Post on 18-Dec-2015
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DICOTS and StackGuard: Two current approaches to
tolerating maliciouscodeCarl Landwehr
Mitretek Systems, Inc.7525 Colshire Dr.McLean, VA 22102
Carl.Landwehr@mitretek.org(703)610-1576
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DICOTS:
Data Integrity for COTS Databases • Research project at Naval Research Lab• Based on Integrity Cluster concept (J.
McDermott)• Developed and prototyped by B.
Montrose and L. Day
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What is the Threat?
Malicious code embedded in COTS products may compromise the integrity of this data.
Select About from help menuCtrl-drag “e” graphic onto EarthRelease Ctrl KeyCtrl-drag “e” again to push text out of the wayPress “unlock” buttonReturn “e” to Earth
Type X97:L97Press Enter then TabHold Ctrl-Shift while you click on the Chart Wizard Button in the toolbar
Open a New SheetPress the F5 key
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Majo
rity
Vote
I tem Stock
9mm Cart. 4Hellfire Missile 8Scud Missile 7
I tem Stock
9mm Cart. 4Hellfire Missile 8Scud Missile 7
I tem Stock
9mm Cart. 412Hellfire Missile 113Scud Missile 51
Technical Approach
Diverse & Redundant Databases with a Trustworthy
Voter
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Supporting Research
Integrity Clusters
Integrity Cluster: diverse, redundant data-storing components accessed and controlled exclusively by an Integrity Controller.
The Integrity Controller replicates transactions to all of its attached data storages in a one-copy serializable fashion.
Data integrity is verified by comparing replicas.
Integrity Cluster Criticisms
Maintaining at least three redundant replicas at one site is impractical. Remedy: DICOTS can be made to work with existing distributed databases.
Performance cost (from serialization) is intuitively high. Remedy: measure the actual performance, assess, seek speed-ups
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Sybase HostSybase
DB
Clien
t H
ost
Sybase ODBC Driver
COTS ODBC
Software
Open Database Connectivity
Technical Solution
Vendor provides ODBC driver designed to run directly on DBMS
Practical implementation of DICOTS requires network between client and database server
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COTS ODBC
Software
Clien
t H
ost
DICOTS Operation
Sybase HostSybase
DB Oracle HostOracle
DB Informix HostInformix
DB
Sybase ODBC Driver
Oracle ODBC Driver
Informix ODBC Driver
Custom ODBC Driver
ODBC Proxy
Interface
ODBC Proxy
Interface
ODBC Proxy
Interface
•DICOTS implements custom ODBC driver as DLL•Application generates std database calls; DLL packages data, initiates TCP/IP connection, and transfers to proxy•Proxy polls incoming links round robin•When transaction found, it accepts and initiates it at all three back ends•Proxy waits for all backends to finish
•If data returned, proxy votes it and reports failure if appropriate (note format issues)•Then accept another transaction
COTS ODBC
Software
Clien
t H
ost
Custom ODBC Driver
COTS ODBC
Software
Clien
t H
ost
Custom ODBC Driver
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DICOTS preliminary results
• Feasible to construct the architecture• Partial set of ODBC API implemented• Some compatibility issues encountered• Benchmarking in progress• Initial evidence: retrieval times considerably
longer than update• Current implementation represents baseline,
near worst case
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StackGuard:Tolerating Stack Smashing Attacks
• Stack Smashing Attacks are Legion • Vulnerabilities attributable to buffer overflow/stack
smashing attacks in CERT advisories since 1997:– 1997: 14 of 28 – 1998: 7 of 12 – 1999: 5 of 12 (to Oct 1)
Effective countermeasure to stack smashing could eliminate many actual attacks
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Problem:Buffer Overflow Attack
• Attacker feeds a big string to an input routine that does not do bounds checking
• String over-writes return address
• String injects code• Function return
jumps to injected code
Program Text[write
protected]
Data (static)
Stack[read/write OK]
Low address
High address
Buffer
Return Address
… Other local varioable ...
Stackframe
Normal Stack
Executionpoint
Calling stack frame
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After a Successful Attack• Return address points into
buffer• Buffer contains attack code• Executing return invokes
attack code
Program Text[write
protected]
Data (static)
Stack[read/write OK]
Low address
High address
Attack code
Return Address
… Other variable ...
Stackframe
Smashed Stack
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Problems for the attacker• Hacker must find: program that runs with system privileges (e.g.,
root privilege) and fails to check the length of its input• Hacker needs to know:
– Address of the start of the buffer within the stack, to set up return address properly
– Distance between start of buffer and the return address in the stack, to write a string long enough to rewrite the return address
– The actual attack code (binary instructions) to create a new process [exec(“sh”)]
• Hacker must be able to enter entire attack string as single string input (typically, no embedded nulls)– May have to specialize machine instruction to avoid nulls in
attack code
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Attacker tricks• By starting the attack
code with NOPs, the rewritten return address only need be approximate
• By writing the guessed return address several times, the guessed displacement from buffer start to return address need only be approximate
Low address
High address
NOP...NOPAttack Code
Return AddressReturn AddressReturn Address
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StackGuard Solution: Detect Stack Smashing As it HappensCrispin Cowan, Calton Pu, Oregon Grad Inst.
• StackGuard is a compiler that emits robust bug tolerant code– Stack smashing vulnerabilities are not
repaired, nor are the attacks prevented• Instead, program detects attack in progress
– Stack smash corrupts running function’s activation record
– StackGuard code generator detects corruption before return, preventing attacker from getting control
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StackGuard 1: Defend Programs with a Stack Integrity Check
• Stack smash goes through– Attack code injected– Return address altered
• But Stack smash also smashes the Canary– Function checks for Canary before returning
– If Canary smashed, program halts instead of yielding control to the attacker
Low address
High address
Buffer
Return Address
Canary
Stackframe
Normal Stack
Calling stack frame
Other local variables ...
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Canary Integrity
• Problem:– attacker customizes attack string so that string
over-write replaces proper canary word• Original solution: random canaries
– Choose random canary values at exec() time• Requires lookup on each function call and return
– Slow– Can’t be used with shared libraries
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Faster, Better Canary Integrity: StackGuard 1.1
• New solution: “terminator” canary– canary word is Null, CR, LF, and -1– I.e. the common termination symbols for C
string functions• Hard for attacker to embed a termination
symbol in the attack string• Improvement: static canary lookup
– Faster– Can be used with shared libraries
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What this solution requires
• Recompile vulnerable programs with StackGuard compiler– Changes call sequence to add canary to stack– Changes return sequence to check canary
before returning• If canary intact, return normally• If canary not intact, abort the process
• Programs not recompiled by Stackguard are not protected, but not affected (no compatibility issue)
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How good is the protection?
• Possible to rewrite return address randomly in some situations, but much more difficult; virtually impossible for this kind of attack
• Overhead for StackGuard 1.1 (fixed canary) is barely measurable (time and space)
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References
McDermott, J. and Froscher, J. “Practical Defenses Against Storage Jamming,” in Proceedings of the 20th National Information Systems Security Conference, Baltimore, MD, October 1997, pp. 162-173
McDermott, J. Integrity Clusters. Submitted for publication, Journal of Computer Security, 1999
Aleph One. Smashing Stacks for Fun and Profit. Phrack V. 7, issue. 49.Http://www.reality.sgi.com/nate/machines/security/P49-14-Aleph-OneCowan, Crispin, Calton Pu, et. Al. StackGuard Automatic Adaptive
Detection and Prevention of Buffer Overflow Attacks. Proceedings 7th USENIX Conf. Available at http://www.cse.ogi.edu/DISC/projects/immunix/StackGuard/