Covering Tracks and Hiding 1

Covering Tracks and Hiding

Covering Tracks and Hiding 2

In This Chapter… Hiding evidence

o Altering log fileso Hidden files

Practical covert channels

Covering Tracks and Hiding 3

Intro Attacks happen

o See zone-h.com Some attackers want attention Recently, more stealthy attacks

o “Silent” attacks (botnets)o Attacker must hide tracks

Covering Tracks and Hiding 4

Altering Event Logs Even rootkits leave traces in log

files With admin privilege

o Attacker could delete log fileso Probably a bad idea…

A better idea: selectively edit logs How?

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Logs in Windows EventLog is logging service

o Files ending with .LOGo E.g., SECURITY, SYSTEM, APPLICATION

This info moved to main event logso SECEVENT.EVT, SYSEVENT.EVT, …o The .EVT files read by admin using

Windows Event Viewer

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Windows Event Viewer

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Windows Logs SECEVENT.EVT

o Failed logins, policy changes, attempts to access files without permission, etc.

SYSEVENT.EVTo E.g., details of driver failure

APPEVENT.EVTo Application-related issues

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Windows Logs Altering event logs

o At minimum, must change SECEVENTs EVT files “locked” and binary

formato Cannot open/edit with usual tools

With physical access…o …boot to Linux and edit logso Not practical in most cases

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Windows Logs Event editing tools

o None for XP (as of writing)o Do exist for NT/2000

WinZappero Attacker can selectively edit EVT fileso But, must reboot machine to restart

EventLog service

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WinZapper

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UNIX Logging Log files usually in ASCII text With privilege, easy to edit Config file tells where log files located Attacker can locate files, and edit Also “accounting files”

o utmp, wtmp, lastlogo Binary files, so harder to edit

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UNIX Logging Tools to edit accounting files

o Many at www.packetstormsecurity.org

o Simple Nomad effect on many versions

o Others similar tools: wtemped, marry, cloak, logwedit, wzap, zapper

Accounting file editing tool is standard part of most rootkits

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Shell History Files List of command line commands issued Attacker would like to edit this Files are in ASCII, easy to edit

o Can insert lines tooo Why might this be useful?

Edit to shell file written to shell historyo When shell is exited gracefullyo How to get around this?

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Defenses Activate logging

o Log according to some specified policy Periodically audit logging Allow plenty of space for logs Restrictive permissions on log files Use separate server for logging

o Logs redirected to logging servero Not everything can be redirected

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Defenses Encrypt log files Make log files append-only

o Little more than a “speed bump” Store logs on unalterable media

o E.g., non-rewritable CD/DVD

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Hidden Files Why would attacker use hidden

files?o Store attack toolso Save sniffed passwords, etc.

What does “hidden” mean?o Maybe just hard to findo Or easily overlooked

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Hidden Files In UNIX, prepend “.” to filename Use “.” followed by space(s)

o What the … ? Other ideas?

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Hidden Files in Windows

Use “hidden” attributeo Very lame

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Hidden Files in

Windows

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Hidden Files in Windows Alternate Data Streams (ADS)

o Available in NTFSo Multiple streams of data can be

associated with a single fileo These streams can store any infoo “Usual” view is just one such streamo Fairly effective means of hiding files

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Defenses File integrity checking Host-based IDS In Windows, use ADS-aware tools

o CrucialADS, LADS, for example

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Covert Channels Suppose attacker has…

o Gotten accesso Installed evil code/toolso Covered their tracks, etc.

Attacker still needs to communicateo How to do this without detection?

Covert channelo “communication path not intended as such

by system’s designers”

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Covert Channels

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Covert Channels In networked systems…

o Covert channels are everywhere! When does a covert channel exist?

1.Sender and receiver have a shared resource2.Sender able to vary property of resource

that receiver can observe3.Communication between sender and

receiver can be synchronized

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Covert Channels Examples of covert channels? How to eliminate covert channels?

o Easy: eliminate all communication and shared resources

o DoD gave up on eliminating covert channelso Instead, try to reduce the capacityo Does this solve the problem?o Does it help?

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Tunneling Q: What is tunneling? A: One protocol carries another

o E.g., SSH used to carry Telneto E.g., TCP/CP (RFC 1149 and RFC 2549

) Tunneling used for covert channel

o We look at Loki, Reverse WWW Shell

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Loki Suppose

o Attacker 0wns servero Server network allows incoming ICMP

(ping/traceroute) Loki pronounced “low key”

o Provides shell access over ICMPo “Better” than TCP/UDP backdoors

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Loki Trudy installs Loki server on server

o Lokid (“low key dee”)o Must run as rooto Grabs incoming ICMP packets from kernel

Trudy installs Loki client on her machineo Data sent to Lokid using ICMPo Under radar of most backdoor detection

(Why?)o ICMP has no concept of a port

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Loki

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Loki Optionally, uses UDP port 53

o Switch between ICMP/UDP on the fly Supports encryption

o Using Blowfish encryptiono Diffie-Hellman key exchange

Other similar toolso CCTT and MSNShell

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Reverse WWW Shell Covert channel using HTTP Reverse WWW Shell installed on

machine on networko Every 60 seconds, it “phones home”o I.e,. contacts external master servero The “reverse” part: it pulls in

commandso Looks like normal Web traffic

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Reverse WWW Shell

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Reverse WWW Shell Sometimes username/pwd required to

access Webo If known, Reverse WWW Shell can automate

Note that other protocols could be used Reverse WWW Shell idea used by some

legitimate softwareo E.g., remote GUI access to machineo See GoToMyPC.com

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Covert Channels and Malware

Consider spyware to steal passwords

How to exfiltrate passwords?o Piggyback on legitimate outbound

traffico In Windows, IE is a natural choiceo HTTP/HTTPS

Malware often designed as a Browser Helper Object (BHO) for IE

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Headers as Covert Channels

Lots of room for covert channelso E.g., unused bitso But possible to be more clever

Toolso Covert_TCPo Nushu

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IP & TCP Headers

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Covert_TCP Covert_TCP can make use of

o IP identificationo TCP sequence numbero TCP ACK number

Lots of other possible covert channelso Only 3 above used by Covert_TCP

NAT or proxy will cause problemso But IP ID may still work thru NAT

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Covert_TCP IP identification

o Insert one ASCII charactero Read it at other end

TCP sequence numbero Send SYN with ASCII character as initial

sequence numbero Reply with RESETo Ironically, RESET acts as ACK

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Covert_TCP TCP ACK number

o Most sophisticated optiono Involves server (sender), client

(receiver), and unwitting “bounce server”

o Data “bounces” off bounce server

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Covert_TCP TCP ACK number Client send SYN packet to bounce

servero Source address spoofed to client’s addresso ISN is one less than desired ASCII character

Bounce server responds to cliento Either SYN ACK or RESETo Either way, ISN incremented by 1

Server recovers ASCII character (ISN)

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Covert_TCP

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Nushu Uses a “passive” covert channel

o Data sent from host to gateway Embeds info in other (real) packets Alters ISN to contain data Assumes attacker also controls gateway

o At gateway, read data from ISN and forward it

How much data can be transferred?

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Nushu

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Nushu

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Nushu

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Nushu Implemented as Linux kernel

module Creates “issue” with seq numbers Spse the good guys

o …sniff packets on hosto …and same packets elsewhere on

LANo What anomaly will they see?

Covering Tracks and Hiding 47

Defenses No effective defense against

covert channels once attacker has access

So, keep attackers outo Secure configurationo Apply patcheso Antiviruso Monitor for BHOs in IE

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Defenses Know what is normal

o Good luck! Network-based IDS

o Commercial: Sourcefire Intrusion Sensors, ISS RealSecure, Cisco Secure IDS, Network Flight Recorder

o Freeware: Snort

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Conclusions

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Summary