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 "ht_undo_impl()" function in "src/lib/OpenEXRCore/internal_ht.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 "INT_MAX", 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 "ht_undo_impl()" is entered. This is the second distinct integer overflow in "ht_undo_impl()". CVE-2026-34545 addressed a different overflow in the same function — the "int16_t 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 "wcount*nx" overflow in "internal_piz.c") and should be remediated with the same pattern. The CVE-2026-34588 fix did not touch "internal_ht.cpp". Version 3.4.10 contains a remediation that addresses the vulnerability in "internal_ht.cpp".