CVE-2020-36995: Mocha Telnet Lite iOS DoS Vulnerability

CVE-2020-36995 Overview

CVE-2020-36995 is a denial of service vulnerability affecting Mocha Telnet Lite for iOS version 4.2. The vulnerability stems from a buffer overflow condition (CWE-120) that allows attackers to crash the application by manipulating user configuration input. Specifically, attackers can overwrite the 'User' field with 350 bytes of repeated characters to trigger an application crash and prevent normal functionality.

Critical Impact

This vulnerability enables attackers to cause persistent denial of service by crashing the Mocha Telnet Lite iOS application through malicious input manipulation, disrupting users' ability to establish telnet connections.

Affected Products

• Mocha Telnet Lite for iOS version 4.2

Discovery Timeline

• 2026-01-29 - CVE-2020-36995 published to NVD
• 2026-01-29 - Last updated in NVD database

Technical Details for CVE-2020-36995

Vulnerability Analysis

This vulnerability is classified as a classic buffer overflow (CWE-120: Buffer Copy without Checking Size of Input). The application fails to properly validate the length of user-supplied input in the 'User' configuration field. When an attacker provides an excessively long string (350 bytes of repeated characters), the application attempts to copy this data into a fixed-size buffer without boundary checking, resulting in memory corruption and subsequent application crash.

The local attack vector requires user interaction, meaning an attacker would need to either have physical access to the device or convince the user to enter malicious configuration data. While the impact is limited to application availability rather than confidentiality or integrity compromise, the vulnerability can be reliably triggered to prevent legitimate use of the telnet client.

Root Cause

The root cause of CVE-2020-36995 lies in improper input validation within the user configuration handling code. The application allocates a fixed-size buffer for storing the username but fails to verify that the input length does not exceed the buffer's capacity before performing the copy operation. This missing bounds check allows attackers to overflow the buffer with 350+ bytes of data, corrupting adjacent memory and causing the application to crash.

Attack Vector

The attack requires local access and user interaction. An attacker exploits this vulnerability by modifying the 'User' configuration field within the Mocha Telnet Lite application settings. By entering a string consisting of approximately 350 bytes (such as repeated 'A' characters), the buffer overflow is triggered when the application processes this malformed input, leading to immediate application termination.

The exploitation is straightforward and requires no authentication or special privileges beyond the ability to modify application settings. Technical details and proof-of-concept information can be found in the Exploit-DB #48728 advisory.

Detection Methods for CVE-2020-36995

Indicators of Compromise

• Unexpected crashes of the Mocha Telnet Lite application during configuration changes
• Application configuration files containing abnormally long username values (350+ characters)
• iOS crash reports indicating memory corruption in the Mocha Telnet Lite process

Detection Strategies

• Monitor iOS device logs for repeated crash events from the Mocha Telnet Lite application
• Implement mobile device management (MDM) policies to detect unusual application behavior patterns
• Review application crash analytics for buffer overflow signatures in telnet client applications

Monitoring Recommendations

• Enable iOS crash reporting and review logs for Mocha Telnet Lite crash events
• Deploy endpoint detection solutions capable of monitoring mobile application stability
• Configure alerts for repeated application crashes that may indicate exploitation attempts

How to Mitigate CVE-2020-36995

Immediate Actions Required

• Remove or restrict use of Mocha Telnet Lite version 4.2 on managed iOS devices
• Consider migrating to alternative telnet clients with proper input validation
• Educate users about the risks of entering untrusted configuration data into the application

Patch Information

No vendor patch information is currently available for this vulnerability. Users should monitor the Apple App Store listing for potential updates from the vendor. Additional advisory information is available from the VulnCheck Advisory.

Workarounds

• Uninstall Mocha Telnet Lite version 4.2 until a patched version is available
• Use alternative iOS telnet clients from reputable vendors with active security maintenance
• Implement MDM policies to restrict application installation on enterprise-managed devices
• Avoid entering configuration data from untrusted sources