PYSEC-2026-2843 
### Summary
While fuzzing `openexr_exrcheck_fuzzer`, Valgrind reports a conditional branch depending on uninitialized data inside `generic_unpack`. This indicates a use of uninitialized memory (CWE-457). The issue is reproducible with the current OSS-Fuzz harness and a single-file PoC.

### Details

**Environment:**
- Tooling: `valgrind --tool=memcheck --track-origins=yes`
- Target: `openexr_exrcheck_fuzzer`
- OS: Ubuntu 20.04.6 LTS focal x86_64
- openexr version and Git-commit hash: ` openexr 3.4.2 | commit fd657e8a41e157e5841c7cc2e2a5efe094b069a1 (grafted, HEAD -> main, origin/main, origin/HEAD)`

Function: `generic_unpack`

Possible root cause (based on observed symptoms):
The unpacker is branching on bytes in a scratch buffer that were never written because the decode step didn’t fully populate it.
- The first use flagged is in `generic_unpack()`. That function reads from the decompressed/expanded pixel buffer to scatter data into the framebuffer. A “conditional jump depends on uninitialised value(s)” means it’s consulting bytes in that buffer before they were written.
- Valgrind says the uninitialised value “was created by a heap allocation (malloc)”, not the stack: this matches a per-tile/per-scanline decode scratch buffer allocated in `exr_decoding_run()`.

**Valgrind Trace (top frames):**
```bash
==454== Conditional jump or move depends on uninitialised value(s)
==454==    at 0x4539BE: generic_unpack (in /out/openexr_exrcheck_fuzzer)
==454==    by 0x44B85F: exr_decoding_run (in /out/openexr_exrcheck_fuzzer)
==454==    by 0x38BC5F: Imf_4_0::(anonymous namespace)::TileProcess::run_decode(_priv_exr_context_t const*, int, Imf_4_0::FrameBuffer const*, std::__1::vector<Imf_4_0::Slice, std::__1::allocator<Imf_4_0::Slice> > const&) (in /out/openexr_exrcheck_fuzzer)
==454==    by 0x388BE1: Imf_4_0::TiledInputFile::Data::readTiles(int, int, int, int, int, int) (in /out/openexr_exrcheck_fuzzer)
==454==    by 0x388619: Imf_4_0::TiledInputFile::readTiles(int, int, int, int, int, int) (in /out/openexr_exrcheck_fuzzer)
==454==    by 0x353755: Imf_4_0::InputFile::Data::bufferedReadPixels(int, int) (in /out/openexr_exrcheck_fuzzer)
==454==    by 0x352286: Imf_4_0::InputFile::readPixels(int) (in /out/openexr_exrcheck_fuzzer)
==454==    by 0x3190FA: Imf_4_0::(anonymous namespace)::readMultiPart(Imf_4_0::MultiPartInputFile&, bool, bool) (in /out/openexr_exrcheck_fuzzer)
==454==    by 0x314C4D: Imf_4_0::checkOpenEXRFile(char const*, unsigned long, bool, bool, bool) (in /out/openexr_exrcheck_fuzzer)
==454==  Uninitialised value was created by a heap allocation at 0x483B7F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
```

### PoC
In the attached archive, you will find:
- The executable used for our tests.
- The testcase used to trigger the bug.

To observe the bug, simply run the OSS-Fuzz helper script:
```bash
git clone https://github.com/google/oss-fuzz.git
cd oss-fuzz

python3 infra/helper.py build_image openexr
python3 infra/helper.py build_fuzzers --sanitizer=none openexr
python3 infra/helper.py shell openexr

apt update && apt install -y valgrind
ulimit -n 65535
valgrind --tool=memcheck --track-origins=yes /out/openexr_exrcheck_fuzzer /path/to/poc
```

### Impact
- Undefined Behavior
- Potential crash
- Denial of Service

**Credit:** Aldo Ristori
[archive0.zip](https://github.com/user-attachments/files/23024726/archive0.zip)



### Update Note:
Other saved testcases from the fuzzing campaign trigger the same underlying bug, but with a different manifestation. So there is one root cause (missing post-decode validation / zero-init before any unpack), with different call-sites. Below there are several archives, formatted like the previous one, that reproduce the other test cases.

**Other observed sinks (distinct manifestations of the same bug):**

**Deep pointers path:**
generic_unpack_deep_pointers (deep scanline/tiled)
[archive1.zip](https://github.com/user-attachments/files/23024736/archive1.zip)


**Deep sample table path:**
unpack_sample_table (deep scanline)
[archive2.zip](https://github.com/user-attachments/files/23024740/archive2.zip)


**Half conversion path:**
half_to_float_buffer_f16c via unpack_half_to_float_3chan_planar
[archive3.zip](https://github.com/user-attachments/files/23024744/archive3.zip)


**Deep compositing:**
CompositeDeepScanLine::readPixels → ThreadPool::addTask → LineCompositeTask::execute
[archive4.zip](https://github.com/user-attachments/files/23024746/archive4.zip)
CVSS v4.0 score: 2
CVSS v3.1 score: 7.5