CVE-2021-47827: WebSSH for iOS DoS Vulnerability

CVE-2021-47827 Overview

CVE-2021-47827 is a denial of service vulnerability affecting WebSSH for iOS version 14.16.10. The vulnerability exists in the mashREPL tool component, which fails to properly validate input length before processing. Attackers can exploit this flaw by pasting malformed input into the mashREPL input field, causing the application to crash and become unresponsive.

Critical Impact

This vulnerability allows local attackers with user interaction to crash the WebSSH application, disrupting SSH connectivity and terminal operations for iOS users relying on this client for remote server management.

Affected Products

• WebSSH for iOS version 14.16.10
• mashREPL component within WebSSH

Discovery Timeline

• 2026-01-16 - CVE CVE-2021-47827 published to NVD
• 2026-01-16 - Last updated in NVD database

Technical Details for CVE-2021-47827

Vulnerability Analysis

This denial of service vulnerability stems from improper input validation (CWE-1284) in the mashREPL tool within WebSSH for iOS. The application fails to properly handle boundary conditions when processing user-supplied input in the REPL (Read-Eval-Print Loop) interface.

When a user pastes a specially crafted input string into the mashREPL field, the application cannot properly manage the memory allocation or parsing logic required for such input. The vulnerability requires local access and user interaction, as the attacker must either have physical access to the device or convince the user to paste the malicious content themselves.

The exploitation is straightforward: copying a 300-character buffer consisting of repeated 'A' characters into the mashREPL input field triggers the crash condition. This indicates that the input handler lacks proper bounds checking or sanitization for oversized input strings.

Root Cause

The root cause of CVE-2021-47827 is improper input validation (CWE-1284) in the mashREPL component. The application does not adequately verify the size or content of input data before processing it, allowing specially crafted strings to trigger an unhandled exception or memory issue that crashes the application.

Attack Vector

The attack vector is local, requiring either physical device access or social engineering to convince a user to paste malicious content. The exploitation mechanism involves pasting a 300-character buffer of repeated characters (such as 'AAAAAA...') into the mashREPL input field. Upon submission or processing of this input, the application fails to handle the oversized buffer correctly, resulting in an application crash. Additional technical details and a proof-of-concept are documented in the Exploit-DB #49883 entry.

Detection Methods for CVE-2021-47827

Indicators of Compromise

• Unexpected WebSSH application crashes during mashREPL usage
• iOS crash logs showing the WebSSH process terminating abnormally when processing input
• Repeated application restarts or instability after clipboard paste operations

Detection Strategies

• Monitor iOS crash reports for WebSSH application terminations related to input processing
• Implement application-level logging to detect oversized input attempts in mashREPL
• Use mobile device management (MDM) solutions to track application crash frequency

Monitoring Recommendations

• Enable crash analytics for WebSSH in your organization's mobile fleet management solution
• Set up alerts for unusual patterns of application crashes across managed iOS devices
• Review device logs for evidence of repeated crash-and-restart cycles in WebSSH

How to Mitigate CVE-2021-47827

Immediate Actions Required

• Update WebSSH for iOS to the latest available version from the Apple App Store
• Avoid pasting untrusted content into the mashREPL tool until the application is updated
• Consider using alternative SSH clients for critical remote management tasks until a patch is confirmed

Patch Information

Users should check the Apple App Store listing for WebSSH for the latest version updates that may address this vulnerability. Review the VulnCheck Advisory on WebSSH for additional remediation guidance and patch status information.

Workarounds

• Avoid using the mashREPL feature within WebSSH until a patch is available
• Do not paste content from untrusted sources into any input fields within the WebSSH application
• Use alternative SSH client applications for iOS that are not affected by this vulnerability
• Implement organizational policies restricting clipboard sharing to trusted applications on managed devices