CVE-2021-40729: Adobe Acrobat DC Information Disclosure

CVE-2021-40729 Overview

CVE-2021-40729 is an out-of-bounds read vulnerability affecting Adobe Acrobat Reader DC and related Adobe Acrobat products. The vulnerability allows an attacker to read memory beyond the intended buffer boundaries, potentially disclosing sensitive memory contents. This information disclosure could be leveraged to bypass security mitigations such as Address Space Layout Randomization (ASLR), which is a critical defense mechanism against exploitation of memory corruption vulnerabilities.

Exploitation of this vulnerability requires user interaction, specifically that a victim must open a maliciously crafted PDF file. This attack vector is commonly used in targeted phishing campaigns where attackers deliver weaponized PDF documents via email or compromised websites.

Critical Impact

This out-of-bounds read vulnerability enables disclosure of sensitive memory contents and can be used to bypass ASLR mitigations, potentially facilitating further exploitation when chained with other vulnerabilities.

Affected Products

• Adobe Acrobat DC (Continuous) version 21.007.20095 and earlier
• Adobe Acrobat Reader DC (Continuous) version 21.007.20096 and earlier
• Adobe Acrobat (Classic 2020) version 20.004.30015 and earlier
• Adobe Acrobat Reader (Classic 2020) version 20.004.30015 and earlier
• Adobe Acrobat (Classic 2017) version 17.011.30202 and earlier
• Adobe Acrobat Reader (Classic 2017) version 17.011.30202 and earlier

Discovery Timeline

• 2021-10-15 - CVE-2021-40729 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-40729

Vulnerability Analysis

CVE-2021-40729 is classified as CWE-125 (Out-of-Bounds Read), a memory corruption vulnerability class that occurs when software reads data past the end or before the beginning of an intended buffer. In the context of Adobe Acrobat products, this vulnerability exists in the PDF parsing or rendering functionality, where specially crafted PDF content can trigger the application to read memory locations outside of allocated buffer boundaries.

The vulnerability's primary impact is information disclosure. When triggered, the out-of-bounds read operation exposes contents of adjacent memory regions that should not be accessible to the attacker. This leaked information can include memory addresses, heap metadata, or other sensitive data that undermines memory protection mechanisms.

A significant concern with this vulnerability is its potential use as a building block in exploit chains. By disclosing memory layout information, attackers can defeat ASLR, making it easier to exploit other vulnerabilities that require precise memory addresses for successful exploitation.

Root Cause

The root cause of this vulnerability lies in improper boundary validation when processing PDF document structures. The Adobe Acrobat parsing engine fails to adequately verify that memory read operations remain within the bounds of allocated buffers when handling certain malformed or specially crafted PDF elements. This insufficient bounds checking allows read operations to access memory beyond the intended buffer, leading to information disclosure.

Attack Vector

The attack requires local access and user interaction. An attacker must craft a malicious PDF file containing specific content designed to trigger the out-of-bounds read condition. The victim must then be convinced to open this PDF file using a vulnerable version of Adobe Acrobat or Acrobat Reader.

Common delivery mechanisms for such attacks include:

• Phishing emails with malicious PDF attachments
• Drive-by downloads from compromised or malicious websites
• Social engineering tactics to encourage users to open untrusted PDF documents

The vulnerability manifests when the PDF parsing engine processes malformed content within the document structure. The specific technical details of the triggering conditions are documented in the Adobe Security Advisory APSB21-104.

Detection Methods for CVE-2021-40729

Indicators of Compromise

• Unusual PDF files with anomalous internal structures or embedded objects
• Unexpected memory access patterns in Adobe Acrobat processes
• Crash dumps or error logs indicating buffer read violations in Acrobat components
• PDF files from untrusted sources that trigger application errors or unexpected behavior

Detection Strategies

• Deploy endpoint detection and response (EDR) solutions capable of monitoring Adobe Acrobat process behavior for anomalous memory access patterns
• Implement email security gateways with PDF sandboxing capabilities to analyze incoming PDF attachments before delivery
• Use file integrity monitoring to detect suspicious PDF files on endpoints and file shares
• Enable application crash reporting and analyze crash dumps for signs of exploitation attempts

Monitoring Recommendations

• Monitor Adobe Acrobat process activity for unusual memory access patterns or unexpected crashes
• Track PDF file downloads and email attachments, flagging files from untrusted sources for additional analysis
• Implement logging for Adobe Acrobat application events and correlate with security information and event management (SIEM) systems
• Review endpoint telemetry for signs of exploitation attempts targeting PDF readers

How to Mitigate CVE-2021-40729

Immediate Actions Required

• Update Adobe Acrobat and Acrobat Reader to the latest versions immediately
• Enable automatic updates for Adobe Acrobat products to ensure timely patching
• Implement application allowlisting to prevent execution of untrusted PDF readers
• Educate users about the risks of opening PDF files from untrusted sources

Patch Information

Adobe has addressed this vulnerability in security bulletin APSB21-104. Users should update to the following versions or later:

• Acrobat DC (Continuous): Update to version 21.007.20099 or later
• Acrobat Reader DC (Continuous): Update to version 21.007.20099 or later
• Acrobat 2020 (Classic): Update to version 20.004.30017 or later
• Acrobat Reader 2020 (Classic): Update to version 20.004.30017 or later
• Acrobat 2017 (Classic): Update to version 17.011.30204 or later
• Acrobat Reader 2017 (Classic): Update to version 17.011.30204 or later

Workarounds

• Enable Protected View in Adobe Acrobat Reader to sandbox PDF rendering and limit the impact of exploitation
• Disable JavaScript in Adobe Acrobat Reader preferences if not required for business operations
• Use alternative PDF readers for opening untrusted documents until patching is complete
• Implement network-level controls to scan and sanitize PDF attachments before they reach end users

# Enable Protected View via Windows Registry for Adobe Acrobat Reader DC
# Sets Protected View to "All files" mode
reg add "HKCU\Software\Adobe\Acrobat Reader\DC\TrustManager" /v bProtectedMode /t REG_DWORD /d 1 /f
reg add "HKCU\Software\Adobe\Acrobat Reader\DC\TrustManager" /v iProtectedView /t REG_DWORD /d 2 /f