Detect Exfiltration Over Symmetric Encrypted Non-C2 Protocol in Splunk
Adversaries may steal data by exfiltrating it over a symmetrically encrypted network protocol other than that of the existing command and control channel. Symmetric encryption algorithms (RC4, AES, ChaCha20, Blowfish) use shared keys on both ends of the channel. Attackers may implement custom encryption over protocols not natively encrypted (HTTP, FTP, DNS) or add extra encryption layers over already-encrypted protocols (HTTPS, SFTP) to obscure data contents from network inspection tools. This technique is distinguished from asymmetric exfiltration by the pre-shared key requirement, often resulting in artifacts such as key material embedded in scripts, configuration files, or command-line arguments.
MITRE ATT&CK
- Tactic
- Exfiltration
- Sub-technique
- T1048.001 Exfiltration Over Symmetric Encrypted Non-C2 Protocol
- Canonical reference
- https://attack.mitre.org/techniques/T1048/001/
SPL Detection Query
index=wineventlog sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=1
| eval CommandLine=lower(CommandLine)
| eval Image_lower=lower(Image)
| eval IsEncryptionTool=if(
match(Image_lower, "(openssl|ccrypt|mcrypt|cryptcat|aescrypt|gpg)") OR
(match(Image_lower, "python") AND match(CommandLine, "(aes|rc4|des|chacha|blowfish|pycrypto|cryptography|cipher|encrypt)")),
1, 0
)
| eval HasAESIndicator=if(match(CommandLine, "(enc\s+-aes|aes-256|aes-128|aes256|aes128|-aes)"), 1, 0)
| eval HasRC4Indicator=if(match(CommandLine, "(rc4|\-rc4|arcfour)"), 1, 0)
| eval HasKeyMaterial=if(match(CommandLine, "(-k\s+\S+|-pass\s+pass:|-passout|-passin|-password|--passphrase|secret|shared.?key)"), 1, 0)
| eval HasNetworkTarget=if(match(CommandLine, "(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}|https?://|ftp://|-connect|s_client|s_server|netcat|nc\s)"), 1, 0)
| eval HasNoSalt=if(match(CommandLine, "-nosalt"), 1, 0)
| eval SuspicionScore=IsEncryptionTool + HasAESIndicator + HasRC4Indicator + HasKeyMaterial + HasNetworkTarget + HasNoSalt
| where SuspicionScore >= 2
| eval EncryptionAlgo=case(
HasAESIndicator=1, "AES",
HasRC4Indicator=1, "RC4",
match(CommandLine, "des"), "DES",
match(CommandLine, "chacha"), "ChaCha20",
1=1, "Unknown"
)
| table _time, host, User, Image, CommandLine, ParentImage, ParentCommandLine, IsEncryptionTool, HasAESIndicator, HasRC4Indicator, HasKeyMaterial, HasNetworkTarget, HasNoSalt, SuspicionScore, EncryptionAlgo
| sort - _time
| appendcols [
search index=wineventlog sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=3
(Image="*openssl*" OR Image="*cryptcat*" OR Image="*ccrypt*")
NOT (DestinationIp="10.*" OR DestinationIp="172.16.*" OR DestinationIp="172.17.*" OR DestinationIp="172.18.*" OR DestinationIp="172.19.*" OR DestinationIp="172.20.*" OR DestinationIp="172.21.*" OR DestinationIp="172.22.*" OR DestinationIp="172.23.*" OR DestinationIp="172.24.*" OR DestinationIp="172.25.*" OR DestinationIp="172.26.*" OR DestinationIp="172.27.*" OR DestinationIp="172.28.*" OR DestinationIp="172.29.*" OR DestinationIp="172.30.*" OR DestinationIp="172.31.*" OR DestinationIp="192.168.*" OR DestinationIp="127.*")
| table _time, host, User, Image, DestinationIp, DestinationPort, CommandLine
]Detects exfiltration over symmetric encrypted non-C2 protocols using Sysmon Event ID 1 (Process Creation) and Event ID 3 (Network Connection). Assigns a suspicion score based on detection of encryption tools, algorithm indicators (AES, RC4), key material in command lines, network targets, and nosalt flags. Score threshold of 2 or higher triggers alerting. Also appends network connection events for known encryption tools communicating to public IPs.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Legitimate OpenSSL usage for certificate operations, key generation, and TLS debugging by network or system administrators
- Backup and archival software using GPG or AES encryption for data-at-rest protection before upload to cloud storage
- Security tools performing vulnerability assessments or penetration tests with authorized scope
- DevOps pipelines encrypting build artifacts or secrets using OpenSSL with symmetric keys
- Network monitoring tools using encryption to protect captured traffic during analysis
Other platforms for T1048.001
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 1Exfiltrate File Using OpenSSL AES Encryption over HTTP
Expected signal: Sysmon Event ID 1: Process Create for openssl with CommandLine containing 'enc -aes-256-cbc -nosalt -k' and the key material. Sysmon Event ID 1: Process Create for curl with destination URL. Sysmon Event ID 3: Network connection from curl to destination IP:8080. Sysmon Event ID 11: File Create for /tmp/exfil_payload.enc. Auditd records showing execve syscalls for openssl and curl with full argument lists.
- Test 2Symmetric Encrypted Exfiltration via Python Script with RC4
Expected signal: Sysmon Event ID 1: Process Create for python3 with CommandLine containing 'rc4', 'socket', 'encrypt', and 'connect'. Sysmon Event ID 3: Network connection from python3 to 192.168.1.200:9001 (non-standard port, high-confidence exfiltration indicator). Auditd execve syscall with full Python -c command argument.
- Test 3OpenSSL Encrypted Data Transfer via Named Pipe to Netcat
Expected signal: Sysmon Event ID 1: Process Create for openssl with '-aes-256-cbc -k <key> -nosalt' in CommandLine. Sysmon Event ID 1: Process Create for nc targeting 192.168.1.200:4444 (classic reverse shell port). Sysmon Event ID 3: Network connection from nc to external IP on port 4444. Possible Sysmon Event ID 1 for tar as parent/sibling process. Linux auditd: execve records for all three commands (tar, openssl, nc).
- Test 4Exfiltration via OpenSSL s_client Over HTTPS Port with Symmetric Key
Expected signal: Sysmon Event ID 1: First openssl process with 'enc -aes-256-cbc -k TeamPreSharedKey99 -nosalt' in CommandLine. Sysmon Event ID 1: Second openssl process with 's_client -connect 192.168.1.200:443' in CommandLine. Sysmon Event ID 3: Network connection from openssl s_client to port 443. Sysmon Event ID 11: File Create for /tmp/inner_encrypted.bin. The double-process pattern (enc then s_client) is highly distinctive.
References (10)
- https://attack.mitre.org/techniques/T1048/001/
- https://arxiv.org/ftp/arxiv/papers/1408/1408.1136.pdf
- https://www.openssl.org/docs/man3.0/man1/openssl-enc.html
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1048.001/T1048.001.md
- https://www.sans.org/white-papers/33649/
- https://www.mandiant.com/resources/blog/targeted-attack-exfiltration
- https://learn.microsoft.com/en-us/defender-endpoint/advanced-hunting-devicenetworkevents-table
- https://docs.microsoft.com/en-us/sysinternals/downloads/sysmon
- https://github.com/SigmaHQ/sigma/tree/master/rules/network/zeek
- https://attack.mitre.org/techniques/T1573/001/
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