Detect Binary Padding in Microsoft Sentinel
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/
KQL Detection Query
let LargePEThresholdMB = 50;
DeviceFileEvents
| where Timestamp > ago(24h)
| where ActionType == "FileCreated"
| where FileName endswith ".exe" or FileName endswith ".dll" or FileName endswith ".sys"
| where FileSize > (LargePEThresholdMB * 1024 * 1024)
| join kind=leftouter (
DeviceProcessEvents
| where Timestamp > ago(24h)
| project DeviceName, InitiatingProcessFileName, InitiatingProcessCommandLine, ProcessCommandLine, SHA256
) on DeviceName
| where FolderPath !startswith "C:\\Windows\\"
and FolderPath !startswith "C:\\Program Files\\"
and FolderPath !startswith "C:\\Program Files (x86)\\"
| project Timestamp, DeviceName, FolderPath, FileName, FileSize,
InitiatingProcessFileName, InitiatingProcessCommandLine, SHA256
| extend FileSizeMB = round(toreal(FileSize) / (1024*1024), 1)
| sort by FileSizeMB descDetects unusually large PE files (executables, DLLs, drivers) created in non-standard locations using Microsoft Defender for Endpoint DeviceFileEvents. Adversaries pad binaries to exceed AV scanning size limits (e.g., VirusTotal's 650MB cap) and change file hashes. Focuses on files dropped outside standard Windows/Program Files directories, which is where legitimate oversized binaries rarely appear. FileSize threshold set at 50MB; tune upward in environments with large legitimate installers.
Data Sources
Required Tables
False Positives & Tuning
- Large legitimate software installers (e.g., game installers, IDE packages, database engines) dropped in temp or user directories during installation
- Backup or archive tools writing large consolidated files that happen to use .exe or .dll extensions
- Software development workflows where compiled binaries with debug symbols legitimately exceed size thresholds
- Container or VM image export utilities writing large binary blobs with executable extensions
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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