CVE-2021-47784: Cyberfox Web Browser DOS Vulnerability

CVE-2021-47784 Overview

CVE-2021-47784 is a denial of service vulnerability in Cyberfox Web Browser version 52.9.1. The browser fails to enforce limits on input size in its search bar component. An attacker with local access can paste a 9,000,000 byte payload into the search bar to crash the application. The flaw maps to [CWE-770: Allocation of Resources Without Limits or Throttling]. Cyberfox is a discontinued Firefox derivative previously distributed by 8pecxstudios, increasing residual risk for users who still run the browser without vendor support.

Critical Impact
Local attackers with the ability to interact with the browser interface can crash Cyberfox 52.9.1 by submitting an oversized payload to the search bar, disrupting browsing sessions and potentially causing loss of unsaved tab state.

Affected Products

Cyberfox Web Browser 52.9.1
8pecxstudios Cyberfox (discontinued, no vendor patches available)
Legacy Firefox-derived browsers sharing the same search bar input handling

Discovery Timeline

2026-01-15 - CVE-2021-47784 published to the National Vulnerability Database
2026-04-15 - Last updated in NVD database

Technical Details for CVE-2021-47784

Vulnerability Analysis

The vulnerability is a denial of service issue triggered by uncontrolled resource consumption in the browser's search bar input handler. Cyberfox 52.9.1 accepts pasted input without validating length or enforcing an upper bound on memory allocation. When the input crosses approximately 9 million bytes, the application exhausts its processing capacity and terminates.

The attack requires user interaction because the malicious payload must be pasted into the search bar by a user on the local system. Exploitation does not yield code execution, privilege escalation, or data disclosure. The impact is limited to availability of the browser process.

Because Cyberfox is no longer maintained, the affected code path will not receive vendor remediation. Organizations still operating the browser inherit the defect permanently unless they migrate to a supported alternative.

Root Cause

The root cause is the absence of input length validation and resource throttling on the search bar text field. The application allocates memory proportional to pasted content and processes the buffer synchronously, blocking the main thread until the operation fails. This pattern aligns with [CWE-770], where resources are allocated without a defined ceiling.

Attack Vector

The attack vector is local and requires user interaction. An attacker must convince a user to paste a crafted payload into the Cyberfox search bar, or have direct keyboard access to the system. The vulnerability cannot be triggered remotely over the network without an additional social engineering or local access primitive.

A proof-of-concept payload consisting of approximately 9,000,000 bytes of repeated character data is documented in Exploit-DB #50336. The exploit description confirms that pasting the buffer into the search bar reliably terminates the browser process.

Detection Methods for CVE-2021-47784

Indicators of Compromise

Repeated unexpected termination of the cyberfox.exe process on endpoints running version 52.9.1
Crash dumps or Windows Error Reporting events referencing the Cyberfox process following clipboard paste actions
Presence of installed Cyberfox 52.9.1 binaries on managed endpoints, which itself indicates exposure

Detection Strategies

Inventory endpoints for the presence of Cyberfox binaries and flag installations of version 52.9.1
Monitor for abnormal process exit codes from cyberfox.exe correlated with high memory allocation events
Alert on application crashes in close proximity to clipboard paste operations or large process working set spikes

Monitoring Recommendations

Forward application crash telemetry and Windows Error Reporting logs to a centralized analytics platform for correlation
Track software inventory changes to detect new installations of discontinued or unsupported browsers
Establish a baseline for browser process memory usage and alert on outliers exceeding several hundred megabytes

How to Mitigate CVE-2021-47784

Immediate Actions Required

Uninstall Cyberfox 52.9.1 from all managed endpoints, as the product is discontinued and will not receive patches
Migrate users to a maintained browser such as Mozilla Firefox ESR, Google Chrome, or Microsoft Edge
Add Cyberfox executables to application control denylists to prevent reinstallation

Patch Information

No vendor patch is available. Cyberfox development ceased and the project's distribution site is only accessible through archived snapshots. See the Cyberfox Archive Snapshot for historical context. Remediation requires removal of the affected software rather than patching.

Workarounds

Restrict paste operations into the Cyberfox interface by training users to avoid pasting untrusted content of unknown length
Run Cyberfox in a sandboxed environment so that a crash does not affect other workloads
Disable or remove the search bar extension if the deployment supports modifying the browser chrome

bash

# Example: enumerate and remove Cyberfox installations on Windows endpoints
Get-WmiObject -Class Win32_Product | Where-Object { $_.Name -like "*Cyberfox*" } | ForEach-Object { $_.Uninstall() }