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) (1 replies)

---[ 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

data integrity.

--[Background Information]--

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) :

URL: http://www.ysgnet.com/phorum/read.php?f=1&i=341&t=324#reply_341

---[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

Extracting MAIL.DWN

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:


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