Detect Exfiltration Over Symmetric Encrypted Non-C2 Protocol in Elastic Security
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/
Elastic Detection Query
sequence by host.id with maxspan=5m
[process where event.type == "start" and
(
process.name : ("openssl", "ccrypt", "mcrypt", "cryptcat", "aescrypt", "gpg", "7z") or
(process.name : ("python", "python3", "python.exe") and
process.command_line : ("*AES*", "*RC4*", "*Cipher*", "*encrypt*", "*socket*", "*send*"))
) and
process.command_line : (
"*enc -aes*", "*enc -rc4*", "*enc -des*", "*-k *", "*-pass pass:*",
"*-aes-256*", "*-aes-128*", "*aes256*", "*aes128*", "*rc4*", "*arcfour*",
"*chacha20*", "*-nosalt*", "*-base64*", "*-connect*", "*netcat*",
"*socket*", "*curl*", "*wget*", "*ftp*"
)]
[network where event.type in ("start", "connection") and
network.direction : ("outbound", "egress") and
not destination.ip : (
"10.*", "172.16.*", "172.17.*", "172.18.*", "172.19.*",
"172.20.*", "172.21.*", "172.22.*", "172.23.*", "172.24.*",
"172.25.*", "172.26.*", "172.27.*", "172.28.*", "172.29.*",
"172.30.*", "172.31.*", "192.168.*", "127.*", "::1"
)]
/* Standalone: Large outbound transfer from encryption tool process */
// network where event.type in ("start", "connection") and
// network.direction : ("outbound", "egress") and
// process.name : ("openssl", "ccrypt", "mcrypt", "cryptcat", "aescrypt", "gpg") and
// network.bytes > 1000000 and
// not destination.ip : ("10.*", "172.16.*", "172.17.*", "172.18.*", "172.19.*",
// "172.20.*", "172.21.*", "172.22.*", "172.23.*", "172.24.*",
// "172.25.*", "172.26.*", "172.27.*", "172.28.*", "172.29.*",
// "172.30.*", "172.31.*", "192.168.*", "127.*")Detects symmetric encryption tools (openssl, ccrypt, cryptcat, gpg, aescrypt) executing with cipher-specific arguments (AES, RC4, ChaCha20, -nosalt, -pass pass:, embedded key material) followed within 5 minutes by outbound network connections to public IP space. Uses EQL sequence correlation on host.id to join process launch and network telemetry. A secondary standalone query flags large outbound transfers (>1 MB) directly from encryption tool processes. Targets T1048.001 by focusing on the combination of symmetric cipher arguments and network exfiltration indicators.
Data Sources
Required Tables
False Positives & Tuning
- Administrators using openssl s_client or s_server for TLS certificate testing and OCSP validation against public CA endpoints
- Python-based DevOps automation or data pipelines using pycryptodome or cryptography libraries for legitimate API calls to external services
- Backup solutions invoking ccrypt or gpg to encrypt files before transferring to cloud storage buckets over HTTPS
- CI/CD pipelines performing GPG-signed artifact verification with outbound keyserver lookups (keys.openpgp.org, keyserver.ubuntu.com)
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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