GHSA-hff9-5pqw-6w9f

Summary

The ICO loader allocates data_size + BMP_FILE_HEADER_LEN bytes after reading a 32-bit attacker-controlled data_size. For data_size > UINT32_MAX - BMP_FILE_HEADER_LEN, the addition wraps before allocation. imgproxy then writes the ICO prefix and reads up to the original declared size starting 14 bytes into the undersized allocation. A network client can turn this heap overflow into remote denial of service or remote code execution.

Technical details

The vulnerable path is in vips/icoload.c at commit f4fedbb:

uint32_t data_size = GUINT32_FROM_LE(largest_image_header.data_size);

VipsPel *data = (VipsPel *) VIPS_MALLOC(NULL, data_size + BMP_FILE_HEADER_LEN);
void *actual_data = data + BMP_FILE_HEADER_LEN;

if (vips_foreign_load_read_full(ico->source, actual_data, data_size) <= 0) {
  vips_error("vips_foreign_load_ico_header", "unable to read ICO image data from the source");
  return -1;
}

The crafted directory entry contains:

data_size       = 0xfffffff5 (4294967285)
data_offset     = 22
BMP header size = 14
32-bit result   = (0xfffffff5 + 14) mod 2^32 = 3

The allocation is therefore three bytes. actual_data points 14 bytes after the allocation base. The fixture contains 64 bytes after the ICO header, so the source callback copies those 64 bytes to an invalid destination before it can report that the declared multi-gigabyte payload is incomplete.

The complete source-to-sink path is:

1. Attacker-controlled bytes begin with the ICO magic 00 00 01 00.
2. imgproxy identifies the source as ICO and dispatches it to vips_icoload_source_go.
3. vips_foreign_load_ico_header reads the attacker-controlled data_size.
4. The 32-bit addition wraps, and VIPS_MALLOC receives the value three.
5. vips_foreign_load_read_full passes the original data_size to the Go source callback.
6. The callback writes the available source bytes through the invalid pointer, corrupting the process heap.

Impact

A crafted image can terminate the imgproxy process handling it or potentially lead to remote code execution due to heap corruption.