Pillow: Out-of-bounds read via attacker-controlled row stride on Pillow's mmap path (McIdas AREA files)
Summary When Pillow loads an uncompressed image whose tile uses the raw codec and a mode in Image._MAPMODES, and the image was opened from a filename, it memory-maps the file and builds the image's row pointers directly into the mapping via PyImaging_MapBuffer (src/map.c). The per-row spacing (stride) is taken from the tile arguments. map.c validates offset + ysizestride <= buffer_len but never checks that stride is at least the natural row width xsize pixelsize. The McIdas* AREA plugin (McIdasImagePlugin.py) derives stride, offset, xsize, and ysize directly from attacker-controlled 32-bit header words with no validation. By supplying a stride far smaller than the row width, an attacker makes each row pointer read xsizepixelsize bytes that run past the mapped region. Accessing the pixels (e.g. Image.tobytes(), getpixel, convert, save) then reads adjacent process memory (information disclosure) or faults (SIGBUS, denial of service). Complete Code Trace Step 1: McIdasImageFile._open - turns attacker header words into image size, file offset, and row stride with no validation. ``python src/PIL/McIdasImagePlugin.py:41-70 s = self.fp.read(256) if not _accept(s) or len(s) != 256: # _accept: prefix == b"\x00\x00\x00\x00\x00\x00\x00\x04" raise SyntaxError(...) self.area_descriptor = w = [0, struct.unpack("!64i", s)] # w[1..64] = signed BE int32, ALL attacker-controlled if w[11] == 1: mode = rawmode = "L" # pixelsize 1, in _MAPMODES elif w[11] == 2: mode = rawmode = "I;16B" # pixelsize 2, in _MAPMODES ... self._mode = mode self._size = w[10], w[9] # (xsize, ysize) <-- attacker offset = w[34] + w[15] # <-- attacker stride = w[15] + w[10] w[11] w[14] # <-- attacker (set w[14]=0, w[15]=1 => stride=1) self.tile = [ ImageFile._Tile("raw", (0, 0) + self.size, offset, (rawmode, stride, 1)) ] ` Step 2: ImageFile.load (mmap branch) - selects mmap and delegates to map_buffer. `python src/PIL/ImageFile.py:322-348 if use_mmap: # use_mmap = self.filename and len(self.tile) == 1 decoder_name, extents, offset, args = self.tile[0] if (decoder_name == "raw" and isinstance(args, tuple) and len(args) >= 3 and args[0] == self.mode and args[0] in Image._MAPMODES): if offset < 0: # only lower-bound guard on offset raise ValueError("Tile offset cannot be negative") with open(self.filename) as fp: self.map = mmap.mmap(fp.fileno(), 0, access=mmap.ACCESS_READ) if offset + self.size[1] args[1] > self.map.size(): # == offset + ysizestride; NO stride>=linesize check raise OSError("buffer is not large enough") self.im = Image.core.map_buffer( self.map, self.size, decoder_name, offset, args # args = ("L", stride, 1) ) ` Step 3: PyImaging_MapBuffer - builds row pointers at stride spacing into the mmap; validates everything except stride >= row width. `c / src/map.c:65-140 / if (!PyArg_ParseTuple(args, "O(ii)sn(sii)", &target, &xsize, &ysize, &codec, &offset, &mode_name, &stride, &ystep)) return NULL; ... const ModeID mode = findModeID(mode_name); / "L" / if (stride <= 0) { / attacker sets stride=1 (>0) -> NOT recomputed / if (mode == IMAGING_MODE_L || mode == IMAGING_MODE_P) stride = xsize; else if (isModeI16(mode)) stride = xsize 2; else stride = xsize 4; } if (stride > 0 && ysize > PY_SSIZE_T_MAX / stride) {/ overflow guard only / PyErr_SetString(PyExc_MemoryError, "Integer overflow in ysize"); return NULL; } size = (Py_ssize_t)ysize stride; / = 11 = 1 / if (offset > PY_SSIZE_T_MAX - size) { ... } ... if (offset + size > view.len) { / 1 + 1 = 2 <= 256 -> PASSES / PyErr_SetString(PyExc_ValueError, "buffer is not large enough"); PyBuffer_Release(&view); return NULL; } im = ImagingNewPrologueSubtype(mode, xsize, ysize, sizeof(ImagingBufferInstance)); / im->linesize = xsize pixelsize = 200000 (the REAL per-row read width) / / setup file pointers -- NO check that stride >= im->linesize / if (ystep > 0) { for (y = 0; y < ysize; y++) { im->image[y] = (char )view.buf + offset + y stride; / row points into mmap, spacing=1 / } } else { ... } ` im->linesize (the number of bytes any consumer reads per row) is xsize pixelsize = 200000, but the row pointers are only stride = 1 byte apart and the buffer is only offset + ysizestride = 2 bytes "claimed". Nothing reconciles the two. Step 4: pixel access (Image.tobytes() → raw encoder copy1) - reads linesize bytes from im->image[0], i.e. xsize bytes starting at view.buf + offset, running far past the mmap. `c / the raw "L" packer copies linesize (=xsize) bytes per row from im->image[y]; for row 0 that is view.buf+1 .. view.buf+1+200000, vs a 256-byte file. / ` Chain Summary ` SOURCE: McIdas AREA header words w[9],w[10],w[11],w[14],w[15],w[34] (Image.open on a path) ↓ McIdasImagePlugin._open: stride = w[15]+w[10]w[11]w[14] -> attacker sets stride=1 [McIdasImagePlugin.py:66] ↓ tile = ("raw", (0,0,xsize,1), offset, ("L", 1, 1)) [McIdasImagePlugin.py:68] GADGET: ImageFile.load mmap branch -- only checks offset+ysizestride<=len <- BUG: no stride>=linesize check [ImageFile.py:343] ↓ core.map_buffer(map, (xsize,1), "raw", offset, ("L",1,1)) [ImageFile.py:346] SINK: PyImaging_MapBuffer: im->image[0] = view.buf + offset + 0stride; linesize=xsize [map.c:134] ↓ Image.tobytes() raw "L" encoder reads linesize (=xsize) bytes from im->image[0] IMPACT: reads xsize bytes from a tiny mmap -> OOB read of adjacent process memory (leak) or SIGBUS (DoS) ` Proof of Concept See attached poc.zip Impact on a Parent Application Any application that opens image files supplied by users from a path on disk (the common pattern: save upload to a temp file, then Image.open(path)), has the default plugin set (McIdas is registered by default), and subsequently reads/returns/re-encodes the decoded pixels (thumbnailing, format conversion, serving a preview), is exposed: Information disclosure (High): the decoded "image" contains bytes of the worker process's adjacent heap/mapped memory, which the app then serves or stores - potentially leaking secrets, credentials, or other users' data. Denial of service (High): a larger xsize reliably crashes the worker with SIGBUS. Suggested fix Core fix in src/map.c (PyImaging_MapBuffer): reject offset < 0 and stride < im->linesize. Defense-in-depth in McIdasImagePlugin._open: reject offset < 0 or stride < xsize*pixelsize` .