Polymorphic code: Difference between revisions

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{{Short description|Self-modifying program code designed to defeat anti-virus programs or reverse engineering}}

In computer science (or more often, in computer underground terms) '''polymorphic code''' is code that mutates while keeping the original [[algorithm]] intact.

{{distinguish|Polymorphism (computer science)}}

{{refimprove|date=November 2010}}

In computing, '''polymorphic code''' is code that uses a [[polymorphic engine]] to mutate while keeping the original [[algorithm]] intact - that is, the ''code'' changes itself every time it runs, but the ''function'' of the code (its [[semantics]]) stays the same. For example, the simple math expressions 3+1 and 6-2 both achieve the same result, yet run with different [[machine code]] in a [[Central processing unit|CPU]]. This technique is sometimes used by [[computer virus]]es, [[shellcode]]s and [[computer worm]]s to hide their presence.<ref name="rugha">{{cite thesis |last=Raghunathan |first=Srinivasan |date=2007 |title=Protecting anti-virus software under viral attacks |type=M.Sc. |publisher=Arizona State University |citeseerx=10.1.1.93.796}}</ref>

[[Encryption]] is the most common method to hide code. With encryption, the main body of the code (also called its [[Payload (computing)|payload]]) is encrypted and will appear meaningless. For the code to function as before, a decryption function is added to the code. When the code is ''executed'', this function reads the payload and decrypts it before executing it in turn.

This is often used by [[computer virus]]es and [[shellcode]] to keep their [[encryption|de/en-cryption-engines]] from being detected by [[anti virus software]] and [[intrusion-detection system]].

Encryption alone is not polymorphism. To gain polymorphic behavior, the encryptor/decryptor pair is mutated with each copy of the code. This allows different versions of some code which all function the same.<ref name="wongstamp">{{cite journal |last1=Wong |first1=Wing |last2=Stamp |first2=M. |title=Hunting for Metamorphic Engines |journal=Journal in Computer Virology |volume=2 |issue= 3|pages=211–229 |date=2006 |doi=10.1007/s11416-006-0028-7 |citeseerx=10.1.1.108.3878|s2cid=8116065 }}</ref>

== How it works ==

== Malicious code ==

An algorithm that uses, for example, the variables A and B but not the variable C could stay intact even if you added lots of codes that changed the content in the variable C.

Most [[anti-virus software]] and [[intrusion detection system]]s (IDS) attempt to locate malicious code by searching through computer files and data packets sent over a [[computer network]]. If the security software finds patterns that correspond to known computer viruses or worms, it takes appropriate steps to neutralize the threat. Polymorphic algorithms make it difficult for such software to recognize the offending code because it constantly mutates.

The original algorithm:

Start:

GOTO Decryption_Code

Encrypted:

...

lots of encrypted code!!!

...

Decryption_Code:

*A = Encrypted

Loop:

B = *A

B = B [[XOR]] CryptoKey

*A = B

A = A + 1

GOTO Loop IF NOT A = (Decryption_Code - Encrypted)

GOTO Encrypted

CryptoKey:

some_random_number

Malicious [[programmer]]s have sought to protect their encrypted code from this virus-scanning strategy by rewriting the unencrypted decryption engine (and the resulting encrypted payload) each time the virus or worm is propagated. Anti-virus software uses sophisticated pattern analysis to find underlying patterns within the different mutations of the decryption engine, in hopes of reliably detecting such [[malware]].

The same algorithm, but with lots of unnessisary C-altering codes..

Start:

GOTO Decryption_Code

Encrypted:

...

lots of encrypted code!!!

...

Decryption_Code:

C = C + 1

*A = Encrypted

Loop:

B = *A

C = 3214 * A

B = B [[XOR]] CryptoKey

*A = B

C = 1

C = A + B

A = A + 1

GOTO Loop IF NOT A = (Decryption_Code - Encrypted)

C = C^2

GOTO Encrypted

CryptoKey:

some_random_number

Emulation may be used to defeat polymorphic obfuscation by letting the malware demangle itself in a virtual environment before utilizing other methods, such as traditional signature scanning. Such a virtual environment is sometimes called a [[Sandbox (computer security)|sandbox]]. Polymorphism does not protect the virus against such emulation if the decrypted payload remains the same regardless of variation in the decryption algorithm. [[Metamorphic code]] techniques may be used to complicate detection further, as the virus may execute without ever having identifiable code blocks in memory that remains constant from infection to infection.

The code inside "Encrypted" could then search the code between Decryption_Code and [[CryptoKey]] and remove all the code that alters the variable C. Before the next time the encryption engine is used, it could input new unnecessary codes that alters C.

Start:

GOTO Decryption_Code

Encrypted:

...

(when this part is decrypted, it will contain the following...)

...

Then remove all the C-Codes betwen Decryption_engine and CryptoKey

And input new unnessisary C-Codes at other locations in the algorithm

Change the code beneth "Change_this" to RETURN_TO_SYSTEM

...

Do_whatever_this_code_is_suposed_to_do

...

Goto Decryption_Code

Decryption_Code:

C = C + 1

*A = Encrypted

Loop:

B = *A

C = 3214 * A

B = B [[XOR]] CryptoKey

*A = B

C = 1

C = A + B

A = A + 1

GOTO Loop IF NOT A = (Decryption_Code - Encrypted)

C = C^2

Change_this:

GOTO Encrypted

CryptoKey:

some_random_number

The first known polymorphic virus was written by Mark Washburn. The virus, called [[1260 (computer virus)|1260]], was written in 1990.<ref>{{Cite web |title=An Example Decryptor of 1260 |url=https://userpages.umbc.edu/~dgorin1/432/example_decryptor.htm |access-date=2025-03-21 |website=userpages.umbc.edu}}</ref> A better-known polymorphic virus was created in 1992 by the hacker [[Dark Avenger]] as a means of avoiding pattern recognition from antivirus software. A common and very virulent polymorphic virus is the file infecter [[Virut]].

The code above will decrypt the code inside "Encrypted" with a mutated crypto-engine. Then it will transfer control to the decrypted code inside "Encrypted". The code in there will remove all the codes that alters C. Then it will input new codes that alters C at other (random) locations in the crypto-algorithm. The change of "GOTO Encrypted" to "RETURN_TO_SYSTEM" will make it possible to re-use the old encryption engine (as [[XOR]] works in both ways.) then it will do ''something'' (like spreading to other files in the case of a virus) and at last, it will return to the Decryption_Code.

== See also ==

If the code is trying to replicate itself (in the case of a [[computer worm]] or [[computer virus]]) it will need to encrypt the code inside "Encrypted" before it sends it away.

* [[Metamorphic code]]

* [[Self-modifying code]]

* [[Alphanumeric shellcode]]

* [[Shellcode]]

* [[Obfuscated code]]

* [[Oligomorphic code]]

== References ==

see [[self-modifying code]], [[alphanumeric code]], [[shellcode]], [[cracking]]

<references/>

{{refbegin}}

*{{cite journal |author-link= |last=Spinellis |first=Diomidis |url=http://www.spinellis.gr/pubs/jrnl/2002-ieeetit-npvirus/html/npvirus.html |title=Reliable identification of bounded-length viruses is NP-complete |journal=IEEE Transactions on Information Theory |volume=49 |issue=1 |pages=280–4 |date=January 2003 |doi=10.1109/TIT.2002.806137}}

{{refend}}

[[Category:Types of malware]]