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Upgrade to ProDetect Encrypted/Encoded File in Sumo Logic CSE
This detection identifies adversary use of encrypted or encoded files to conceal malicious payloads, configurations, or command-and-control artifacts from static analysis and signature-based detection. Adversaries commonly apply Base64 encoding, XOR ciphers, AES/RC4 encryption, or layered encoding schemes to files dropped on victim hosts, phishing attachments (password-protected documents, self-extracting archives), or scripts that decode and execute payloads at runtime. Detection focuses on processes decoding or decrypting file content, suspicious file entropy indicators surfaced via tooling, script interpreters reading encoded blobs, and the creation of high-entropy files in user-writable directories.
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/
Sumo Detection Query
(_sourceCategory=windows/sysmon OR _sourceCategory=windows/security)
| where EventCode in ("1", "4688")
| parse field=CommandLine "*" as cmd nodrop
| where (
cmd matches "*certutil*-decode*"
OR cmd matches "*certutil*-decodehex*"
OR cmd matches "*-EncodedCommand*"
OR cmd matches "*FromBase64String*"
OR cmd matches "*openssl enc*"
OR cmd matches "*base64 -d*"
OR cmd matches "*[Convert]::FromBase64*"
)
| parse field=Image "*\\*" as path, proc_name nodrop
| where proc_name in ("certutil.exe", "powershell.exe", "pwsh.exe", "python.exe", "wscript.exe", "cscript.exe", "mshta.exe")
| count by _sourceHost, proc_name, cmd, User
| sort by _count descDetects processes executing file decoding and decryption operations on Windows via Sysmon and Security event logs in Sumo Logic, focusing on common LOLBins and interpreters used to decode obfuscated payloads.
Data Sources
Required Tables
False Positives & Tuning
- IT admin scripts using certutil or PowerShell encoded commands for legitimate maintenance
- Application deployment pipelines decoding Base64-encoded configuration data
- Security tools performing live encoding/decoding tests during assessments
Other platforms for T1027.013
Testing Methodology
Validate this detection against 3 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 1certutil Base64 Decode to Disk
Expected signal: DeviceProcessEvents: certutil.exe with -decode argument; Sysmon Event ID 1 with CommandLine containing '-decode'; Sysmon Event ID 11 showing decoded_payload.bin creation in C:\Windows\Temp
- Test 2PowerShell In-Memory Base64 Payload Decode
Expected signal: DeviceProcessEvents: powershell.exe with FromBase64String in command line; PowerShell Script Block Log Event ID 4104 containing FromBase64String and [Convert]::FromBase64; Sysmon Event ID 11 for decoded_ps_payload.bin creation
- Test 3OpenSSL AES Decrypt File on Linux
Expected signal: Linux auditd execve syscall records for base64 and openssl processes; Syslog entries for process execution; Elastic Auditbeat process events showing openssl enc -d with -aes-256-cbc arguments; file creation event for decrypted_payload.bin in /tmp
References (7)
- https://attack.mitre.org/techniques/T1027/013/
- https://www.mandiant.com/resources/apt41-initiating-footholds
- https://www.mcafee.com/blogs/other-blogs/mcafee-labs/operation-north-star-a-job-offer-thats-too-good-to-be-true/
- https://www.trendmicro.com/en_us/research/18/d/new-macos-backdoor-linked-to-oceanlotus-found.html
- https://securelist.com/cloud-atlas-redoctober-apt-is-back-in-style/68083/
- https://www.welivesecurity.com/2014/11/11/sednit-espionage-group-attacking-air-gapped-networks/
- https://www.hhs.gov/sites/default/files/qilin-threat-profile.pdf
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