Threat Group 3390 APT Sample - HttpBrowser RAT Dropper

Hunt Hypothesis

The detection identifies potential deployment of a HttpBrowser RAT dropper associated with Threat Group 3390, leveraging suspicious HTTP-based communication to exfiltrate or execute malicious payloads. SOC teams should proactively hunt for this behavior in Azure Sentinel to detect early-stage APT activity and prevent lateral movement and data exfiltration.

YARA Rule

rule HttpBrowser_RAT_dropper_Gen1 
{

    meta:
        description = "Threat Group 3390 APT Sample - HttpBrowser RAT Dropper"
        author = "Florian Roth"
        reference = "http://snip.ly/giNB"
        date = "2015-08-06"
        score = 70
        hash1 = "808de72f1eae29e3c1b2c32be1b84c5064865a235866edf5e790d2a7ba709907"
        hash2 = "f6f966d605c5e79de462a65df437ddfca0ad4eb5faba94fc875aba51a4b894a7"
        hash3 = "f424965a35477d822bbadb821125995616dc980d3d4f94a68c87d0cd9b291df9"
        hash4 = "01441546fbd20487cb2525a0e34e635eff2abe5c3afc131c7182113220f02753"
        hash5 = "8cd8159f6e4689f572e2087394452e80e62297af02ca55fe221fe5d7570ad47b"
        hash6 = "10de38419c9a02b80ab7bf2f1f1f15f57dbb0fbc9df14b9171dc93879c5a0c53"
        hash7 = "c2fa67e970d00279cec341f71577953d49e10fe497dae4f298c2e9abdd3a48cc"
  
    strings:
        $x1 = "1001=cmd.exe" fullword ascii 
        $x2 = "1003=ShellExecuteA" fullword ascii 
        $x3 = "1002=/c del /q %s" fullword ascii
        $x4 = "1004=SetThreadPriority" fullword ascii

        /* $s1 = "pnipcn.dllUT" fullword ascii
        $s2 = "ssonsvr.exeUT" fullword ascii
        $s3 = "navlu.dllUT" fullword ascii
        $s4 = "@CONOUT$" fullword wide 
        $s5 = "VPDN_LU.exeUT" fullword ascii
        $s6 = "msi.dll.urlUT" fullword ascii
        $s7 = "setup.exeUT" fullword ascii 
        $s8 = "pnipcn.dll.urlUT" fullword ascii
        $s9 = "ldvpreg.exeUT" fullword ascii */

        $op0 = { e8 71 11 00 00 83 c4 10 ff 4d e4 8b f0 78 07 8b } /* Opcode */
        $op1 = { e8 85 34 00 00 59 59 8b 86 b4 } /* Opcode */
        $op2 = { 8b 45 0c 83 38 00 0f 84 97 } /* Opcode */
        $op3 = { 8b 45 0c 83 38 00 0f 84 98 } /* Opcode */
        $op4 = { 89 7e 0c ff 15 a0 50 40 00 59 8b d8 6a 20 59 8d } /* Opcode */
        $op5 = { 56 8d 85 cd fc ff ff 53 50 88 9d cc fc ff ff e8 } /* Opcode */
    
    condition:
        uint16(0) == 0x5a4d and filesize < 400KB and all of ($x*) and 1 of ($op*)
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

Microsoft Defender for Endpoint — Custom indicators / advanced hunting
Email Gateway — Attachment scanning
File Share Monitoring — Periodic scanning of shared drives
YARA CLI — Manual threat hunting on endpoints

This rule contains 19 string patterns in its detection logic.

References

False Positive Guidance

Scenario: Legitimate Scheduled Job Downloading a Browser Extension
Description: A scheduled job runs to download a browser extension from a known CDN, which triggers the rule due to the presence of a suspicious URL pattern.
Filter/Exclusion: Exclude URLs that match known CDN domains (e.g., https://*.cdn.mozilla.net, https://*.google.com), or filter by file hashes of known safe browser extensions.
Scenario: System Administrator Installing a Browser-Based Monitoring Tool
Description: An admin installs a legitimate browser-based monitoring or analytics tool (e.g., Google Analytics, Hotjar) using a script that includes a URL resembling a malicious payload.
Filter/Exclusion: Exclude URLs containing known legitimate analytics or monitoring service domains (e.g., analytics.google.com, hotjar.com), or filter by user-agent strings associated with admin tools.
Scenario: User Accessing a Phishing Page via Browser
Description: A user accesses a phishing page that mimics a legitimate internal portal, and the page contains a script that triggers the rule due to its similarity to a known RAT dropper.
Filter/Exclusion: Exclude URLs that match internal phishing page patterns (e.g., https://*.internalportal.com), or filter by user behavior (e.g., users who have accessed phishing pages in the past).
Scenario: Automated Patching Tool Using a Browser to Download Updates
Description: A patching tool uses a browser to download updates from an internal repository, which includes a URL that matches the rule’s signature.
Filter/Exclusion: Exclude URLs that match internal update servers (e.g., https://updates.corp.example.com), or filter by process names associated with patching tools (e.g., patchmgr.exe).
Scenario: Legitimate Browser-Based Application Deployment
*Description