CVE-2021-47784: Cyberfox Web Browser DoS Vulnerability

CVE-2021-47784 Overview

CVE-2021-47784 is a denial of service vulnerability affecting Cyberfox Web Browser version 52.9.1. The vulnerability allows attackers to crash the application by overflowing the search bar with excessive data. Specifically, an attacker can generate a 9,000,000 byte payload and paste it into the search bar to trigger an application crash, resulting in a complete denial of service.

Critical Impact

Local attackers can crash Cyberfox Web Browser through resource exhaustion by submitting oversized input to the search bar, causing immediate application termination.

Affected Products

• Cyberfox Web Browser version 52.9.1

Discovery Timeline

• 2026-01-15 - CVE CVE-2021-47784 published to NVD
• 2026-01-16 - Last updated in NVD database

Technical Details for CVE-2021-47784

Vulnerability Analysis

This vulnerability is classified under CWE-770 (Allocation of Resources Without Limits or Throttling). The Cyberfox Web Browser fails to implement proper input validation and resource allocation limits for the search bar input field. When an attacker submits an exceptionally large payload (approximately 9MB of data), the application attempts to process this input without adequate memory management controls, leading to resource exhaustion and subsequent application crash.

The local attack vector requires user interaction, as the attacker would need to either convince a user to paste malicious content or have local access to the system running the vulnerable browser. The discontinued nature of Cyberfox (development ended around 2018) means no official patches will be released.

Root Cause

The root cause of this vulnerability is the absence of input size validation and resource throttling mechanisms in the search bar component. The browser's search functionality does not enforce reasonable limits on the amount of data that can be processed, allowing unbounded memory allocation when handling oversized inputs. This lack of defensive programming permits resource exhaustion attacks that crash the application.

Attack Vector

The attack requires local access and user interaction. An attacker can exploit this vulnerability by crafting a large payload (9,000,000 bytes) and pasting it into the Cyberfox search bar. This can be achieved through social engineering tactics where a victim is tricked into copying and pasting malicious content, or by an attacker with local system access directly inputting the payload. Upon submission of the oversized data, the browser's memory allocation routines are overwhelmed, causing the application to crash.

The exploitation is straightforward: generating a large string payload and submitting it through the search interface. Technical details and a proof-of-concept can be found in the Exploit-DB #50336 entry.

Detection Methods for CVE-2021-47784

Indicators of Compromise

• Unexpected Cyberfox browser crashes or termination events
• Large clipboard operations preceding browser crashes
• Memory spike events in the Cyberfox process followed by immediate termination

Detection Strategies

• Monitor for repeated Cyberfox application crashes in Windows Event logs or system crash reporters
• Implement endpoint detection rules for abnormal memory consumption patterns in browser processes
• Track clipboard paste operations involving unusually large data transfers to browser applications

Monitoring Recommendations

• Enable application crash monitoring on endpoints running Cyberfox Web Browser
• Configure SentinelOne to alert on abnormal resource consumption by browser processes
• Review system logs for patterns indicating repeated denial of service attempts against local applications

How to Mitigate CVE-2021-47784

Immediate Actions Required

• Discontinue use of Cyberfox Web Browser 52.9.1 as it is no longer maintained
• Migrate to actively supported web browsers such as Firefox, Chrome, or Edge
• Remove Cyberfox installations from enterprise environments

Patch Information

No official patch is available for this vulnerability. Cyberfox Web Browser is a discontinued project, with development ending around 2018. The archived project page can be found at the Cyberfox Archived Page. Users are strongly advised to migrate to actively maintained browser alternatives.

Workarounds

• Migrate to a currently supported web browser (Firefox, Chrome, Edge, or other maintained alternatives)
• If continued use is unavoidable, restrict clipboard paste operations in the browser where possible
• Implement endpoint protection solutions to monitor for and prevent resource exhaustion attacks
• Educate users about the risks of pasting unknown or untrusted content into browser input fields

# Identify and remove Cyberfox installations
# Windows PowerShell - Find Cyberfox installation
Get-ItemProperty HKLM:\Software\Microsoft\Windows\CurrentVersion\Uninstall\* | Where-Object { $_.DisplayName -like "*Cyberfox*" }

# Recommended: Uninstall Cyberfox and migrate to supported browser