Back to list
Winrar doesn't determine the actual size of compressed files+possibility of DoS attack on server!
Sep 10 2003 03:46AM
hUNTER 007 (door_hunt3r blackcodemail com)
---[ about WinRAR]---
Winrar (http://www.rarsoft.com/) is one of the most popular file
compression utilities for Windows.
Winrar incorrectly determines the actual size of compressed files saved
in .rar format by reading it's header information.
Recently we managed to devise a technique to spoof the header and
creating a valid CRC checksum. Later we found that Winrar only depends on
it's header information and CRC check sum to determine the size and
integrity of .rar files. Before uncompressing .rar files, Winrar pre-
allocates space according to the actual file size specified in the header
to avoid fragmentation.But pre-allocation occurs without checking the
available hdd space. Then it goes extracting, even if the hdd size is
less than the size of the files.We did a test by extracting 1GB files in
a hdd with 700MB free space.
Surprisingly, we later discover that even in detecting of header
corruption WinRAR doesn't enforce to avoid extraction process. this lead
WinRAR to believe that the actual size is correct .We managed to exploit
this and create a proof of concept to demonstrate this problem by
changing the actual file size in it's header. When it starts extracting
it doesn't find any valid data in the archive and on the basis of it's
header it attempts to extract 1 gigabyte of data and simply goes on
writing "0x00" filling up valuable hdd space.
--[Proof of concept]--
The proof of concept is a valid .rar file which is just 100 bytes but
it's header has been forged to fool Winrar into thinking that it's a 1
gigabyte file by forging it's header and creating a valid CRC checksum.
All versions of Winrar (upto 3.20 - latest version till date) seem to be
The proof of concept of .rar file can be obtained from the following URL:
If you extract the file Winrar will try to extract this 100 bytes .rar
file trusting the information in it's header but not on the basis of it's
This bug was originally discovered by hUNT3R, a member of 01 Security
Sumbission. The vendor was notified via email. Further discussion took
place in 01 Security Sumbission's forum with the developer of Winrar
(Eugene Roshal) :
---[about 01 security submission]---
01s.s is a small group having experience as security specialists,
programmers and system administrators
An email from full-disclosure with bug verification and patch!
This looks very bad to me.
I've tested it on a Linux machine with unrar 2.71, which comes with most
distributions. Same unrar binary is used by anti-virus scanner.
Result is the following:
$ unrar x -v test123.rar
UNRAR 2.71 freeware Copyright (c) 1993-2000 Eugene Roshal
Extracting from test123.rar
MAIL.DWN - CRC failed
Total errors: 1
As CRC failed, unrar will delete this file immediately but during the
extraction it'll create nice 1GB file.
As I wrote above, same unrar binary is used by anti-virus scanner
in this case), so this is creates a very nasty possibility of DoS
attack on servers.
Solution is to download and install the latest version from WinRAR's
Particulary, for Unix/Linux get it's source:
[ reply ]
Re: Winrar doesn't determine the actual size of compressed files+possibility of DoS attack on server!
Sep 11 2003 10:47AM
Steve Clement (steve ion lu)
Copyright 2010, SecurityFocus