CVE-2020-36943: aSc TimeTables DoS Vulnerability

CVE-2020-36943 Overview

CVE-2020-36943 is a Denial of Service (DoS) vulnerability affecting aSc TimeTables 2021.6.2. This vulnerability allows attackers to crash the application by overwriting subject title fields with excessive data. Specifically, an attacker can generate a 10,000-character buffer and paste it into the subject title field to trigger application instability and a potential crash.

Critical Impact

Successful exploitation causes application crashes and service disruption for users of aSc TimeTables scheduling software, potentially affecting educational institutions and organizations relying on this tool for timetable management.

Affected Products

• aSc TimeTables 2021.6.2

Discovery Timeline

• 2026-01-28 - CVE CVE-2020-36943 published to NVD
• 2026-01-29 - Last updated in NVD database

Technical Details for CVE-2020-36943

Vulnerability Analysis

This vulnerability is classified as CWE-770 (Allocation of Resources Without Limits or Throttling). The application fails to properly validate or limit the size of input data when users enter information into the subject title field. This allows an attacker with local access to paste excessively large strings (approximately 10,000 characters) into the field, overwhelming the application's ability to handle the data and causing it to crash.

The attack requires local access to the system and user interaction (the attacker or a deceived user must paste the malicious input). While this limits the attack surface, in shared computing environments such as educational computer labs or workstations, an attacker could craft a malicious payload and trigger the crash to disrupt scheduling operations.

Root Cause

The root cause of this vulnerability is improper resource allocation and missing input validation in the subject title handling routines. The application does not enforce a maximum character limit on the subject title input field, allowing unbounded data to be processed. When excessively large input is provided, the application cannot handle the resource allocation demands, leading to instability and eventual crash.

Attack Vector

The attack vector is local, requiring an attacker to have access to a system running aSc TimeTables 2021.6.2. The exploitation process involves:

1. Generating a large buffer of characters (approximately 10,000 characters)
2. Opening the aSc TimeTables application
3. Navigating to a subject title field within the application
4. Pasting the oversized buffer into the field
5. The application becomes unstable and crashes

This attack requires user interaction but can be triggered by social engineering tactics such as convincing a user to paste clipboard contents or through malicious automation scripts if the attacker has local access.

Technical details about this vulnerability can be found in the Exploit-DB #49147 entry and the VulnCheck Advisory on ASC Timetables.

Detection Methods for CVE-2020-36943

Indicators of Compromise

• Unexpected crashes or instability of the aSc TimeTables application
• Application error logs showing memory allocation failures or buffer-related errors
• Reports of unusually large text entries in subject title fields
• Clipboard monitoring showing large text buffers being prepared before application crashes

Detection Strategies

• Monitor application event logs for aSc TimeTables crash events with memory-related error codes
• Implement endpoint detection rules to alert on repeated application crashes within short time periods
• Deploy application monitoring to detect abnormal input sizes in form fields
• Use behavioral analysis to identify attempts to paste unusually large clipboard contents into applications

Monitoring Recommendations

• Configure endpoint protection to monitor aSc TimeTables process stability and resource usage
• Enable Windows Error Reporting analysis for crash patterns related to TimeTables.exe
• Establish baseline metrics for application behavior to detect anomalies indicative of exploitation attempts
• Implement user activity monitoring in shared computing environments where the application is deployed

How to Mitigate CVE-2020-36943

Immediate Actions Required

• Identify all systems running aSc TimeTables version 2021.6.2
• Check for updated versions of aSc TimeTables on the official download page
• Restrict local access to systems running the vulnerable application where possible
• Educate users about the risks of pasting untrusted clipboard contents into application fields

Patch Information

Users should check the aSc TimeTables homepage and download page for updated versions that may address this vulnerability. Contact the vendor directly for information about available patches or fixes for this denial of service issue.

Workarounds

• Implement application control policies to restrict who can access and use aSc TimeTables in shared environments
• Configure group policies to limit clipboard size or monitor clipboard activity on critical systems
• Consider running the application in a sandboxed environment to contain potential crashes
• Establish backup procedures for timetable data to minimize disruption if crashes occur

# Example: Windows Event Log monitoring for application crashes
# Check for aSc TimeTables crash events in Event Viewer
wevtutil qe Application /q:"*[System[(EventID=1000) and (Provider[@Name='Application Error'])]] and *[EventData[Data[1]='TimeTables.exe']]" /c:10 /f:text