Detect Encrypted/Encoded File in CrowdStrike LogScale
Adversaries encrypt or encode files to conceal malicious content and evade static signature detection. Techniques include XOR (single-byte and multi-byte), RC4, AES, 3DES, Base64, and custom encoding schemes applied to malware payloads, configuration files, C2 communication blobs, and dropped files. The full content or only specific values (such as C2 addresses or strings) may be obfuscated, sometimes in multiple redundant layers. Common delivery vectors include password-protected ZIP/Word documents and self-extracting (SFX) archives. Threat actors ranging from APT28 and Inception Group to ransomware families like Qilin and RansomHub consistently use encrypted/encoded files to defeat antivirus and EDR static analysis.
MITRE ATT&CK
- Tactic
- Defense Evasion
- Technique
- T1027 Obfuscated Files or Information
- Sub-technique
- T1027.013 Encrypted/Encoded File
- Canonical reference
- https://attack.mitre.org/techniques/T1027/013/
LogScale Detection Query
#event_simpleName=ProcessRollup2
| (
(FileName = /(?i)certutil\.exe/ AND CommandLine = /(?i)-(decode|decodehex)/)
OR (FileName = /(?i)powershell\.exe/ AND CommandLine = /(?i)FromBase64String/ AND CommandLine = /(?i)(Invoke-Expression|\biex\b|&\(|\.Invoke)/)
OR (FileName = /(?i)powershell\.exe/ AND CommandLine = /(?i)\bbxor\b/)
OR (FileName = /(?i)(7z|7za|winrar|wrar)\.exe/ AND CommandLine = /-p\S+/)
OR (FileName = /(?i)(mshta|wscript|cscript)\.exe/ AND CommandLine = /(?i)(Base64|FromBase64|\bbxor\b|Encoding)/)
)
| case {
FileName = /(?i)certutil\.exe/ |
detection_type := "certutil_decode" |
risk_score := 75;
FileName = /(?i)powershell\.exe/ AND CommandLine = /(?i)\bbxor\b/ |
detection_type := "powershell_xor_decode" |
risk_score := 80;
FileName = /(?i)powershell\.exe/ |
detection_type := "powershell_b64_decode_exec" |
risk_score := 85;
FileName = /(?i)(7z|7za|winrar|wrar)\.exe/ |
detection_type := "password_protected_archive" |
risk_score := 55;
* |
detection_type := "scripting_engine_encoded_exec" |
risk_score := 75
}
| table(
[timestamp, ComputerName, UserName, UserSid, detection_type, risk_score,
FileName, CommandLine, ParentBaseFileName, SHA256HashData]
)
| sort(field=risk_score, order=desc)CrowdStrike LogScale (Falcon) CQL query for T1027.013 operating on ProcessRollup2 events from the Falcon EDR telemetry stream. Uses FileName (process base name) and CommandLine fields to identify certutil decode operations, PowerShell Base64-to-exec attack chains, PowerShell XOR decode via -bxor, password-protected archive extraction with 7z/WinRAR, and scripting engine invocations with encoded content references. Assigns risk scores per detection subtype and surfaces SHA256 hash for file reputation lookups. A companion query on FileCreateInfo events should be deployed to cover encoded file staging detection.
Data Sources
Required Tables
False Positives & Tuning
- CrowdStrike Falcon sensor policy deployment and RTR (Real Time Response) operations may internally invoke PowerShell with encoded parameters on managed endpoints, producing matches for powershell_b64_decode_exec that originate from the Falcon sensor process tree
- Enterprise software packaging workflows using tools like Chocolatey, Scoop, or WinGet wrappers that call certutil for checksum verification or invoke 7z/WinRAR with password-protected archives during application installation will trigger certutil_decode and password_protected_archive detections on software distribution hosts
- Authorized red team engagements and Atomic Red Team test executions (e.g., T1027.013 atomics using certutil, PowerShell FromBase64String with IEX, or -bxor patterns) will produce true-positive-equivalent alerts during scheduled test windows — correlate with change management records to suppress
Other platforms for T1027.013
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 1Base64 Encode, Drop, and Decode with certutil
Expected signal: Sysmon EventCode 1 (ProcessCreate) for certutil.exe with '-decode' in CommandLine. EventCode 11 for both the .b64 source file and decoded output file creation in %TEMP%.
- Test 2PowerShell XOR Decryption Simulating Single-Byte XOR Malware
Expected signal: Sysmon EventCode 1 (ProcessCreate) for powershell.exe. PowerShell Script Block Logging EventCode 4104 will capture the deobfuscated script including the -bxor operator and Invoke-Expression call.
- Test 3Password-Protected ZIP Delivery and Extraction
Expected signal: Sysmon EventCode 1 for 7z.exe invocations with '-p' flag in CommandLine. EventCode 11 for .zip creation and extracted file creation in %TEMP%.
- Test 4Base64 Blob Decode and Execute via PowerShell
Expected signal: Sysmon EventCode 1 for powershell.exe. PowerShell Script Block Logging EventCode 4104 will capture both the FromBase64String call and the decoded Invoke-Expression content — this is the primary high-value telemetry source.
References (7)
- https://attack.mitre.org/techniques/T1027/013
- https://www.crowdstrike.com/blog/self-extracting-archives-decoy-files-and-their-hidden-payloads/
- https://www.crowdstrike.com/blog/shlayer-malvertising-campaigns-still-using-flash-update-disguise/
- https://www.mandiant.com/resources/apt41-dual-espionage-and-cyber-crime-operation
- https://securelist.com/the-darkhotel-apt/66779/
- https://unit42.paloaltonetworks.com/sofacy-attacks-multiple-government-entities/
- https://www.secureworks.com/research/threat-profiles/gold-southfield
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