fix: guard against null prev_val in JsonPrinter::PrintOffset for unions (fixes #9033) - #9236
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fix: guard against null prev_val in JsonPrinter::PrintOffset for unions (fixes #9033)#9236jdymitarai wants to merge 1 commit into
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…ns (fixes google#9033) In JsonPrinter::PrintOffset(), union fields require reading the union type discriminator byte via `prev_val`. Previously, the code only checked `FLATBUFFERS_ASSERT(prev_val)`. In release builds (-DNDEBUG), asserts are compiled out. If a malformed or corrupt buffer has the union value field present in the vtable but the union type discriminator absent, `prev_val` is nullptr, causing an immediate segmentation fault (CWE-476). Similarly, if vector_index is out of range for the union type vector, it could lead to an out-of-bounds read. This commit adds runtime validation for `prev_val` and vector bounds before dereferencing, returning descriptive error messages and propagating them safely through GenText(). Also adds a unit test in tests/test.cpp to ensure corrupt buffers are rejected cleanly. Fixes google#9033.
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Summary of Changes
Fixes #9033 (CWE-476 Null Pointer Dereference in
JsonPrinter::PrintOffset()for union types).Root Cause
In
src/idl_gen_text.cpp,JsonPrinter::PrintOffset()dereferencesprev_valto obtain the union type discriminator:In release builds (
-DNDEBUG),FLATBUFFERS_ASSERTis removed. When processing an untrusted or malformed FlatBuffer where the union value field is present in the table vtable but the union type discriminator is absent (vtable offset zeroed),prev_valisnullptr. Attempting*prev_valimmediately causes a segmentation fault (SIGSEGV), terminatingflatcor any library converting unverified/corrupted binary buffers to JSON.Additionally, for vectors of unions (
vector_index >= 0), ifvector_index >= type_vec->size(), accessingtype_vec->Get(vector_index)can result in an out-of-bounds read.Fix
FLATBUFFERS_ASSERT(prev_val)with a runtime check returning"corrupt buffer: missing union type field".static_cast<uoffset_t>(vector_index) >= type_vec->size()) returning"corrupt buffer: union type vector out of range".tests/test.cpp(UnionVectorTest) that crafts a corrupted vtable with missing union type discriminator and verifies thatGenText()returns an error string instead of crashing.