or hold everything until a fix is ready. No disclosure deadline from me; I will not publish anything while this is being worked on. ### Impact Any application that calls `Image.open()` / `im.load()` on an image it did not produce — the overwhelmingly common case for upload handlers, thumbnailers and preview pipelines — can be made to perform an out-of-bounds heap write by supplying one crafted file. **Floor, already certain.** On a stock non-sanitised wheel the overflow performs 245 writes walking to +5856 bytes past a 24-byte allocation, and glibc's own heap-metadata checker aborts the process (`corrupted size vs. prev_size`, `corrupted double-linked list`, SIGABRT). One file reliably kills a long-lived worker. **Ceiling, demonstrated as a primitive but not as a finished exploit.** With same-size-class objects alive on both sides — an ordinary busy heap — the walk crosses `opj_event_mgr_t`, which holds three `opj_msg_callback` function pointers, and `opj_event_msg` then calls one (event.c:136): ``` rip 0x4000 <- value chosen by the attacker's file layout rsi 0x3ff2 <- m_error_data, written by the same walk ``` Shifting the codestream with a COM marker moves `rip` with it (`0x16ce` → `0x4000`), so the instruction pointer is attacker-**chosen** rather than incidentally invalid. A benign control image leaves the same groomed heap untouched. To be explicit about what I did not do: I have **not** built a working exploit. That additionally needs remote heap grooming and an ASLR defeat, and I attempted neither. I am reporting a controlled control-flow primitive, not a finished chain. The CVSS AC:H reflects that.