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Malformed User Agent

yara LOW Yara-Rules
community
This rule was pulled from an open-source repository and enriched with AI. Validate in a test environment before deploying to production.
View original rule at Yara-Rules →
Retrieved: 2026-07-01T23:00:00Z · Confidence: medium

Hunt Hypothesis

This detection identifies potential reconnaissance or evasion activities where adversaries manipulate User-Agent strings to bypass standard filtering mechanisms or mimic legitimate traffic patterns. Proactively hunting for these anomalies in Azure Sentinel is critical because even low-severity malformed agents can serve as early indicators of sophisticated threat actors attempting to obscure their identity before executing more impactful attacks.

YARA Rule

rule volgmer
{
meta:
    description = "Malformed User Agent"
    ref = "https://www.us-cert.gov/ncas/alerts/TA17-318B"
strings:
    $s = "Mozillar/"
condition:
    (uint16(0) == 0x5A4D and uint16(uint32(0x3c)) == 0x4550) and $s
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

This rule contains 1 string patterns in its detection logic.

False Positive Guidance

Here are 5 specific false positive scenarios for the Malformed User Agent detection rule in an enterprise environment, along with suggested filters or exclusions:

Original source: https://github.com/Yara-Rules/rules/blob/main/malware/MALW_Volgmer.yar