CVE-2021-21044: Adobe Acrobat RCE Vulnerability

CVE-2021-21044 Overview

CVE-2021-21044 is an Out-of-Bounds Write vulnerability affecting Adobe Acrobat Reader DC and Adobe Acrobat products. The vulnerability exists in the JPEG file parsing functionality, where a specially crafted JPEG file can trigger an out-of-bounds memory write operation. An unauthenticated attacker could exploit this vulnerability to achieve arbitrary code execution in the context of the current user. Exploitation requires user interaction, specifically that the victim must open a malicious file.

Critical Impact

Successful exploitation allows attackers to execute arbitrary code with the privileges of the current user, potentially leading to complete system compromise, data theft, or malware installation through malicious PDF documents containing crafted JPEG images.

Affected Products

• Adobe Acrobat DC (Continuous) versions 2020.013.20074 and earlier
• Adobe Acrobat Reader DC (Continuous) versions 2020.013.20074 and earlier
• Adobe Acrobat 2020 (Classic) versions 2020.001.30018 and earlier
• Adobe Acrobat Reader 2020 (Classic) versions 2020.001.30018 and earlier
• Adobe Acrobat 2017 (Classic) versions 2017.011.30188 and earlier
• Adobe Acrobat Reader 2017 (Classic) versions 2017.011.30188 and earlier

Discovery Timeline

• 2021-02-11 - CVE-2021-21044 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-21044

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-bounds Write), a memory corruption flaw that occurs when a program writes data past the end of an allocated memory buffer. In the context of Adobe Acrobat Reader, this occurs during the parsing of JPEG image data embedded within PDF documents. When processing a maliciously crafted JPEG file, the parser fails to properly validate boundary conditions, allowing an attacker to write data beyond the intended memory region.

The vulnerability requires local access, meaning an attacker must convince a user to open a malicious file. No special privileges are required to exploit this vulnerability, but user interaction is necessary. Successful exploitation can result in high impact to confidentiality, integrity, and availability of the affected system.

Root Cause

The root cause of CVE-2021-21044 lies in insufficient bounds checking within Adobe Acrobat's JPEG parsing routines. When processing JPEG image data, the application allocates a memory buffer based on header information. However, the parsing logic fails to adequately verify that subsequent write operations remain within the allocated buffer boundaries. A crafted JPEG file with manipulated header values or malformed compressed data segments can trigger writes outside the designated memory region, corrupting adjacent memory structures.

Attack Vector

The attack vector for CVE-2021-21044 is local, requiring user interaction to execute. An attacker would typically:

1. Craft a malicious PDF document containing a specially constructed JPEG image designed to trigger the out-of-bounds write condition
2. Distribute the malicious file via email attachments, malicious websites, or file-sharing platforms
3. Social engineer the victim into opening the malicious PDF file with a vulnerable version of Adobe Acrobat Reader
4. Upon opening the file, the JPEG parsing routine processes the crafted image data, triggering the memory corruption
5. The attacker gains arbitrary code execution with the privileges of the user running Adobe Acrobat Reader

The vulnerability manifests during JPEG file parsing when boundary validation fails to account for malformed image dimensions or corrupted compressed data segments. For detailed technical information, refer to the Adobe Acrobat Security Advisory.

Detection Methods for CVE-2021-21044

Indicators of Compromise

• Unexpected crashes or abnormal behavior in Adobe Acrobat or Acrobat Reader processes
• PDF files containing suspicious or malformed JPEG image data with unusual header values
• Adobe Acrobat processes spawning unexpected child processes or network connections
• Memory access violations or application faults logged in system event logs related to Acrobat processes

Detection Strategies

• Deploy endpoint detection rules to monitor for suspicious behavior from AcroRd32.exe or Acrobat.exe processes
• Implement file inspection capabilities to scan PDF documents for malformed embedded JPEG images before delivery to end users
• Configure application whitelisting to detect when Adobe Acrobat processes attempt to execute unauthorized code
• Monitor for exploitation indicators such as heap spray patterns or ROP chain artifacts in memory

Monitoring Recommendations

• Enable enhanced logging for Adobe Acrobat applications to capture detailed crash and error information
• Configure SIEM rules to alert on patterns of PDF-related application crashes across multiple endpoints
• Implement sandbox analysis for PDF attachments received via email before delivery to end users
• Monitor for suspicious process creation events originating from Adobe Acrobat applications

How to Mitigate CVE-2021-21044

Immediate Actions Required

• Update all Adobe Acrobat and Acrobat Reader installations to the latest patched versions immediately
• Enable Protected View (sandbox mode) in Adobe Acrobat Reader to limit the impact of potential exploitation
• Implement email filtering to quarantine suspicious PDF attachments pending security review
• Educate users about the risks of opening PDF documents from untrusted sources

Patch Information

Adobe has released security updates to address CVE-2021-21044 as part of security bulletin APSB21-09. Organizations should update to the following minimum versions:

• Adobe Acrobat DC (Continuous): version 2021.001.20135 or later
• Adobe Acrobat Reader DC (Continuous): version 2021.001.20135 or later
• Adobe Acrobat 2020 (Classic): version 2020.001.30020 or later
• Adobe Acrobat 2017 (Classic): version 2017.011.30190 or later

For complete patch details, see the Adobe Acrobat Security Advisory.

Workarounds

• Enable Protected View for all files from untrusted sources in Adobe Acrobat Reader preferences
• Disable JavaScript execution in Adobe Acrobat Reader to reduce potential attack surfaces
• Use alternative PDF readers in high-risk environments until patches can be applied
• Implement application control policies to restrict document handling in sensitive environments

# Windows Registry configuration to enable Protected View
# Run in elevated PowerShell
reg add "HKCU\Software\Adobe\Acrobat Reader\DC\TrustManager" /v bEnableProtectedModeByDefault /t REG_DWORD /d 1 /f
reg add "HKCU\Software\Adobe\Acrobat Reader\DC\TrustManager" /v iProtectedView /t REG_DWORD /d 2 /f