GHSA-3h9h-qfvw-98hq

### 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)