CVE-2026-39886: OpenEXR has HTJ2K Signed Integer Overflow in ht_undo_impl()

OpenEXR provides the specification and reference implementation of the EXR file format, an image storage format for the motion picture industry. Versions 3.4.0 through 3.4.9 have a signed integer overflow vulnerability in OpenEXR's HTJ2K (High-Throughput JPEG 2000) decompression path. The htundoimpl() function in src/lib/OpenEXRCore/internalht.cpp accumulates a bytes-per-line value (bpl) using a 32-bit signed integer with no overflow guard. A crafted EXR file with 16,385 FLOAT channels at the HTJ2K maximum width of 32,767 causes bpl to overflow INTMAX, producing undefined behavior confirmed by UBSan. On an allocator-permissive host where the required ~64 GB allocation succeeds, the wrapped negative bpl value would subsequently be used as a per-scanline pointer advance, which would produce a heap out-of-bounds write. On a memory-constrained host, the allocation fails before htundoimpl() is entered. This is the second distinct integer overflow in htundoimpl(). CVE-2026-34545 addressed a different overflow in the same function — the int16t p pixel-loop counter at line ~302 that overflows when iterating over channels whose width exceeds 32,767. The CVE-2026-34545 fix did not touch the int bpl accumulator at line 211, which is the subject of this advisory. The bpl accumulator was also not addressed by any of the 8 advisories in the 2026-04-05 v3.4.9 release batch. This finding is structurally identical to CVE-2026-34588 (PIZ wcountnx overflow in internalpiz.c) and should be remediated with the same pattern. The CVE-2026-34588 fix did not touch internalht.cpp. Version 3.4.10 contains a remediation that addresses the vulnerability in internalht.cpp.