Defaulttab

Overview

Adware:Win32/Defaulttab is a particularly aggressive and persistent Potentially Unwanted Program (PUP) categorized as a Browser Hijacker. It is notorious for not just changing the user's default search engine and homepage to a revenue-generating portal, but for employing rootkit-like techniques to actively defend its registry keys and files against removal attempts by both users and standard antivirus software.

Understanding Defaulttab (Aggressive Adware)
To an end-user, Defaulttab makes the browser completely unusable, constantly redirecting searches and spawning pop-ups. Every attempt to manually change the settings back is instantly reverted. For a security analyst, it represents a complex removal challenge, demonstrating that adware can utilize the same advanced defense evasion techniques as high-level trojans (`T1562.001`).

Execution and Persistence Mechanics
Defaulttab is primarily distributed through deceptive software bundling (`T1189`). Upon execution, it installs malicious extensions into Chrome, Firefox, and Internet Explorer. Crucially, it establishes deep persistence by installing a Windows Service (`T1543.003`) or a scheduled task that acts as a 'watchdog'. This watchdog process continuously monitors the specific registry keys controlling the browser's homepage and default search provider (`T1112`). If an antivirus program or the user attempts to delete or modify these keys, the watchdog process instantly restores them to the hijacked values.

Indicators of Compromise & Impact
The impact is severe disruption of productivity and exposure to potentially malicious third-party advertisements. Host-based IoCs include the presence of the `Defaulttab` process running in Task Manager, a suspicious newly registered Windows Service, and EDR alerts for continuous, rapid modifications to browser registry keys (e.g., `HKCU\Software\Microsoft\Internet Explorer\Main\Start Page`). Network IoCs include DNS requests resolving to known adware tracking domains and forced search redirects.

MITRE ATT&CK Techniques

Observed techniques used by this family, mapped to the MITRE ATT&CK framework:

Generated Detections (Boilerplate)

These YARA and Sigma rules are auto-generated based on the family name and aliases. They must be heavily tuned before deployment in a production environment.

YARA Rule

rule MALWARE_WIN_DEFAULTTAB {
    meta:
        description = "Detects Defaulttab (trojan)"
        author = "SystemHelpdesk Boilerplate Generator"
        date = "2026-07-06"
    strings:
        $s1 = "defaulttab" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

Sigma Rule

title: Suspicious Defaulttab Activity
id: 0d252286c85ac97a99a0a591c537dc12
status: experimental
description: Detects generic indicators of the defaulttab malware family.
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith:
            - '\cmd.exe'
            - '\powershell.exe'
        CommandLine|contains:
            - "*defaulttab*"
    condition: selection
level: medium

Containment & Response Steps

Ordered checklist for responders. Adapt to your environment and engage professional support for active incidents.

1. Reboot the infected machine into 'Safe Mode' to prevent the Defaulttab watchdog service from loading during remediation.
2. Use `services.msc` or Autoruns to identify and disable the malicious Windows Service protecting the adware.
3. Run a full scan with a specialized Adware/PUP removal tool to permanently delete the registry keys, extensions, and watchdog executables.
4. Reset all installed web browsers to their default settings to clear out the hijacked search providers and homepages.

What to Avoid

Common mistakes during response to this family that can destroy evidence, spread the infection, or worsen recovery.

1. Do not attempt manual registry edits while booted in normal Windows mode; the watchdog process will instantly revert your changes.
2. Avoid relying solely on the browser's built-in extension manager to remove Defaulttab, as the persistence mechanism exists outside the browser.

References & External Analysis

• Search "defaulttab" on VirusTotal (External Analysis)

Frequently Asked Questions

Related Families (Category: trojan)

Explore other malware families in the same category:

• Micronames
• Cliptoshuffler
• Installflash
• Mozla
• Gamethief

Machine-readable

Get this profile as JSON: https://jordanricky1604-ship-it.github.io/malware-families-catalog/api/defaulttab.json

Ecosystem & Interactive Environments

This profile is part of the Malware Families Catalog, a public dataset of 2,899 malware families. The catalog is also published across our ecosystem: Hugging Face, Kaggle, Zenodo, Replit, StackBlitz, CodeSandbox, and CodePen.