CVE-2021-47876: GeoGebra Classic DOS Vulnerability

CVE-2021-47876 Overview

CVE-2021-47876 is a denial of service vulnerability affecting GeoGebra Classic 5.0.631.0-d. The vulnerability exists in the application's input field handling mechanism, which fails to properly validate the size of user-supplied input. Attackers can exploit this flaw by sending oversized buffer content to the application, specifically by pasting approximately 800,000 repeated characters into the 'Entrada:' input field, causing the application to crash.

This vulnerability is classified under CWE-770 (Allocation of Resources Without Limits or Throttling), indicating that the application does not implement proper resource allocation controls when processing input data.

Critical Impact

Attackers can cause complete application crashes through oversized input buffer attacks, disrupting user workflows and potentially causing data loss for unsaved work.

Affected Products

• GeoGebra Classic 5.0.631.0-d

Discovery Timeline

• 2026-01-21 - CVE CVE-2021-47876 published to NVD
• 2026-01-21 - Last updated in NVD database

Technical Details for CVE-2021-47876

Vulnerability Analysis

This denial of service vulnerability stems from improper resource allocation in GeoGebra Classic's input handling routines. The application fails to implement appropriate bounds checking or input length validation when processing user-supplied data through the 'Entrada:' input field. When an attacker provides an extremely large input buffer containing approximately 800,000 characters, the application attempts to process this data without adequate memory management controls, leading to resource exhaustion and subsequent application termination.

The local attack vector requires user interaction, as the malicious content must be pasted or entered into the application's input field. While this limits the attack surface compared to network-exploitable vulnerabilities, it still poses a significant risk in scenarios where users may unknowingly process malicious data from untrusted sources.

Root Cause

The root cause of this vulnerability is the lack of input length validation and resource allocation limits in the input processing routines (CWE-770: Allocation of Resources Without Limits or Throttling). The application does not enforce maximum input size restrictions, allowing arbitrarily large data to be submitted to the input field. This absence of defensive input handling causes the application to allocate excessive memory resources when attempting to process the oversized input, ultimately resulting in a crash.

Attack Vector

The attack vector is local, requiring an attacker to either directly interact with the victim's GeoGebra instance or convince a user to paste malicious content into the application. An attacker can generate a buffer containing approximately 800,000 repeated characters (such as the character 'A') and deliver this payload through various means:

1. Social engineering to convince users to paste content from a malicious source
2. Exploiting scenarios where users copy content from untrusted documents or websites
3. Direct access to a shared system running the vulnerable application

The attack does not require any special privileges and relies on standard user interaction with the application's input field. For technical details on the exploitation methodology, refer to the Exploit-DB #49654 entry and the VulnCheck Advisory on Geogebra DoS.

Detection Methods for CVE-2021-47876

Indicators of Compromise

• Unexpected GeoGebra application crashes, particularly when processing pasted content
• Process memory spikes immediately before application termination
• Windows Event Log entries indicating GeoGebra process failures or unresponsive behavior
• User reports of application hangs when working with large input data

Detection Strategies

• Monitor for abnormal memory allocation patterns in GeoGebra processes
• Implement endpoint detection rules to alert on repeated application crashes from the same executable
• Review crash dump files for evidence of buffer-related memory exhaustion
• Deploy application whitelisting policies to control GeoGebra execution contexts

Monitoring Recommendations

• Configure endpoint protection to monitor GeoGebra process stability and resource consumption
• Set up alerts for applications exceeding normal memory thresholds
• Maintain logs of application crash events for forensic analysis
• Consider implementing clipboard monitoring in high-security environments to detect oversized paste operations

How to Mitigate CVE-2021-47876

Immediate Actions Required

• Update GeoGebra Classic to the latest available version that addresses this vulnerability
• Educate users about the risks of pasting untrusted content into applications
• Implement application sandboxing where possible to limit crash impact
• Review and restrict GeoGebra usage in environments where untrusted content may be encountered

Patch Information

Users should check the official GeoGebra website for updated versions that address this vulnerability. Ensure that automatic updates are enabled where available, and verify that deployed versions are newer than 5.0.631.0-d.

Workarounds

• Avoid pasting content from untrusted sources into GeoGebra input fields
• Use input sanitization tools or scripts to validate content size before pasting into the application
• Configure application-level restrictions to limit input field sizes where supported
• Implement endpoint protection solutions to monitor for and potentially block abnormal application behavior
• Consider using the web-based version of GeoGebra which may have different input handling characteristics