# Null Pointer Dereference in ExecuTorch FlatTensorDataMap::get_key() via Missing NamedData.key (.ptd)

**Target:** ExecuTorch 1.3.1 (.ptd, huntr Model File Vulnerability program)
**Severity:** Low-Medium (Denial of Service)
**CWE:** CWE-476 (NULL Pointer Dereference)
**Component:** `extension/flat_tensor/flat_tensor_data_map.cpp`
**Authentication Required:** No — only requires a victim application to load an attacker-supplied `.ptd` file and call a public enumeration API.

## Summary

`FlatTensorDataMap::get_key()` is a public API for enumerating the names of tensors/blobs stored in a `.ptd` file by index — the standard pattern for an application to discover what data a file contains before requesting it by name. It dereferences `NamedData.key` without checking it for null, even though `key` is an ordinary (non-required) field in the schema. A `.ptd` file with a `NamedData` entry present in the `named_data` vector (so it is counted by `get_num_keys()`) but whose own `key` field is omitted crashes any process that enumerates keys by index — a completely standard, expected usage pattern, not an edge case.

**Confirmed against the pristine, unmodified ExecuTorch 1.3.1 source.**

## Vulnerability Details

`extension/flat_tensor/serialize/flat_tensor.fbs`:

```
table NamedData {
  key: string;              // not required
  segment_index: uint32;
  tensor_layout: TensorLayout;
}
```

`extension/flat_tensor/flat_tensor_data_map.cpp` (pristine 1.3.1 source, lines 197–211):

```cpp
ET_NODISCARD Result<uint32_t> FlatTensorDataMap::get_num_keys() const {
  return flat_tensor_->named_data()->size();
}

ET_NODISCARD Result<const char*> FlatTensorDataMap::get_key(
    uint32_t index) const {
  uint32_t num_keys = get_num_keys().get();
  ET_CHECK_OR_RETURN_ERROR(
      index >= 0 && index < num_keys,
      InvalidArgument,
      "Index %u out of range of size %u",
      index,
      num_keys);
  return flat_tensor_->named_data()->Get(index)->key()->c_str();   // <-- crash site, line 201 (offset within function)
}
```

`FlatTensorDataMap::load()`'s validation only checks that the `named_data` **vector itself** is non-null at the top level — it never inspects the fields of individual `NamedData` elements. `get_named_data()` (the linear-search helper backing `get_tensor_layout()`/`get_data()`/`load_data_into()`) happens to implicitly avoid crashing on a null `key` in some cases due to how its comparison is structured, but `get_key()` is a **completely separate, simpler code path** that enumerates by index directly — it does not go through `get_named_data()` at all, so it inherits none of that incidental behavior.

## Steps to Reproduce

### Environment
Linux x86-64, ExecuTorch 1.3.1 pristine source, clang-16, CMake, Ninja. No authentication, no host access.

### 1. Build ExecuTorch with sanitizers

Same build as REPORT-01 Step 1 (`EXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR=ON`).

### 2. Build the harness (`poc/harness_flat_tensor_fuzzer.cpp`, included — identical to REPORT-01's harness, which already calls `get_key()` for every index up to `get_num_keys()`)

```bash
export ET_PARENT=/path/to/parent-of-executorch
C10_INC="$ET_SRC/runtime/core/portable_type/c10"
INCLUDES="-I$ET_PARENT -I$ET_BUILD -I$ET_BUILD/schema/include -I$ET_BUILD/extension/flat_tensor/include -I$ET_BUILD/third-party/flatc_ep/include -I$C10_INC"

clang++-16 -std=c++17 -fsanitize=fuzzer,address,undefined -fno-omit-frame-pointer -fno-sanitize-recover=all \
  $INCLUDES -DFLATBUFFERS_MAX_ALIGNMENT=1024 -DC10_USING_CUSTOM_GENERATED_MACROS \
  -c poc/harness_flat_tensor_fuzzer.cpp -o harness.o

clang++-16 -fsanitize=fuzzer,address,undefined -o poc_harness harness.o \
  "$ET_BUILD/extension/flat_tensor/libextension_flat_tensor.a" \
  "$ET_BUILD/extension/data_loader/libextension_data_loader.a" \
  "$ET_BUILD/libexecutorch_core.a"
```

### 3. PoC file

`poc/poc_get_key_null.ptd` (272 bytes, **included in this report — sha256 `427bc13020be820b41aafec2c6e00de360fe97185d3879d5ec770594c21ac39`**) contains a `named_data` vector with at least one `NamedData` entry whose `key` field is omitted.

### 4. Trigger the crash

```bash
export ASAN_OPTIONS="abort_on_error=1:symbolize=0"
export UBSAN_OPTIONS="halt_on_error=1:print_stacktrace=0"
./poc_harness -timeout=5 -runs=0 poc/poc_get_key_null.ptd
```

### Expected result (secure behavior)
`get_key()` should return a clean `Error` (e.g. `InvalidExternalData`) for an entry with no `key`, rather than crashing.

### Actual result — verified against the pristine, unmodified ExecuTorch 1.3.1 source

```
Running: poc/poc_get_key_null.ptd
extension/flat_tensor/flat_tensor_data_map.cpp:201:57: runtime error: member call on null pointer of type 'flatbuffers::String'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior extension/flat_tensor/flat_tensor_data_map.cpp:201:57 in
==<pid>== ERROR: libFuzzer: deadly signal
```

**Reproduced 3/3 identical runs against the pristine source** (re-verified live for this report):
```
run 1: extension/flat_tensor/flat_tensor_data_map.cpp:201:57: runtime error: member call on null pointer of type 'flatbuffers::String'
run 2: (identical)
run 3: (identical)
```

## Impact

**Who is affected:** Any application enumerating tensor/blob names in a `.ptd` file via `get_num_keys()` + `get_key()` — the standard discovery pattern for this format.

**What the attacker can do:** Cause a deterministic crash simply by having one entry in the `named_data` vector with no `key` — the file still loads successfully (the vector itself is present), so the crash only surfaces during ordinary enumeration.

**What's at risk:** Availability only.

**Why not Critical:** Controlled null-pointer dereference, no memory corruption or code execution.

## Suggested Remediation

```cpp
ET_NODISCARD Result<const char*> FlatTensorDataMap::get_key(
    uint32_t index) const {
  uint32_t num_keys = get_num_keys().get();
  ET_CHECK_OR_RETURN_ERROR(
      index >= 0 && index < num_keys,
      InvalidArgument,
      "Index %u out of range of size %u",
      index,
      num_keys);
  auto* named_data = flat_tensor_->named_data()->Get(index);
  ET_CHECK_OR_RETURN_ERROR(
      named_data != nullptr && named_data->key() != nullptr,
      InvalidExternalData,
      "NamedData entry %u or its key is null",
      index);
  return named_data->key()->c_str();
}
```

**Design recommendation:** this is one of several findings in this file (see companion reports on `get_tensor_layout()`/`create_tensor_layout()`) where the file validates the top-level presence of `named_data`/`segments` vectors at `load()` time but never validates the fields within each element. A single validation pass over `named_data()` at load time — or a shared per-entry validator called from every accessor — would close this entire class of findings at once rather than requiring a separate patch per accessor as each is independently discovered.

A regression test should build a `.ptd` with a `NamedData` entry that has `key` omitted, asserting `get_key()` returns a clean `Error` rather than crashing.

## Files Included in This Report

- `poc/poc_get_key_null.ptd` — the 272-byte PoC file (sha256 `427bc13020be820b41aafec2c6e00de360fe97185d3879d5ec770594c21ac39`)
- `poc/harness_flat_tensor_fuzzer.cpp` — the harness used to trigger and reproduce the crash (same binary as REPORT-01/02/04, which all target `extension/flat_tensor/flat_tensor_data_map.cpp`)

## huntr Submission Note

Per the huntr MFV program's submission requirements, this PoC needs to be uploaded to a public HuggingFace repository before filing. `poc/poc_get_key_null.ptd` is ready for that upload.