Data hiding and finding on Linux

Data hiding and finding on Linux
Anton Chuvakin, Ph.D.
WRITTEN: 2002-2003
DISCLAIMER:
Security is a rapidly changing field of human endeavor. Threats we face literally change every day; moreover, many security professionals consider the rate of change to be accelerating. On top of that, to be able to stay in touch with such ever-changing reality, one has to evolve with the space as well. Thus, even though I hope that this document will be useful for to my readers, please keep in mind that is was possibly written years ago. Also, keep in mind that some of the URL might have gone 404, please Google around.
The article briefly touches upon hiding, finding and destroying data
on Linux file systems. It should become clear that the area of computer
forensics, aimed at recovering the evidence from captured disk drives,
has many challenges, requiring knowledge of hardware, operating
systems and application software.
It is common knowledge, that what is deleted from the computer can
sometimes be brought back. Recent analysis of security implications
of " alternative datastreams" on Windows NT by Kurt Seifried
(http://seifried.org/security/advisories/kssa-003.html) has shown that
Windows NTFS filesystem allows data hiding in " alternative
datastreams" connected to files. These datastreams are not destroyed
by many file wiping utilities that promise irrecoverable removal of
information. Wiping the file means " securely" deleting it from disk
(unlike the usual removal of file entries from directories), so that
file restoration becomes extremely expensive or impossible.
Some overview of what remains on disk after file deletion, how it can
be discovered and how such discovery can be prevented are provided
here http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html The
author recommends overwriting files multiple times with special
patterns. Against casual adversaries, simply overwriting the file with
zeros once will help.
Linux has no alternative data streams, but files removed using /bin/rm
still remain on the disk. Most likely, Linux system uses ext2
filesystem (or its journaling version, ext3 by RedHat). A casual look
at the design of ext2 filesystem
(http://e2fsprogs.sourceforge.net/ext2intro.html) shows several places
where data can be hidden.
Let us start with the classic method to hide material on UNIX
filesystems (not even ext2 specific): run a process that keeps the
file open and then remove the file. The file contents are still on
disk and the space will not be reclaimed by other programs. It is
worthwhile to note, that if an executable erases itself, its contents
can be retrieved from /proc memory image: command " cp /proc/$PID/exe
/tmp/file" creates a copy of a file in /tmp.
If the file is removed by /bin/rm, its content still remains on disk,
unless overwritten by other files. Several Linux unerase utilities
(e2undel - http://e2undel.sourceforge.net/ or recover -
http://recover.sourceforge.net/) attempt automated recovery of
files. They are based on " Linux Ext2fs Undeletion mini-HOWTO"
(http://www.linuxdoc.org/HOWTO/mini/Ext2fs-Undeletion.html) that
provides a nice guide to file recovery from Linux partitions. Recovery
can also be performed manually using debugfs Linux utility (as
described in the above HOWTO).
Overall, if recovery is attempted shortly after file removal and the
partition is promptly unmounted, chances of complete recovery are
high. If the system was heavily used, the probability of successful
data undeletion significantly decreases. However, if we are to look at
the problem from the forensics point of view, the chances of
recovering *something* (such as a small part of the illegal image for
the prosecution) is still very high. It was reported that sometimes
parts of files from several years ago are found by forensic examiners.
Thus files can be hidden in free space. If many copies of the same
file are saved and then erased, the chance of getting the contents
back becomes higher using the above recovery methods. However, due to
the intricacies of ext2 filesystem, the process can only be reliably
automated for small files.
A more detailed look at ext2 internals reveals the existence of slack
space. Filesystem uses addressable parts of disk called blocks, that
have the same size. Ext2 filesystems typically use 1,2 or 4 KB
blocks. If a file is smaller than the block size, the remaining space
is wasted. It is called slack space. This problem long plagued early
Windows 9x users with FAT16 filesystems, which had to use block sizes
of up to 32K, thus wasting a huge amount of space if storing small
files.
On a 4GB Linux partition the block size might be 4K (chosen
automatically when the " mke2fs" utility is run to create a
filesystem). Thus one can reliably hide up to 4KB of data per file if
using a small file. The data will be invulnerable to disk usage,
invisible from the filesystem, and, which is more exciting for some
people, undetectable by file integrity checkers using file
checksumming algorithms and MAC times. Ext2 floppy (with a block size
of 1KB) allows hiding data as well, albeit in smaller chunks.
The obscure tool bmap (available
ftp://ftp.scyld.com/pub/forensic_computing/bmap/) exists to jam data
in slack space, take it out and also wipe the slack space, if
needed. Some of the examples follow:
# echo " evil data is here" | bmap --mode putslack /etc/passwd
puts the data in slack space produced by /etc/password file
# bmap --mode slack /etc/passwd
getting from block 887048
file size was: 9428
slack size: 2860
block size: 4096
evil data is here
shows the data
# bmap --mode wipeslack /etc/passwd
cleans the slack space.
Hiding data in slack space can be used to store secrets, plant
evidence (forensics software will find it, but the suspect probably
will not) and maybe hide tools from integrity checkers (if automated
splitting of the larger file into slack-sized chunks is implemented).
Now lets turn to discovering what is out there on the vast expanses of
the disk drive? If looking for strings of text, a simple " strings
/dev/hdaX | grep 'string we need'" will confirm the presence of string
on the partition (the process *will* take along time). Using a hex
editor on the raw partition can sometimes shed some light on the disk
contents, but the process is extremely messy. Thus further analysis
puts us firmly in the field of computer forensics. The great tools for
snooping around on the disk is Coroner Toolkit by Dan Farmer and
Wietse Venema (tct, can be obtained from
http://www.porcupine.org/forensics/tct.html) and a set of utilities
for it by Brian Carrier (tctutils, available at
http://www.cerias.purdue.edu/homes/carrier/forensics/). The software
provides functionality for gathering forensics data, recovering files,
analyzing drive contents for changes (using file timestamps), locating
content on disks (using inode and block numbers) and for other fun
forensics tasks. Detailed description of the toolkit goes well beyond
the scope of this paper.

Now lets briefly review how to prevent the adversaries from finding
private data Several Linux file wipe utilities exist. All but one can
only be used to wipe files, rather than empty disk space. GNU shred
(by Colin Plumb), srm (by Todd Burgess), wipe (by Tom Vier) and srm
from thc kit (by THC group, see
http://packetstorm.widexs.nl/groups/thc/). Some use the multiple
random passes as recommended in the above paper and some simply
overwrite the file with zeros once. Some does not work under certain
circumstances or for specific filesystems. As reported in shred man
page " shred relies on a very important assumption: that the filesystem
overwrites data in place" . If this condition is not met, no secure
deletion will be performed (with no error message!).
To eliminate the traces of old removed files, one might want to wipe
the empty space. The simple method is to use a standard Linux " dd"
utility. To wipe the empty space on /home partition use:
1. dd if=/dev/zero of=/home/bigfile
2. sync
3. rm /home/bigfile
4. sync
The commands will zero out the empty space on the partition. Doing the
same for /tmp partition might cause some applications to crash, thus
one must be cautious. Utility " sfill" from thc THC secure_delete
package (available at http://packetstorm.widexs.nl/groups/thc/) can
overwrite empty space using more stringent algorithm.
It should be noted that swap space can also contain pieces of private
data and should be wiped as well if additional security is
desired. Another program (sswap) from THC toolkit can be utilized for
the purpose.
The important fact to note is that when empty space is wiped, slack
space for all files remains intact. If file is wiped (at least using
current version of GNU shred), the associated slack space is NOT wiped
with it!
ABOUT THE AUTHOR:
This is an updated author bio, added to the paper at the time of reposting in 2009.
Dr. Anton Chuvakin (http://www.chuvakin.org) is a recognized security expert in the field of log management and PCI DSS compliance. He is an author of books " Security Warrior" and " PCI Compliance" and a contributor to " Know Your Enemy II" , " Information Security Management Handbook" and others. Anton has published dozens of papers on log management, correlation, data analysis, PCI DSS, security management (see list www.info-secure.org) . His blog http://www.securitywarrior.org is one of the most popular in the industry.
In addition, Anton teaches classes and presents at many security conferences across the world; he recently addressed audiences in United States, UK, Singapore, Spain, Russia and other countries. He works on emerging security standards and serves on the advisory boards of several security start-ups.
Currently, Anton is developing his security consulting practice, focusing on logging and PCI DSS compliance for security vendors and Fortune 500 organizations. Dr. Anton Chuvakin was formerly a Director of PCI Compliance Solutions at Qualys. Previously, Anton worked at LogLogic as a Chief Logging Evangelist, tasked with educating the world about the importance of logging for security, compliance and operations. Before LogLogic, Anton was employed by a security vendor in a strategic product management role. Anton earned his Ph.D. degree from Stony Brook University.

Data hiding and finding on Linux

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