CVE-2021-47764: AbsoluteTelnet DOS Vulnerability

CVE-2021-47764 Overview

CVE-2021-47764 is a denial of service vulnerability affecting AbsoluteTelnet version 11.24, a terminal emulator and SSH/Telnet client developed by Celestial Software. The vulnerability allows local attackers to crash the application by manipulating DialUp connection and license name fields with oversized input payloads. By generating a 1000-character payload and pasting it into specific input fields, attackers can trigger application crashes and force unexpected termination.

Critical Impact

Local attackers can reliably crash AbsoluteTelnet 11.24 by exploiting improper input validation in the DialUp connection and license name fields, resulting in denial of service conditions that disrupt legitimate terminal operations.

Affected Products

• Celestial Software AbsoluteTelnet version 11.24

Discovery Timeline

• 2026-01-15 - CVE CVE-2021-47764 published to NVD
• 2026-01-21 - Last updated in NVD database

Technical Details for CVE-2021-47764

Vulnerability Analysis

This vulnerability is classified under CWE-787 (Out-of-Bounds Write), indicating that the application fails to properly validate the length of user-supplied input before writing it to memory buffers. When an attacker provides an excessively long string (approximately 1000 characters) to the DialUp connection or license name input fields, the application attempts to process this data without adequate bounds checking.

The out-of-bounds write condition corrupts adjacent memory regions, leading to application instability and eventual crash. While this vulnerability requires local access and user interaction to exploit, it can be leveraged to disrupt workflow for users who rely on AbsoluteTelnet for critical remote administration tasks.

Root Cause

The root cause of CVE-2021-47764 lies in insufficient input validation within the AbsoluteTelnet application. Specifically, the input handling routines for the DialUp connection settings and license name fields do not enforce proper length restrictions on user-supplied data. When data exceeding the allocated buffer size is provided, the application writes beyond the intended memory boundaries, corrupting adjacent memory structures and triggering a crash condition.

Attack Vector

The attack vector for this vulnerability is local, requiring the attacker to have direct access to the system running AbsoluteTelnet. The exploitation process involves:

1. Opening AbsoluteTelnet 11.24 on the target system
2. Navigating to the DialUp connection settings or license registration dialog
3. Generating a payload of approximately 1000 characters
4. Pasting the oversized payload into the vulnerable input field
5. The application crashes upon processing the malformed input

The vulnerability does not require elevated privileges to exploit, though it does require user interaction to navigate to the vulnerable input fields and submit the malicious payload.

Technical details and proof-of-concept information can be found at Exploit-DB #50511.

Detection Methods for CVE-2021-47764

Indicators of Compromise

• Unexpected AbsoluteTelnet application crashes or terminations
• Windows Event Log entries indicating absolutetelnet.exe process failures with access violation errors
• Crash dump files generated in the application directory or Windows temp folders
• User reports of application instability when modifying connection or license settings

Detection Strategies

• Monitor Windows Application Event Logs for repeated crash events associated with absolutetelnet.exe
• Implement endpoint detection rules to identify abnormal process termination patterns for terminal emulation software
• Configure crash monitoring tools to alert on AbsoluteTelnet process failures
• Use SentinelOne's behavioral AI to detect anomalous application crashes that may indicate exploitation attempts

Monitoring Recommendations

• Enable detailed application crash logging on systems running AbsoluteTelnet
• Configure centralized log collection to aggregate crash events from endpoints using this software
• Establish baseline metrics for normal AbsoluteTelnet operation to identify deviations
• Deploy SentinelOne Singularity platform for real-time endpoint visibility and crash pattern analysis

How to Mitigate CVE-2021-47764

Immediate Actions Required

• Identify all systems running AbsoluteTelnet version 11.24 within your environment
• Restrict local access to affected systems to trusted users only
• Check the Celestial Software Homepage for updated versions that address this vulnerability
• Consider temporary removal of AbsoluteTelnet from critical systems until a patched version is available

Patch Information

No official patch information is currently available in the CVE data. Users should monitor the Celestial Software Homepage for security updates and newer versions of AbsoluteTelnet that may address this input validation vulnerability. Contact Celestial Software support directly for guidance on remediation options.

Workarounds

• Limit access to AbsoluteTelnet configuration dialogs to authorized administrators only
• Implement application whitelisting policies to prevent unauthorized modifications to AbsoluteTelnet settings
• Consider using alternative terminal emulation software that is not affected by this vulnerability
• Deploy endpoint protection solutions like SentinelOne to monitor for exploitation attempts and application crashes

# Configuration example - Restrict AbsoluteTelnet access using Windows Group Policy
# Limit application execution to specific user groups
# Path: Computer Configuration > Windows Settings > Security Settings > Application Control Policies
# Note: Consult your organization's security team for proper implementation