CVE-2026-34588: OpenEXR has a signed 32-bit Overflow in PIZ Decoder Leads to OOB Read/Write

Summary

internalexrundopiz() advances the working wavelet pointer with signed 32-bit arithmetic:

c wavbuf += nx ny wcount;

Because nx, ny, and wcount are int, a crafted EXR file can make this product overflow and wrap. The next channel then decodes from an incorrect address. The wavelet decode path operates in place, so this yields both out-of-bounds reads and out-of-bounds writes.

Tested on commit 7820b7e1b93405ba1d551c43a945018226b75bc5

Technical Details

The vulnerable decode path is:

1. internalexrundopiz() sets wavbuf = decode->scratchbuffer1. 2. For each channel, it calls wav2Ddecode (wavbuf + j, ...). 3. It then advances wavbuf with wavbuf += nx ny wcount.

The overflow happens in step 3. Once wavbuf is wrapped, the next channel's wavelet decode runs on the wrong address.

In the 14-bit wavelet path, wdec144() first reads:

- px - p10 - p01 - p11

and then writes back to the same locations:

- px = ... - p01 = ... - p10 = ... - p11 = ...

As a result, the bug is not just a crash-only invalid read. It is an out-of-bounds read/write condition.

Reproduction

pizscanlineredzone.zip

Build exrcheck with ASAN and run:

❯ ./build-asan/bin/exrcheck /tmp/pizscanlineredzone.exr file /tmp/pizscanlineredzone.exr /home/pop/sec/openexr/src/lib/OpenEXRCore/internalpiz.c:373:19: runtime error: signed integer overflow: 134217724 32 cannot be represented in type 'int' ================================================================= ==1711239==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x7bedc3934700 at pc 0x7bf1f100f498 bp 0x7ffe032d8f00 sp 0x7ffe032d8ef0 READ of size 2 at 0x7bedc3934700 thread T0 #0 0x7bf1f100f497 in wdec144 /home/pop/sec/openexr/src/lib/OpenEXRCore/internalpiz.c:148 #1 0x7bf1f100f497 in wav2Ddecode /home/pop/sec/openexr/src/lib/OpenEXRCore/internalpiz.c:403 #2 0x7bf1f100f497 in internalexrundopiz /home/pop/sec/openexr/src/lib/OpenEXRCore/internalpiz.c:727 #3 0x7bf1f115b038 in exruncompresschunk /home/pop/sec/openexr/src/lib/OpenEXRCore/compression.c:546 #4 0x7bf1f1161168 in exrdecodingrun /home/pop/sec/openexr/src/lib/OpenEXRCore/decoding.c:580 #5 0x7bf1f2a71add in rundecode /home/pop/sec/openexr/src/lib/OpenEXR/ImfScanLineInputFile.cpp:586 #6 0x7bf1f2a83dc4 in Imf40::ScanLineInputFile::Data::readPixels(Imf40::FrameBuffer const&, int, int) /home/pop/sec/openexr/src/lib/OpenEXR/ImfScanLineInputFile.cpp:500 #7 0x7bf1f28c6a81 in Imf40::InputFile::Data::readPixels(int, int) /home/pop/sec/openexr/src/lib/OpenEXR/ImfInputFile.cpp:458 #8 0x7bf1f3bfe2dc in readScanline<Imf40::InputPart> /home/pop/sec/openexr/src/lib/OpenEXRUtil/ImfCheckFile.cpp:239 #9 0x7bf1f3c05b04 in readMultiPart /home/pop/sec/openexr/src/lib/OpenEXRUtil/ImfCheckFile.cpp:905 #10 0x7bf1f3c126fd in runChecks<char const> /home/pop/sec/openexr/src/lib/OpenEXRUtil/ImfCheckFile.cpp:1171 #11 0x7bf1f3c146b9 in Imf40::checkOpenEXRFile(char const, bool, bool, bool) /home/pop/sec/openexr/src/lib/OpenEXRUtil/ImfCheckFile.cpp:1835 #12 0x5d9675fce8f8 in exrCheck(char const, bool, bool, bool, bool) /home/pop/sec/openexr/src/bin/exrcheck/main.cpp:96 #13 0x5d9675fcb2b1 in main /home/pop/sec/openexr/src/bin/exrcheck/main.cpp:164 #14 0x7bf1efe2a1c9 in libcstartcallmain ../sysdeps/nptl/libcstartcallmain.h:58 #15 0x7bf1efe2a28a in libcstartmainimpl ../csu/libc-start.c:360 #16 0x5d9675fcc844 in start (/home/pop/sec/openexr/build-asan/bin/exrcheck+0xe844) (BuildId: 087c972343a5372940c42c0a2e7bce4a84288aec)

0x7bedc3934700 is located 256 bytes before 8590720784-byte region [0x7bedc3934800,0x7befc39f4710) allocated by thread T0 here: #0 0x7bf1f40fd9c7 in malloc ../../../../src/libsanitizer/asan/asanmalloclinux.cpp:69 #1 0x7bf1f115883e in internaldecodeallocbuffer /home/pop/sec/openexr/src/lib/OpenEXRCore/coding.c:256 #2 0x7bf1f100da97 in internalexrundopiz /home/pop/sec/openexr/src/lib/OpenEXRCore/internalpiz.c:643 #3 0x7bf1f115b038 in exruncompresschunk /home/pop/sec/openexr/src/lib/OpenEXRCore/compression.c:546 #4 0x7bf1f1161168 in exrdecodingrun /home/pop/sec/openexr/src/lib/OpenEXRCore/decoding.c:580 #5 0x7bf1f2a71add in rundecode /home/pop/sec/openexr/src/lib/OpenEXR/ImfScanLineInputFile.cpp:586 #6 0x7bf1f2a83dc4 in Imf40::ScanLineInputFile::Data::readPixels(Imf40::FrameBuffer const&, int, int) /home/pop/sec/openexr/src/lib/OpenEXR/ImfScanLineInputFile.cpp:500 #7 0x7bf1f28c6a81 in Imf40::InputFile::Data::readPixels(int, int) /home/pop/sec/openexr/src/lib/OpenEXR/ImfInputFile.cpp:458 #8 0x7bf1f3bfe2dc in readScanline<Imf40::InputPart> /home/pop/sec/openexr/src/lib/OpenEXRUtil/ImfCheckFile.cpp:239 #9 0x7bf1f3c05b04 in readMultiPart /home/pop/sec/openexr/src/lib/OpenEXRUtil/ImfCheckFile.cpp:905 #10 0x7bf1f3c126fd in runChecks<char const> /home/pop/sec/openexr/src/lib/OpenEXRUtil/ImfCheckFile.cpp:1171 #11 0x7bf1f3c146b9 in Imf40::checkOpenEXRFile(char const, bool, bool, bool) /home/pop/sec/openexr/src/lib/OpenEXRUtil/ImfCheckFile.cpp:1835 #12 0x5d9675fce8f8 in exrCheck(char const, bool, bool, bool, bool) /home/pop/sec/openexr/src/bin/exrcheck/main.cpp:96 #13 0x5d9675fcb2b1 in main /home/pop/sec/openexr/src/bin/exrcheck/main.cpp:164 #14 0x7bf1efe2a1c9 in libcstartcallmain ../sysdeps/nptl/libcstartcallmain.h:58 #15 0x7bf1efe2a28a in libcstartmainimpl ../csu/libc-start.c:360 #16 0x5d9675fcc844 in start (/home/pop/sec/openexr/build-asan/bin/exrcheck+0xe844) (BuildId: 087c972343a5372940c42c0a2e7bce4a84288aec)

SUMMARY: AddressSanitizer: heap-buffer-overflow /home/pop/sec/openexr/src/lib/OpenEXRCore/internalpiz.c:148 in wdec144 Shadow bytes around the buggy address: 0x7bedc3934480: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa 0x7bedc3934500: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa 0x7bedc3934580: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa 0x7bedc3934600: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa 0x7bedc3934680: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa =>0x7bedc3934700:[fa]fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa 0x7bedc3934780: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa 0x7bedc3934800: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0x7bedc3934880: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0x7bedc3934900: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0x7bedc3934980: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 Shadow byte legend (one shadow byte represents 8 application bytes): Addressable: 00 Partially addressable: 01 02 03 04 05 06 07 Heap left redzone: fa Freed heap region: fd Stack left redzone: f1 Stack mid redzone: f2 Stack right redzone: f3 Stack after return: f5 Stack use after scope: f8 Global redzone: f9 Global init order: f6 Poisoned by user: f7 Container overflow: fc Array cookie: ac Intra object redzone: bb ASan internal: fe Left alloca redzone: ca Right alloca redzone: cb ==1711239==ABORTING

To prove this is both READ and WRITE, we can also memcheck against non-ASAN release build:

sh valgrind --tool=memcheck --leak-check=no --track-origins=no \ --error-limit=no --num-callers=20 \ ./build-relwithdebinfo/bin/exrcheck /tmp/pizscanlineredzone.exr

Observed result:

- Invalid read of size 2 at internalpiz.c:150 - Invalid write of size 2 at internalpiz.c:171

This confirms the bug is an OOB read/write, not only a read-first crash.

Redzone-Oriented File

- width: 67108862 - height: 32 - channel A: FLOAT, sampling 1 x 1 - channel B: HALF, sampling 33554431 x 16

This makes:

text width 32 2 = 4294967168

which wraps signed 32-bit arithmetic to -128.

That places the next wavbuf access just before the allocated buffer, producing a clean heap-overflow report.

Impact

A crafted EXR file can trigger out-of-bounds memory access during PIZ decompression. The primitive includes both invalid reads and invalid writes. Depending on allocator layout and surrounding memory, this could lead to process crash, memory corruption, or potentially stronger exploitation outcomes.

Recommended Fix

- compute channel span in 64-bit arithmetic - reject any overflow in nx ny wcount - validate cumulative per-channel decoded footprint against outsz before wavelet decode - fail decompression if channel-derived layout does not exactly fit the decompression buffer -------- Found by: Quang Luong of Calif.io