I guess the answer is: There is no easy, readymade solution. Here is what I use now:
class SafeUncompressor(object):
"""Small proxy class that enables external file object
support for uncompressed, bzip2 and gzip files. Works transparently, and
supports a maximum size to avoid zipbombs.
"""
blocksize = 16 * 1024
class FileTooLarge(Exception):
pass
def __init__(self, fileobj, maxsize=10*1024*1024):
self.fileobj = fileobj
self.name = getattr(self.fileobj, "name", None)
self.maxsize = maxsize
self.init()
def init(self):
import bz2
import gzip
self.pos = 0
self.fileobj.seek(0)
self.buf = ""
self.format = "plain"
magic = self.fileobj.read(2)
if magic == '\037\213':
self.format = "gzip"
self.gzipobj = gzip.GzipFile(fileobj = self.fileobj, mode = 'r')
elif magic == 'BZ':
raise IOError, "bzip2 support in SafeUncompressor disabled, as self.bz2obj.decompress is not safe"
self.format = "bz2"
self.bz2obj = bz2.BZ2Decompressor()
self.fileobj.seek(0)
def read(self, size):
b = [self.buf]
x = len(self.buf)
while x < size:
if self.format == 'gzip':
data = self.gzipobj.read(self.blocksize)
if not data:
break
elif self.format == 'bz2':
raw = self.fileobj.read(self.blocksize)
if not raw:
break
# this can already bomb here, to some extend.
# so disable bzip support until resolved.
# Also monitor http://stackoverflow.com/questions/13622706/how-to-protect-myself-from-a-gzip-or-bzip2-bomb for ideas
data = self.bz2obj.decompress(raw)
else:
data = self.fileobj.read(self.blocksize)
if not data:
break
b.append(data)
x += len(data)
if self.pos + x > self.maxsize:
self.buf = ""
self.pos = 0
raise SafeUncompressor.FileTooLarge, "Compressed file too large"
self.buf = "".join(b)
buf = self.buf[:size]
self.buf = self.buf[size:]
self.pos += len(buf)
return buf
def seek(self, pos, whence=0):
if whence != 0:
raise IOError, "SafeUncompressor only supports whence=0"
if pos < self.pos:
self.init()
self.read(pos - self.pos)
def tell(self):
return self.pos
It does not work well for bzip2, so that part of the code is disabled. The reason is that bz2.BZ2Decompressor.decompress
can already produce an unwanted large chunk of data.