CVE-2021-47765: AbsoluteTelnet DOS Vulnerability

CVE-2021-47765 Overview

CVE-2021-47765 is a denial of service vulnerability in AbsoluteTelnet version 11.24 that allows local attackers to crash the application through input manipulation. The vulnerability is classified as CWE-787 (Out-of-Bounds Write), indicating improper memory handling when processing oversized input in specific application fields.

Attackers can exploit this vulnerability by inserting 1000 characters into the username or email address fields within the error report functionality, causing the application to become unresponsive and ultimately crash. This represents a classic input validation failure that leads to memory corruption.

Critical Impact

Local attackers can render AbsoluteTelnet completely unavailable by triggering application crashes through malformed input, disrupting terminal and SSH connectivity for affected users.

Affected Products

• Celestial Software AbsoluteTelnet version 11.24
• AbsoluteTelnet Windows Terminal/SSH Client

Discovery Timeline

• 2026-01-15 - CVE CVE-2021-47765 published to NVD
• 2026-01-21 - Last updated in NVD database

Technical Details for CVE-2021-47765

Vulnerability Analysis

This denial of service vulnerability stems from improper input validation in AbsoluteTelnet's user interface components. When processing username or email address fields within the application's error reporting functionality, the software fails to properly validate the length of user-supplied input before copying it to fixed-size memory buffers.

The vulnerability requires local access to the system where AbsoluteTelnet is installed, meaning an attacker must either have physical access or remote access to the machine through other means. User interaction is required as the attacker needs to manipulate input fields within the running application. While this limits the attack surface, it still poses a significant risk in shared computing environments or scenarios where users may be socially engineered into triggering the crash.

The out-of-bounds write (CWE-787) classification indicates that when the boundary condition is exceeded, the application writes data beyond the allocated buffer, corrupting adjacent memory and causing the application to crash.

Root Cause

The root cause is insufficient input validation and boundary checking in AbsoluteTelnet's error reporting module. The application allocates a fixed-size buffer for username and email address fields but does not enforce length limits before processing user input. When input exceeding 1000 characters is provided, the data overflows the allocated buffer, leading to memory corruption and application termination.

This is a common programming error in applications that do not implement proper bounds checking or use unsafe string handling functions without length validation.

Attack Vector

The attack vector is local with no special privileges required, though user interaction is necessary. An attacker with access to the AbsoluteTelnet application can:

1. Open the error reporting dialog or access the username/email configuration fields
2. Insert an oversized string (1000+ characters) into the username or email address field
3. Trigger the input processing, which causes the buffer overflow
4. The application crashes, resulting in denial of service

The attack is straightforward to execute and requires no sophisticated techniques. A proof of concept is publicly available through Exploit-DB #50510, demonstrating the exact steps to reproduce the crash condition.

Detection Methods for CVE-2021-47765

Indicators of Compromise

• AbsoluteTelnet application crashes with memory access violations or unhandled exceptions
• Windows Event Log entries showing absolutetelnet.exe application faults
• Crash dump files in system temporary directories related to AbsoluteTelnet
• Repeated application restarts in a short time period

Detection Strategies

• Monitor for abnormal application terminations of absolutetelnet.exe through endpoint detection and response (EDR) solutions
• Configure Windows Error Reporting to capture crash events for AbsoluteTelnet
• Implement application monitoring to detect repeated crash patterns that may indicate exploitation attempts
• Review system event logs for WER (Windows Error Reporting) events with faulting module absolutetelnet.exe

Monitoring Recommendations

• Enable SentinelOne's application stability monitoring to detect abnormal crash patterns
• Configure alerts for repeated application failures within short time windows
• Monitor process creation and termination events for AbsoluteTelnet to identify exploitation patterns
• Establish baseline crash rates to detect anomalous spikes in application failures

How to Mitigate CVE-2021-47765

Immediate Actions Required

• Update AbsoluteTelnet to the latest available version from Celestial Software
• Restrict access to systems running AbsoluteTelnet to trusted users only
• Implement application whitelisting to prevent unauthorized access to AbsoluteTelnet
• Consider using alternative terminal/SSH clients if a patched version is not available

Patch Information

Users should check the Celestial Software website for the latest version of AbsoluteTelnet that addresses this vulnerability. Ensure that automatic updates are enabled if available, or subscribe to vendor security notifications for future patches.

At the time of this publication, specific patch version information was not available in the CVE data. Contact Celestial Software directly for patch availability and upgrade instructions.

Workarounds

• Restrict access to the AbsoluteTelnet application to trusted users only through file system permissions
• Disable or restrict access to the error reporting functionality if not required for normal operations
• Implement endpoint protection solutions like SentinelOne to monitor for and prevent exploitation attempts
• In multi-user environments, consider application sandboxing to limit the impact of crashes

# Restrict AbsoluteTelnet access to specific users (Windows)
# Run in elevated PowerShell
icacls "C:\Program Files\AbsoluteTelnet\absolutetelnet.exe" /inheritance:r /grant:r "DOMAIN\TrustedUsers:(RX)"