CVE-2021-28640

What it actually does

The described flaw is a use-after-free memory corruption vulnerability in Adobe Acrobat Reader DC. When a specially crafted document is opened, the application may reuse freed memory in an unsafe way, allowing attacker-controlled data to influence execution flow. Successful exploitation can execute arbitrary code with the privileges of the user running Acrobat Reader, enabling data theft, file modification, persistence, or further local privilege escalation through additional vulnerabilities. The provided product metadata also lists discourse/rails_multisite with a fixed Git commit, which conflicts with the Acrobat Reader description and should be verified before package-specific remediation is applied.

Why it matters to the business

If users open malicious documents delivered through phishing, file sharing, or compromised websites, attackers may gain control of the user session and access sensitive files, business applications, and internal resources available to that user. This can lead to credential theft, malware installation, operational disruption, and lateral movement. The vulnerability is not listed in CISA KEV and is not marked as ransomware-associated, but the EPSS value is high and stable, indicating meaningful exploitation likelihood despite no provided public exploit maturity.

How an attack unfolds

An attacker prepares a malicious document designed to trigger the use-after-free condition in Acrobat Reader. The attacker then convinces an authenticated user to open the file, commonly through phishing email, a shared document link, a collaboration platform, or a downloaded attachment. When the vulnerable reader processes the crafted content, memory corruption occurs and attacker-controlled code may run in the context of the current user. The attacker can then execute commands, drop additional payloads, steal documents or credentials accessible to that user, and attempt persistence or privilege escalation depending on endpoint controls and user privileges.

How to fix it

Upgrade Adobe Acrobat Reader DC to a vendor-supported version newer than the affected releases: 2021.005.20054 and earlier, 2020.004.30005 and earlier, and 2017.011.30197 and earlier. Enforce automatic Adobe updates where possible and remove unsupported legacy Reader versions. Apply endpoint controls that reduce document-exploitation impact, including protected view, sandboxing, attachment detonation, EDR behavioral blocking, and least-privilege user access. Because the supplied affected-product metadata lists discourse/rails_multisite rather than Acrobat Reader, verify the vulnerability-to-product mapping in asset and dependency inventories; if that package is in scope for the advisory source being used, update discourse/rails_multisite to commit c6785cdb5c9277dd2c5ac8d55180dd1ece440ed0 or later.

How to confirm the fix worked

Confirm installed Acrobat Reader versions are newer than the affected release lines and that automatic update policy is enabled. On Windows, validate through installed application inventory, Adobe update logs, software management tooling, and executable file versions for AcroRd32.exe or Acrobat.exe. Test that email and web security controls block or detonate suspicious document attachments. If the discourse/rails_multisite package metadata is relevant to your inventory, verify the deployed Git revision is c6785cdb5c9277dd2c5ac8d55180dd1ece440ed0 or later. Re-scan endpoints and dependency inventories after remediation to ensure no vulnerable versions remain.

Typical attack path

1. Identify organizations or users likely to open PDF or document attachments.
2. Craft or obtain a malicious file that triggers the Acrobat Reader use-after-free condition.
3. Deliver the file through phishing, a file-sharing link, messaging platform, or compromised website.
4. Rely on the victim to open the malicious file in a vulnerable version of Acrobat Reader.
5. Execute code under the victim user's privileges after the memory corruption is triggered.
6. Drop follow-on malware, steal files or credentials, establish persistence, or move laterally using the compromised user context.

How to detect it

Review endpoint telemetry, email security logs, proxy logs, and EDR events for evidence that users received or opened suspicious documents before Acrobat Reader crashes or unusual child processes. Key signs include Acrobat Reader spawning cmd.exe, powershell.exe, wscript.exe, cscript.exe, mshta.exe, rundll32.exe, regsvr32.exe, certutil.exe, curl.exe, or other tools not normally launched by a PDF reader. Correlate suspicious attachment delivery with Windows Application Error events for AcroRd32.exe or Acrobat.exe, newly written executables or scripts in user-writable directories, persistence changes, credential-access tooling, and outbound network connections shortly after document opening. Search mail gateways for external PDF attachments or document links associated with user reports, blocked malware verdicts, or sandbox detections. No specific public exploit indicators, hashes, or actor-specific artifacts were provided, so detections should focus on behavioral evidence of document exploitation.

• AcroRd32.exe
• Acrobat.exe
• Faulting application name: AcroRd32.exe
• Faulting application name: Acrobat.exe
• AcroRd32.exe spawned cmd.exe
• AcroRd32.exe spawned powershell.exe
• AcroRd32.exe spawned wscript.exe
• AcroRd32.exe spawned cscript.exe
• AcroRd32.exe spawned mshta.exe
• AcroRd32.exe spawned rundll32.exe
• AcroRd32.exe spawned regsvr32.exe
• AcroRd32.exe spawned certutil.exe
• Acrobat.exe spawned cmd.exe
• Acrobat.exe spawned powershell.exe
• Acrobat.exe spawned wscript.exe
• Acrobat.exe spawned cscript.exe
• Acrobat.exe spawned mshta.exe
• Acrobat.exe spawned rundll32.exe
• Acrobat.exe spawned regsvr32.exe
• Acrobat.exe spawned certutil.exe
• %APPDATA%\Adobe\Acrobat\DC\
• %LOCALAPPDATA%\Temp\
• c6785cdb5c9277dd2c5ac8d55180dd1ece440ed0

What your security team should do

• Prioritize patching endpoints with Acrobat Reader installed, especially systems used by finance, legal, HR, executives, and other users who frequently receive external documents.
• Verify the product mapping because the supplied affected-product metadata conflicts with the Acrobat Reader vulnerability description.
• Harden document-handling controls: enable protected view/sandboxing, block macros and active content where applicable, and route external attachments through detonation.
• Hunt for suspicious Acrobat Reader child processes, crashes, and file-open activity around phishing events.
• Use EDR rules to alert when Acrobat Reader spawns command shells, scripting engines, archive tools, download utilities, or writes executable content to user-writable paths.
• Educate users to report unexpected PDF or document attachments and links, especially from external senders.
• If discourse/rails_multisite is present and confirmed in scope, update to commit c6785cdb5c9277dd2c5ac8d55180dd1ece440ed0 or later.