Detect Browser Fingerprint in Splunk
Adversaries may attempt to blend in with legitimate traffic by spoofing browser and system attributes like operating system, system language, platform, user-agent string, resolution, time zone, etc. The HTTP User-Agent request header is a string that lets servers and network peers identify the application, operating system, vendor, and/or version of the requesting user agent. Adversaries may gather this information through System Information Discovery or by users navigating to adversary-controlled websites, and then use that information to craft their web traffic to evade defenses. This technique was documented in the FatDuke malware, which mimics a compromised user's traffic by using the same user agent as the installed browser.
MITRE ATT&CK
- Tactic
- Defense Evasion
- Technique
- T1036 Masquerading
- Sub-technique
- T1036.012 Browser Fingerprint
- Canonical reference
- https://attack.mitre.org/techniques/T1036/012/
SPL Detection Query
index=proxy OR index=web sourcetype="stream:http" OR sourcetype="bluecoat" OR sourcetype="squid"
| eval ua=lower(http_user_agent)
| eval process=lower(process_name)
| where isnotnull(ua) AND len(ua) > 10
| eval non_browser=if(NOT match(process, "^(chrome|msedge|firefox|iexplore|opera|brave|vivaldi|safari)"), 1, 0)
| where non_browser=1 OR isnull(process)
| eval outdated_browser=if(match(ua, "(msie [5-9]\.|firefox/[1-3][0-9]\.|chrome/[1-5][0-9]\.)"), 1, 0)
| eval missing_fields=if(NOT match(ua, "mozilla/5\.0") AND match(ua, "^mozilla/"), 1, 0)
| eval known_malware_ua=if(match(ua, "(mozilla/4\.0.*compatible.*msie 7\.0.*windows nt 5\.1|mozilla/5\.0.*compatible.*msie 10\.0.*trident/6\.0)"), 1, 0)
| eval SuspicionScore=non_browser + outdated_browser + missing_fields + known_malware_ua
| where SuspicionScore > 0
| table _time, src_ip, dest_ip, dest_port, http_user_agent, uri_path, process_name, SuspicionScore
| sort - SuspicionScore - _timeDetects browser fingerprint spoofing by analyzing HTTP User-Agent strings from proxy and web traffic logs. Identifies non-browser processes making HTTP requests with browser-like User-Agent strings, outdated browser version strings commonly used by older malware families, and known malicious User-Agent patterns. Uses proxy/web logs (Bluecoat, Squid, Splunk Stream) to capture the actual HTTP User-Agent header rather than relying on process-level telemetry alone.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Legitimate software updaters using HTTP with browser-like User-Agent strings
- System administration scripts using curl/wget with custom User-Agent strings
- Monitoring agents making health-check HTTP requests
- Development and testing automation tools setting custom User-Agent headers
Other platforms for T1036.012
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 1HTTP Request with Spoofed Browser User-Agent via curl
Expected signal: Sysmon Event ID 1: Process Create for curl.exe with '-A' and the full User-Agent string in CommandLine. Sysmon Event ID 3: Network Connection from curl.exe to httpbin.org IP on port 80. DeviceNetworkEvents with InitiatingProcessFileName=curl.exe connecting to external IP. Proxy logs show HTTP request with Chrome User-Agent from a non-browser process.
- Test 2PowerShell HTTP Request with Spoofed User-Agent
Expected signal: Sysmon Event ID 1: Process Create for powershell.exe with 'User-Agent' and 'MSIE 10.0' in CommandLine. Sysmon Event ID 3: Network Connection from powershell.exe to httpbin.org. PowerShell ScriptBlock Log Event ID 4104 with the full script content. Proxy logs show IE10 User-Agent from PowerShell process.
- Test 3Python HTTP Request Mimicking Browser Fingerprint
Expected signal: Auditd EXECVE record for python3 with the HTTP request script. Network connection from python3 to httpbin.org on port 80. Proxy logs show full browser-like header set (User-Agent, Accept, Accept-Language, Accept-Encoding) originating from a Python process rather than a browser.
References (7)
- https://attack.mitre.org/techniques/T1036/012/
- https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/User-Agent
- https://arxiv.org/pdf/2110.10129
- https://www.welivesecurity.com/wp-content/uploads/2019/10/ESET_Operation_Ghost_Dukes.pdf
- https://engineering.salesforce.com/tls-fingerprinting-with-ja3-and-ja3s-247362855967/
- https://learn.microsoft.com/en-us/defender-endpoint/advanced-hunting-devicenetworkevents-table
- https://github.com/SigmaHQ/sigma/tree/master/rules/network
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