CVE-2021-21017: Adobe Acrobat Buffer Overflow Vulnerability

CVE-2021-21017 Overview

CVE-2021-21017 is a heap-based buffer overflow vulnerability affecting Adobe Acrobat Reader DC and Adobe Acrobat products. An unauthenticated attacker could leverage this vulnerability to achieve arbitrary code execution in the context of the current user. Exploitation of this issue requires user interaction in that a victim must open a malicious file, typically a crafted PDF document.

Critical Impact

This vulnerability is actively exploited in the wild and is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog. Successful exploitation enables attackers to execute arbitrary code with the privileges of the current user, potentially leading to complete system compromise.

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

Discovery Timeline

• 2021-02-11 - CVE-2021-21017 published to NVD
• 2025-10-23 - Last updated in NVD database

Technical Details for CVE-2021-21017

Vulnerability Analysis

This vulnerability is classified under CWE-122 (Heap-based Buffer Overflow) and CWE-787 (Out-of-bounds Write). The flaw exists within the PDF parsing and rendering functionality of Adobe Acrobat and Acrobat Reader applications. When a specially crafted PDF file is processed, the application fails to properly validate user-supplied data, leading to a heap-based buffer overflow condition.

The vulnerability requires user interaction—specifically, the victim must open a malicious PDF file. Once triggered, the attacker can write data beyond the allocated heap buffer boundaries, corrupting adjacent memory structures. This memory corruption can be weaponized to gain control of program execution flow, ultimately achieving arbitrary code execution within the context of the current user.

Given the widespread deployment of Adobe Acrobat products across both Windows and macOS platforms in enterprise environments, this vulnerability presents a significant attack surface for threat actors conducting targeted attacks via spear-phishing campaigns with malicious PDF attachments.

Root Cause

The root cause of CVE-2021-21017 stems from insufficient bounds checking during the processing of PDF document structures. Specifically, when parsing certain elements within a PDF file, the application allocates a heap buffer based on attacker-controllable values without properly validating that subsequent write operations remain within the allocated boundaries.

This improper input validation allows an attacker to supply crafted data that exceeds the expected buffer size, resulting in an out-of-bounds write condition on the heap. The corrupted heap memory can then be manipulated to overwrite critical data structures, function pointers, or other memory allocations, enabling the attacker to redirect code execution.

Attack Vector

The attack vector for CVE-2021-21017 is network-based with user interaction required. A typical attack scenario involves:

1. An attacker crafts a malicious PDF document containing specially designed structures that trigger the heap-based buffer overflow
2. The malicious PDF is delivered to the victim via email attachment, malicious website download, or file-sharing platform
3. When the victim opens the PDF using a vulnerable version of Adobe Acrobat or Acrobat Reader, the heap overflow is triggered
4. The attacker-controlled data corrupts heap memory, allowing for arbitrary code execution
5. Code executes with the privileges of the user running the application

The vulnerability's presence in CISA's Known Exploited Vulnerabilities catalog confirms active exploitation in the wild, making this a priority for immediate remediation.

Detection Methods for CVE-2021-21017

Indicators of Compromise

• Unusual Adobe Acrobat or Reader process behavior, including unexpected child process spawning
• Crash dumps or error reports from AcroRd32.exe or Acrobat.exe indicating heap corruption
• Suspicious PDF files with abnormal internal structures or embedded JavaScript
• Network connections originating from Adobe Acrobat processes to unknown external hosts
• Endpoint detection alerts for memory corruption or code injection in Adobe processes

Detection Strategies

• Deploy endpoint detection and response (EDR) solutions capable of monitoring Adobe Acrobat process memory operations
• Implement file inspection rules to identify malformed PDF documents before they reach end users
• Enable crash reporting and analysis to identify potential exploitation attempts
• Monitor for suspicious process chains where Adobe Acrobat spawns unexpected child processes
• Utilize memory protection features such as heap integrity validation

Monitoring Recommendations

• Configure SIEM rules to alert on Adobe Acrobat application crashes correlated with recent PDF file access
• Enable Windows Event Logging for Application Crash events related to Acrobat processes
• Monitor email gateway logs for PDF attachments delivered around the time of detected anomalies
• Implement behavioral analytics to detect post-exploitation activities following PDF file opens
• Review endpoint telemetry for signs of privilege escalation following Adobe process anomalies

How to Mitigate CVE-2021-21017

Immediate Actions Required

• Update all Adobe Acrobat and Acrobat Reader installations to the latest patched versions immediately
• Enable Protected View in Adobe Acrobat Reader to sandbox PDF rendering
• Block or quarantine PDF attachments from untrusted sources at the email gateway
• Implement application control policies to prevent unauthorized code execution from Adobe processes
• Review and update endpoint protection signatures for known exploitation patterns

Patch Information

Adobe has released security updates addressing CVE-2021-21017 in security bulletin APSB21-09. Organizations should update to the following minimum versions:

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

This vulnerability is tracked in the CISA Known Exploited Vulnerabilities Catalog, requiring federal agencies to apply remediation by the specified deadline.

Workarounds

• Enable Protected View for all files by navigating to Edit > Preferences > Security (Enhanced) and selecting "Files from potentially unsafe locations" or "All files"
• Disable JavaScript in Adobe Acrobat Reader via Edit > Preferences > JavaScript > uncheck "Enable Acrobat JavaScript"
• Configure email gateways to strip or quarantine PDF attachments from external sources
• Implement network segmentation to limit the impact of potential code execution
• Use application sandboxing technologies to isolate Adobe Acrobat processes from critical system resources

# Registry configuration to enable Protected View (Windows)
# Sets Protected View to "All files" mode
reg add "HKCU\Software\Adobe\Acrobat Reader\DC\TrustManager" /v bEnhancedSecurityInBrowser /t REG_DWORD /d 1 /f
reg add "HKCU\Software\Adobe\Acrobat Reader\DC\TrustManager" /v bEnhancedSecurityStandalone /t REG_DWORD /d 1 /f

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