REPORT.md

Null Pointer Dereference in ExecuTorch BackendDelegate::PopulateCompileSpecs() via Missing CompileSpec.key (.pte)

Target: ExecuTorch 1.3.1 (.pte, huntr Model File Vulnerability program) Severity: Low-Medium (Denial of Service) CWE: CWE-476 (NULL Pointer Dereference) Component: runtime/executor/method.cpp Authentication Required: No — requires a victim application to load a .pte file whose method has a backend delegate with a compile_specs list, with at least one backend registered/available (a routine deployment configuration).

Summary

BackendDelegate::PopulateCompileSpecs() dereferences each CompileSpec entry's key field unconditionally via ->c_str(), without checking it for null. CompileSpec.key is declared as an optional string in the schema. A .pte file with a delegate whose compile_specs list contains an entry with a value but no key crashes the moment Method::init() resolves that delegate — before any execution begins.

This is the third distinct null-pointer-dereference finding reached through the same Method::init() delegate-resolution code path in this investigation (following the Operator.overload and Program.backend_delegate_data findings), reinforcing that this area of the codebase was written with less consistent defensive guarding than the tensor-parsing paths.

Vulnerability Details

runtime/executor/method.cpp's BackendDelegate::PopulateCompileSpecs():

static Error PopulateCompileSpecs(
    const flatbuffers::Vector<flatbuffers::Offset<
        executorch_flatbuffer::CompileSpec>>* compile_specs_in_program,
    BackendInitContext& backend_init_context,
    CompileSpec** out_spec) {
  auto number_of_compile_specs = compile_specs_in_program->size();
  CompileSpec* compile_specs_list = ...;
  ...
  for (size_t j = 0; j < number_of_compile_specs; j++) {
    auto compile_spec_in_program = compile_specs_in_program->Get(j);

    compile_specs_list[j].key = compile_spec_in_program->key()->c_str();  // <-- crash site
    compile_specs_list[j].value = {
        static_cast<void*>(const_cast<uint8_t*>(compile_spec_in_program->value()->Data())),
        compile_spec_in_program->value()->size(),
    };
  }
  ...

schema/program.fbs:

table CompileSpec {
  // One compile spec. There are can be multiple specs for one method
  key: string; // like max_value
  value: [ubyte]; // like 4, or other types based on needs.
}

PopulateCompileSpecs() is called from BackendDelegate::Init() whenever delegate.compile_specs() != nullptr — a normal, documented way for a delegate to pass backend-specific compilation options.

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-11/12 Step 1.

2. Build the execute-level harness (poc/harness_execute_fuzzer.cpp, same harness as REPORT-11/12, included in this report)

This harness registers fake backends under common names (XnnpackBackend, QnnBackend, etc.) that trivially succeed on init(), exercising the real BackendDelegate::Init()/PopulateCompileSpecs() code path.

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_execute_fuzzer.cpp -o harness.o

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

3. PoC file

poc/poc_compilespec_key_null.pte (44,800 bytes, included in this report — sha256 673c5e5f95788827bf24bef50f7a2de39aab9e63b17278a5415769746bd417bb). This PoC was originally discovered by coverage-guided fuzzing (a 1,355,528-byte input) and minimized via libFuzzer's built-in crash minimizer to this 44,800-byte reproducer while preserving the exact same crash.

4. Trigger the crash

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_compilespec_key_null.pte

Expected result (secure behavior)

Method::init() should return a clean Error (or substitute an empty string) when a CompileSpec entry has no key, rather than crashing.

Actual result — verified against BOTH a patched build AND an independently rebuilt, wholly unmodified pristine ExecuTorch 1.3.1 source tree

Running: poc/poc_compilespec_key_null.pte
runtime/executor/method.cpp:181:67: runtime error: member call on null pointer of type 'flatbuffers::String'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior runtime/executor/method.cpp:181:67 in
==<pid>== ERROR: libFuzzer: deadly signal
    #0 ... (abort machinery)
    BackendDelegate::PopulateCompileSpecs (method.cpp:181)
    BackendDelegate::Init (method.cpp)
    Method::init (method.cpp:971)

Reproduced 3/3 identical runs against the independently rebuilt pristine source (re-verified live for this report):

run 1: member call on null pointer of type 'flatbuffers::String'
run 2: (identical)
run 3: (identical)

Impact

Who is affected: Any application calling Program::load_method() on a method whose delegates specify compile_specs with a null key, where the victim has registered/made available a backend matching the delegate's id — a routine configuration for any deployment with at least one hardware-acceleration backend.

What the attacker can do: Cause a deterministic crash during method loading, unconditionally (no build-flag dependency).

What's at risk: Availability only.

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

Suggested Remediation

compile_specs_list[j].key = (compile_spec_in_program->key() != nullptr)
    ? compile_spec_in_program->key()->c_str()
    : "";

Design recommendation: consider a single validation pass over all CompileSpec entries in a delegate's compile_specs list at the top of PopulateCompileSpecs(), checking both key and value for null in one place, consistent with the recommendation made in the companion Operator.overload (REPORT-11) and Program.backend_delegate_data (REPORT-12) findings — all three sit in the same delegate-resolution code area and would benefit from a shared, consistent validation approach.

A regression test should build a .pte with one BackendDelegate (using a registered fake backend) whose compile_specs contains an entry with a value but no key, asserting Method::load() returns a clean Error rather than crashing.

Files Included in This Report

• poc/poc_compilespec_key_null.pte — the 44,800-byte minimized PoC file (sha256 673c5e5f95788827bf24bef50f7a2de39aab9e63b17278a5415769746bd417bb)
• poc/harness_execute_fuzzer.cpp — the harness used to trigger and reproduce the crash, including fake backend registration for exercising BackendDelegate::Init()/PopulateCompileSpecs()

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_compilespec_key_null.pte is ready for that upload.