CVE-2020-24435: Adobe Acrobat Buffer Overflow Vulnerability

CVE-2020-24435 Overview

CVE-2020-24435 is a heap-based buffer overflow vulnerability affecting Adobe Acrobat Reader DC and related products. The vulnerability exists in the submitForm function and can potentially allow arbitrary code execution in the context of the current user. Successful exploitation requires user interaction, specifically that a victim must open a maliciously crafted PDF file in a vulnerable version of Acrobat Reader.

Critical Impact

This vulnerability enables attackers to execute arbitrary code with user-level privileges through specially crafted PDF documents, potentially leading to complete system compromise.

Affected Products

• Adobe Acrobat DC (Continuous and Classic tracks) versions 2020.012.20048 and earlier
• Adobe Acrobat Reader DC (Continuous and Classic tracks) versions 2020.001.30005 and earlier
• Adobe Acrobat Reader versions 2017.011.30175 and earlier
• Affected on both Microsoft Windows and Apple macOS platforms

Discovery Timeline

• November 5, 2020 - CVE-2020-24435 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-24435

Vulnerability Analysis

This vulnerability is classified as CWE-122 (Heap-based Buffer Overflow). The flaw resides in the submitForm JavaScript function within Adobe Acrobat Reader, which is used for form submission functionality in PDF documents. When processing specially crafted PDF files, the function fails to properly validate input data, allowing attackers to overflow heap-allocated buffers and corrupt adjacent memory regions.

The exploitation requires local access where a user must open a malicious PDF file. No authentication is required from the attacker's perspective, but user interaction is mandatory for successful exploitation. The impact is severe, affecting confidentiality, integrity, and availability of the target system.

Root Cause

The root cause of CVE-2020-24435 is insufficient bounds checking in the submitForm function when handling form data within PDF documents. The function allocates heap memory for form submission data but does not properly validate the size of input before copying it into the allocated buffer. This allows an attacker to supply oversized data that exceeds the buffer boundaries, causing heap memory corruption.

Attack Vector

The attack vector requires local user interaction. An attacker must craft a malicious PDF document containing specially formatted form data designed to trigger the buffer overflow condition in the submitForm function. The attack workflow typically involves:

1. Attacker creates a PDF document with embedded JavaScript calling submitForm with crafted parameters
2. The malicious PDF is distributed via email attachments, malicious websites, or other social engineering methods
3. Victim opens the PDF in a vulnerable version of Adobe Acrobat Reader
4. The submitForm function processes the malformed data, triggering the heap overflow
5. Attacker gains code execution with the privileges of the user running Acrobat Reader

The vulnerability is particularly dangerous in enterprise environments where PDF documents are routinely exchanged and opened without suspicion.

Detection Methods for CVE-2020-24435

Indicators of Compromise

• Unexpected crashes or abnormal behavior in Adobe Acrobat Reader processes
• PDF files containing obfuscated JavaScript with calls to submitForm function
• Memory access violations or heap corruption errors in Acrobat Reader logs
• Unusual child processes spawned from AcroRd32.exe or Acrobat.exe

Detection Strategies

• Deploy endpoint detection and response (EDR) solutions to monitor for suspicious process behavior from Adobe Acrobat processes
• Implement email gateway scanning to detect malicious PDF attachments before delivery
• Use sandbox analysis for PDF documents from untrusted sources to identify exploitation attempts
• Monitor for JavaScript execution patterns in PDF documents that call form submission functions with abnormal parameters

Monitoring Recommendations

• Enable detailed logging for Adobe Acrobat Reader and monitor for crash events or error conditions
• Implement file integrity monitoring on systems where Acrobat Reader is installed
• Configure security information and event management (SIEM) rules to alert on suspicious PDF-related activity
• Monitor network traffic for data exfiltration attempts following potential exploitation

How to Mitigate CVE-2020-24435

Immediate Actions Required

• Update Adobe Acrobat Reader DC and Adobe Acrobat to the latest available versions immediately
• Enable Protected View mode in Adobe Acrobat Reader to sandbox PDF document processing
• Disable JavaScript execution in Adobe Acrobat Reader if not required for business operations
• Implement application whitelisting to prevent unauthorized code execution

Patch Information

Adobe has released security updates to address this vulnerability as documented in Adobe Security Advisory APSB20-67. Organizations should update to:

• Acrobat DC and Acrobat Reader DC Continuous track: Version 2020.013.20064 or later
• Acrobat 2020 and Acrobat Reader 2020 Classic track: Version 2020.001.30010 or later
• Acrobat 2017 and Acrobat Reader 2017 Classic track: Version 2017.011.30180 or later

Additional technical details are available in the Talos Intelligence Vulnerability Report.

Workarounds

• Configure Adobe Acrobat Reader to open PDF documents in Protected View mode by default
• Disable JavaScript in Adobe Acrobat Reader via Edit > Preferences > JavaScript > uncheck "Enable Acrobat JavaScript"
• Use alternative PDF readers that are not affected by this vulnerability until patching is complete
• Implement network segmentation to limit the impact of potential exploitation on critical systems

# Disable JavaScript in Adobe Acrobat Reader via Registry (Windows)
reg add "HKCU\Software\Adobe\Acrobat Reader\DC\JSPrefs" /v bEnableJS /t REG_DWORD /d 0 /f

# Enable Protected View for all files (Windows)
reg add "HKCU\Software\Adobe\Acrobat Reader\DC\TrustManager" /v iProtectedView /t REG_DWORD /d 2 /f