Full Report
Detecting stealthy command-line activity that may indicate dark web access or anonymized traffic is a growing challenge for security teams. Tools like curl.exe—while entirely legitimate—can be leveraged by advanced threats to route traffic through proxy networks or TOR. This is where Uncoder AI’s Full Summary capability provides crucial context. When applied to SentinelOne Query Language […] The post Investigating Curl-Based TOR Proxy Access with Uncoder AI and SentinelOne Query Language appeared first on SOC Prime.
Analysis Summary
# Tool/Technique: Curl.exe Activity with TOR/Proxy Indicators
## Overview
This analysis focuses on detecting malicious use of the legitimate command-line tool `curl.exe` when it attempts to connect via TOR proxies or references `.onion` domains. This activity is often indicative of threat actors attempting to bypass traditional network visibility, communicate with hidden Command and Control (C2) infrastructure on the dark web, or exfiltrate data discreetly.
## Technical Details
- Type: Technique / Malicious Tool Usage
- Platform: Windows (where `curl.exe` is used, often leveraging bundled or external libraries)
- Capabilities: Facilitates network connections (HTTP/S, etc.) through configured proxies or directly to Tor hidden services.
- First Seen: Not specified in the context, but the technique is continually relevant.
## MITRE ATT&CK Mapping
- T1090 - Proxy
- T1090.003 - Multi-hop Proxy (Potentially applicable if chaining proxies, including TOR)
- T1105 - Ingress Tool Transfer (If using curl to download secondary payloads)
## Functionality
### Core Capabilities
- Establishing network connections using the `curl` utility.
- Utilizing SOCKS proxies (often configured for TOR) to anonymize traffic.
- Resolving and connecting to `.onion` domains to access hidden services/C2 infrastructure.
### Advanced Features
- Attackers leverage this method to bypass standard logging and visibility checks.
- Used for reaching out to C2 servers hosted on the dark web.
- Used for data exfiltration without detection across standard network perimeters.
- Association with advanced tooling like the **Kalambur backdoor** when TOR is involved.
## Indicators of Compromise
- File Hashes: N/A (Focus is on process behavior)
- File Names: `curl.exe`
- Registry Keys: N/A
- Network Indicators: Presence of SOCKS proxy configuration parameters or attempts to resolve/connect to `.onion` addresses within `curl` arguments.
- Behavioral Indicators: Execution of `curl.exe` that includes arguments referencing `socks5://` or `.onion` domains, especially when executed outside of expected system or administrative tasks.
## Associated Threat Actors
- Threat actors utilizing TOR-enabled implants (e.g., mention of **Kalambur backdoor**).
## Detection Methods
- Signature-based detection: Requires rules that specifically look for command-line arguments of `curl.exe` containing TOR/proxy indicators.
- Behavioral detection: Monitoring for anomalous execution patterns of `curl.exe`, particularly when unusual network destinations (like onion services) are accessed.
- YARA rules: Not explicitly mentioned, but Sigma/Detection-as-Code rules targeting command-line arguments are suggested (leveraging tools like Uncoder AI to generate these).
## Mitigation Strategies
- Deep packet inspection of suspected process activity.
- Reviewing endpoint history and persistence mechanisms if activity is confirmed.
- Correlating suspicious connectivity across multiple hosts to identify broader compromise.
- Restricting the execution of `curl.exe` or enforcing strict application whitelisting if it is not required for legitimate operations.
## Related Tools/Techniques
- TOR (The Onion Router) network usage.
- Other network tunneling/proxying tools.
- Backdoors utilizing network obfuscation (e.g., Kalambur).