Obfuscated Files or Information: Binary Padding

Adversaries may use binary padding to add junk data and change the on-disk representation of malware. This can be done without affecting the functionality or behavior of a binary, but can increase the size of the binary beyond what some security tools are capable of handling due to file size limitations.

Binary padding effectively changes the checksum of the file and can also be used to avoid hash-based blocklists and static anti-virus signatures.[1] The padding used is commonly generated by a function to create junk data and then appended to the end or applied to sections of malware.[2] Increasing the file size may decrease the effectiveness of certain tools and detection capabilities that are not designed or configured to scan large files. This may also reduce the likelihood of being collected for analysis. Public file scanning services, such as VirusTotal, limits the maximum size of an uploaded file to be analyzed.[3]

ID: T1027.001
Sub-technique of:  T1027
Tactic: Defense Evasion
Platforms: Linux, Windows, macOS
Defense Bypassed: Anti-virus, Signature-based detection
Contributors: Martin Jirkal, ESET
Version: 1.2
Created: 05 February 2020
Last Modified: 15 October 2021

Procedure Examples

ID Name Description
G0016 APT29

APT29 has used large file sizes to avoid detection.[4]

G0050 APT32

APT32 includes garbage code to mislead anti-malware software and researchers.[1][5]

S0268 Bisonal

Bisonal has appended random binary data to the end of itself to generate a large binary.[6]


BRONZE BUTLER downloader code has included "0" characters at the end of the file to inflate the file size in a likely attempt to evade anti-virus detection.[7][8]

S0244 Comnie

Comnie appends a total of 64MB of garbage data to a file to deter any security products in place that may be scanning files on disk.[9]


CORESHELL contains unused machine instructions in a likely attempt to hinder analysis.[10]

S0614 CostaBricks

CostaBricks has added the entire unobfuscated code of the legitimate open source application Blink to its code.[11]

S0082 Emissary

A variant of Emissary appends junk data to the end of its DLL file to create a large file that may exceed the maximum size that anti-virus programs can scan.[12]

S0512 FatDuke

FatDuke has been packed with junk code and strings.[13]

S0182 FinFisher

FinFisher contains junk code in its functions in an effort to confuse disassembly programs.[14][15]

G0047 Gamaredon Group

Gamaredon Group has obfuscated .NET executables by inserting junk code.[16]

S0666 Gelsemium

Gelsemium can use junk code to hide functions and evade detection.[17]

S0477 Goopy

Goopy has had null characters padded in its malicious DLL payload.[18]

S0531 Grandoreiro

Grandoreiro has added BMP images to the resources section of its Portable Executable (PE) file increasing each binary to at least 300MB in size.[19]

S0632 GrimAgent

GrimAgent has the ability to add bytes to change the file hash.[20]

G0126 Higaisa

Higaisa performed padding with null bytes before calculating its hash.[21]

S0528 Javali

Javali can use large obfuscated libraries to hinder detection and analysis.[22]

S0236 Kwampirs

Before writing to disk, Kwampirs inserts a randomly generated string into the middle of the decrypted payload in an attempt to evade hash-based detections.[23]

G0065 Leviathan

Leviathan has inserted garbage characters into code, presumably to avoid anti-virus detection.[24]

S0449 Maze

Maze has inserted large blocks of junk code, including some components to decrypt strings and other important information for later in the encryption process.[25]

G0002 Moafee

Moafee has been known to employ binary padding.[26]

G0129 Mustang Panda

Mustang Panda has used junk code within their DLL files to hinder analysis.[27]

G0040 Patchwork

Patchwork apparently altered NDiskMonitor samples by adding four bytes of random letters in a likely attempt to change the file hashes.[28]


POWERSTATS has used useless code blocks to counter analysis.[29]

S0650 QakBot

QakBot can use large file sizes to evade detection.[30][31]

S0433 Rifdoor

Rifdoor has added four additional bytes of data upon launching, then saved the changed version as C:\ProgramData\Initech\Initech.exe.[32]

S0370 SamSam

SamSam has used garbage code to pad some of its malware components.[33]


TAINTEDSCRIBE can execute FileRecvWriteRand to append random bytes to the end of a file received from C2.[34]

S0612 WastedLocker

WastedLocker contains junk code to increase its entropy and hide the actual code.[35]

S0117 XTunnel

A version of XTunnel introduced in July 2015 inserted junk code into the binary in a likely attempt to obfuscate it and bypass security products.[36]

S0248 yty

yty contains junk code in its binary, likely to confuse malware analysts.[37]

S0230 ZeroT

ZeroT has obfuscated DLLs and functions using dummy API calls inserted between real instructions.[38]


This type of attack technique cannot be easily mitigated with preventive controls since it is based on the abuse of system features.


ID Data Source Data Component
DS0022 File File Metadata

Depending on the method used to pad files, a file-based signature may be capable of detecting padding using a scanning or on-access based tool. When executed, the resulting process from padded files may also exhibit other behavior characteristics of being used to conduct an intrusion such as system and network information Discovery or Lateral Movement, which could be used as event indicators that point to the source file.


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