Detect Binary Padding in Elastic Security
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. Known threat actors including APT29, Kimsuky, Emotet, QakBot, Black Basta, and Akira have employed this technique to inflate file sizes and change file hashes.
MITRE ATT&CK
- Tactic
- Defense Evasion
- Technique
- T1027 Obfuscated Files or Information
- Sub-technique
- T1027.001 Binary Padding
- Canonical reference
- https://attack.mitre.org/techniques/T1027/001/
Elastic Detection Query
file where event.action == "creation" and
file.extension in~ ("exe", "dll", "sys") and
file.size > 52428800 and
not file.path : (
"C:\\Windows\\*",
"C:\\Program Files\\*",
"C:\\Program Files (x86)\\*"
)Detects creation of large PE files (>50MB) outside trusted Windows system directories using Elastic Common Schema file events. Binary padding inflates file sizes to evade hash-based detection and bypass file-size limits in security tooling (T1027.001). Covers .exe, .dll, and .sys file types consistent with tradecraft observed in APT29, Kimsuky, Emotet, QakBot, Black Basta, and Akira campaigns. File size threshold of 52,428,800 bytes (50MB) is expressed in raw bytes as required by ECS.
Data Sources
Required Tables
False Positives & Tuning
- Large legitimate software installers (e.g., game engines, enterprise IDEs, CAD suites) downloaded and extracted to user temp or downloads directories by package managers or browser processes
- Virtual machine guest additions or driver packages written to staging directories by hypervisor tools (VMware Tools, VirtualBox Guest Additions) with .sys extension components
- Enterprise software deployment agents (SCCM, Intune, Tanium) extracting large update packages to local staging folders prior to installation under Program Files
Other platforms for T1027.001
Testing Methodology
Validate this detection against 4 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Inflate Executable with Null Bytes using fsutil
Expected signal: Sysmon Event ID 11: File Create for padded_calc.exe in %TEMP%. Sysmon Event ID 1: Process Create for fsutil.exe with 'file seteof' command. Security Event ID 4688 (if enabled): fsutil.exe execution. The file hash of padded_calc.exe will differ from the original calc.exe hash.
- Test 2Append Random Bytes to Binary Using PowerShell
Expected signal: Sysmon Event ID 11: File Create for padded_notepad.exe. PowerShell Script Block Logging Event ID 4104 with the Add-Content and RandomNumberGenerator commands. The file will be approximately original size + 10MB.
- Test 3Binary Padding with DD Command on Linux
Expected signal: Syslog/auditd: execve syscall for cp and dd commands with their arguments. File creation event for /tmp/padded_ls. The file will be approximately 20MB larger than the original /bin/ls. The SHA256 hash will differ from the original.
- Test 4Verify Hash Change After Padding
Expected signal: Sysmon Event ID 1: certutil.exe process creation with '-hashfile' argument (twice). Sysmon Event ID 11: File modification of test_calc.exe. The two certutil executions will produce different SHA256 hashes, demonstrating the hash change.
References (6)
- https://attack.mitre.org/techniques/T1027/001/
- https://www.welivesecurity.com/2018/03/13/oceanlotus-ships-new-backdoor/
- https://securelist.com/old-malware-tricks-to-bypass-detection-in-the-age-of-big-data/78010/
- https://www.virustotal.com/en/faq/
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1027.001/T1027.001.md
- https://www.mandiant.com/resources/blog/golang-internals-symbol-recovery
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