CVE-2021-39839

What it actually does

CVE-2021-39839 is a use-after-free vulnerability in Adobe Acrobat and Acrobat Reader DC when processing the AcroForm getItem action inside a PDF. A specially crafted PDF can trigger unsafe memory reuse and allow attacker-controlled code execution within the Acrobat or Reader process. Because the process runs as the current user, successful exploitation can give the attacker the same file, network, and application access as the victim. The CVSS 7.8 rating reflects local exploitation with required user interaction but high confidentiality, integrity, and availability impact.

Why it matters to the business

An attacker who convinces a user to open a malicious PDF can compromise that user’s workstation, steal documents or credentials accessible to the user, install persistence, move laterally, or stage additional malware. This is especially relevant for organizations that routinely receive external PDF attachments, legal documents, invoices, resumes, forms, or other business files. Although this issue is not listed in CISA KEV and has no confirmed ransomware association in the provided data, the EPSS score of 0.65249 indicates elevated likelihood of exploitation compared with many vulnerabilities, so unpatched Acrobat and Reader installations should be treated as meaningful endpoint risk.

How an attack unfolds

An attacker prepares a malicious PDF containing crafted AcroForm content that abuses the vulnerable getItem action. The file is delivered to a target through phishing email, a shared download link, messaging platform, compromised website, or other document-sharing workflow. When the victim opens the PDF in an affected version of Adobe Acrobat or Acrobat Reader DC, the malformed AcroForm action triggers a use-after-free condition. With a reliable exploit, the attacker gains arbitrary code execution inside the Acrobat or Reader process as the logged-in user. The payload may then launch a child process, drop malware, retrieve a second-stage payload, steal files, harvest credentials or browser data available to the user, or establish persistence through user-level startup mechanisms.

How to fix it

Upgrade Adobe Acrobat and Adobe Acrobat Reader DC to a version newer than the affected releases: Acrobat Reader DC 2021.005.20060 and earlier, Acrobat/Reader 2020.004.30006 and earlier, and Acrobat/Reader 2017.011.30199 and earlier are vulnerable. Apply the relevant Adobe security update for the Acrobat/Reader track in use, or move to the current supported Continuous or Classic release. Where immediate patching is not possible, reduce exposure by disabling JavaScript in Acrobat/Reader where operationally feasible, enforcing Protected View/Protected Mode, blocking PDFs from untrusted sources at email and web gateways, opening external PDFs in isolated environments, and restricting Acrobat/Reader child-process creation through endpoint protection, application control, or attack surface reduction rules. Remove unsupported Acrobat/Reader versions from managed endpoints.

How to confirm the fix worked

Confirm that all installed Adobe Acrobat and Acrobat Reader instances report versions later than the vulnerable releases for their respective release tracks. Validate through endpoint management inventory, software asset scans, vulnerability scanners, or local version checks in the application About dialog and installed-program metadata. Test that Adobe update deployment completed successfully on Windows and macOS endpoints. Review configuration baselines to confirm Protected Mode and Protected View are enabled where supported. For exposure validation, search for endpoints running Acrobat Reader DC 2021.005.20060 or earlier, Acrobat/Reader 2020.004.30006 or earlier, or Acrobat/Reader 2017.011.30199 or earlier, and prioritize systems that receive external PDFs or are used by high-risk business functions.

Typical attack path

1. Identify users or business functions likely to open externally supplied PDFs, such as finance, HR, legal, procurement, or customer support.
2. Create or obtain a malicious PDF that triggers the AcroForm getItem use-after-free vulnerability in affected Acrobat or Acrobat Reader DC versions.
3. Deliver the PDF through email, file-sharing services, messaging platforms, web downloads, or social engineering.
4. Convince the victim to open the file in a vulnerable Acrobat or Reader version.
5. Trigger memory corruption during AcroForm processing and execute attacker-controlled code in the context of the current user.
6. Run a payload or script from the Acrobat or Reader process, often by spawning system utilities or dropping an executable.
7. Establish persistence, steal accessible data, collect credentials, or use the compromised endpoint for lateral movement.

How to detect it

To determine whether exploitation has already occurred, review endpoint detection telemetry, process creation logs, email security logs, proxy logs, file-system events, and Acrobat/Reader crash reports around the time suspicious PDFs were opened. Focus on Adobe Acrobat or Reader processes spawning unusual child processes, writing executables or scripts to user-writable locations, initiating unexpected outbound connections, or crashing shortly before suspicious activity. On Windows, analyze Sysmon Event ID 1/process creation, Event ID 11/file creation, Event ID 3/network connection, Windows Security Event ID 4688, PowerShell logs, and EDR process trees where AcroRd32.exe, Acrobat.exe, or acrord32.exe is the parent. On macOS, review EndpointSecurity/EDR telemetry, Unified Logs, file quarantine events, and child processes launched by Adobe Reader or Acrobat. Correlate with email gateway records for inbound PDFs, especially unsolicited attachments or links, and inspect any PDFs opened immediately before suspicious endpoint behavior. Because no unique public exploit string is provided in the input, detection should emphasize behavioral indicators rather than a single static signature.

• AcroRd32.exe spawning cmd.exe
• AcroRd32.exe spawning powershell.exe
• AcroRd32.exe spawning wscript.exe
• AcroRd32.exe spawning cscript.exe
• AcroRd32.exe spawning mshta.exe
• AcroRd32.exe spawning rundll32.exe
• AcroRd32.exe spawning regsvr32.exe
• AcroRd32.exe spawning certutil.exe
• AcroRd32.exe spawning bitsadmin.exe
• AcroRd32.exe spawning curl.exe
• AcroRd32.exe spawning wget.exe
• Acrobat.exe spawning cmd.exe
• Acrobat.exe spawning powershell.exe
• Acrobat.exe spawning wscript.exe
• Acrobat.exe spawning cscript.exe
• Acrobat.exe spawning mshta.exe
• Acrobat.exe spawning rundll32.exe
• Acrobat.exe spawning regsvr32.exe
• Acrobat.exe spawning certutil.exe
• Acrobat.exe spawning bitsadmin.exe
• Acrobat.exe spawning curl.exe
• Acrobat.exe spawning wget.exe
• ParentImage=AcroRd32.exe Image=powershell.exe
• ParentImage=AcroRd32.exe Image=cmd.exe
• ParentImage=Acrobat.exe Image=powershell.exe
• ParentImage=Acrobat.exe Image=cmd.exe
• ParentImage=*AcroRd32.exe* EventCode=4688
• ParentImage=*Acrobat.exe* EventCode=4688
• /Acrobat Reader DC/Reader/AcroRd32.exe
• /Adobe Acrobat DC/Acrobat/Acrobat.exe
• AcroRd32.exe creating file in %TEMP%
• Acrobat.exe creating file in %TEMP%
• AcroRd32.exe creating file in %APPDATA%
• Acrobat.exe creating file in %APPDATA%
• AcroRd32.exe outbound network connection
• Acrobat.exe outbound network connection

What your security team should do

• Prioritize patching Acrobat and Acrobat Reader installations because the vulnerability enables user-context code execution and has a high EPSS score.
• Inventory all Acrobat and Reader versions across Windows and macOS and identify systems at or below the vulnerable version thresholds.
• Apply current Adobe security updates or migrate users to supported Acrobat/Reader release tracks.
• Enable or enforce Acrobat/Reader Protected Mode and Protected View, especially for files from the internet, email, or network shares.
• Harden endpoints to detect or block suspicious Acrobat/Reader child processes such as command shells, scripting engines, LOLBins, archive tools, or unknown executables.
• Add email and web-gateway controls to detonate or quarantine suspicious PDFs, especially those containing forms, embedded scripts, or anomalous AcroForm structures.
• Hunt historical endpoint telemetry for Acrobat or Reader spawning unexpected child processes, creating executables or scripts in user-writable directories, or making unusual outbound network connections.
• Educate users in high-risk roles to treat unsolicited PDFs, invoices, forms, and document-signing requests as potentially malicious.
• Remove obsolete Acrobat/Reader versions and unsupported tracks from the environment.