CVE-2021-34833

What it actually does

CVE-2021-34833 is a memory safety vulnerability in Foxit PDF Reader and related Foxit PDF products affecting handling of PDF Annotation objects. The flaw is caused by failure to validate that an object still exists before operating on it, creating a use-after-free condition. A malicious PDF document or malicious web content can trigger memory corruption and allow attacker-controlled code to run inside the Foxit application process. Successful exploitation gives the attacker the privileges of the user who opened the file or visited the malicious page, enabling data access, payload execution, and potential follow-on compromise of the Windows host.

Why it matters to the business

An attacker can compromise a workstation by convincing a user to open a crafted PDF or visit a malicious page. Because PDF readers are commonly deployed and PDFs are frequently exchanged through email, file-sharing platforms, ticketing systems, and web downloads, this vulnerability creates a realistic phishing and document-based intrusion risk. The high EPSS score indicates broad exploitability interest and a higher likelihood of exploitation in the wild even though this CVE is not currently listed in CISA KEV and no ransomware association is reported. A successful compromise can lead to credential theft, malware installation, unauthorized access to business files, and disruption of endpoint operations.

How an attack unfolds

An attacker prepares a specially crafted PDF or web page containing malformed Annotation object structures designed to trigger the use-after-free condition in a vulnerable Foxit PDF Reader or Foxit PDF Editor installation. The attacker then delivers the content through phishing email, a download link, a compromised website, instant messaging, or another social-engineering channel. When the victim opens the malicious PDF or browses to content that invokes the vulnerable Foxit component, Foxit mishandles the freed object and corrupts process memory. With a reliable exploit, the attacker gains code execution in the context of the current user. The initial code may launch a downloader, drop malware, spawn scripting tools, connect to command-and-control infrastructure, or steal local files and credentials. Follow-on activity depends on the user’s privileges and endpoint defenses; if the user has elevated privileges or if additional privilege-escalation flaws are chained, compromise can expand beyond the initial application process.

How to fix it

Upgrade affected Foxit PDF Reader and Foxit PDF Editor installations to a vendor-supported release that contains the fix for this vulnerability. The provided fixed-version data does not specify exact minimum versions, so organizations should deploy the latest available Foxit Reader/Foxit PDF Editor builds from Foxit’s official update channels and remove unsupported versions such as Foxit PDF Reader 11.0.0.49893. Where immediate patching is not possible, reduce exposure by disabling automatic opening of PDFs from browsers and email clients, blocking unsolicited PDF attachments from untrusted sources, enforcing Protected View/sandboxing features where available, restricting script execution and child-process creation from PDF reader processes, and applying endpoint attack-surface-reduction controls. Ensure users do not run PDF readers with administrative privileges.

How to confirm the fix worked

Confirm exposure by inventorying installed Foxit products and versions across Windows endpoints and identifying Foxit PDF Reader 11.0.0.49893 or other unpatched 11.x-era builds. Validate remediation by confirming that Foxit update logs, installed application metadata, software inventory, or endpoint management tooling show a current vendor-supported version. On Windows, check installed application records, executable file versions, and package inventory for FoxitReader.exe, FoxitPDFReader.exe, FoxitPhantomPDF.exe, and FoxitPDFEditor.exe. Test policy mitigations by verifying that PDFs do not auto-open from untrusted locations, browser PDF handoff behavior is controlled, and EDR/application-control rules alert on or block suspicious child processes spawned by Foxit applications.

Typical attack path

1. Identify targets likely to use vulnerable Foxit PDF Reader, Foxit PDF Editor, or older Foxit-branded PDF products on Windows.
2. Create a malicious PDF or malicious web page that triggers the vulnerable Annotation object handling path.
3. Deliver the malicious content through phishing, file sharing, a web link, a compromised site, or another user-driven channel.
4. Convince the user to open the PDF or visit the malicious page.
5. Trigger memory corruption in the Foxit process and execute attacker-controlled code as the current user.
6. Spawn or inject a secondary payload, such as a downloader, remote access trojan, credential stealer, or script interpreter.
7. Establish persistence, collect credentials or files, and perform lateral movement if additional access is available.

How to detect it

To determine whether this vulnerability was already exploited, hunt for user-driven Foxit activity followed by abnormal process behavior. Key log sources include EDR process telemetry, Windows Security event logs, Sysmon logs, application crash logs, email gateway attachment records, web proxy logs, DNS logs, endpoint file creation events, and Windows Error Reporting records. Look for Foxit PDF Reader/Foxit PDF Editor opening a recently downloaded or emailed PDF, then crashing or spawning unusual child processes such as cmd.exe, powershell.exe, wscript.exe, cscript.exe, mshta.exe, rundll32.exe, regsvr32.exe, certutil.exe, bitsadmin.exe, curl.exe, or wget.exe. Review outbound connections initiated directly by Foxit processes or by child processes shortly after a PDF was opened. Correlate suspicious PDFs from email/web download telemetry with endpoint execution, quarantine events, and any files dropped in user-writable paths such as Downloads, AppData, Temp, or Startup folders. Also investigate Foxit application faults, access-violation crashes, or repeated crashes followed by successful execution of other binaries, as exploitation attempts may be unreliable and can leave crash artifacts before successful code execution.

• FoxitPDFReader.exe
• FoxitReader.exe
• FoxitPhantomPDF.exe
• FoxitPDFEditor.exe
• FoxitPDFReader.exe -> cmd.exe
• FoxitPDFReader.exe -> powershell.exe
• FoxitPDFReader.exe -> wscript.exe
• FoxitPDFReader.exe -> cscript.exe
• FoxitPDFReader.exe -> mshta.exe
• FoxitPDFReader.exe -> rundll32.exe
• FoxitPDFReader.exe -> regsvr32.exe
• FoxitPDFReader.exe -> certutil.exe
• FoxitPDFReader.exe -> bitsadmin.exe
• FoxitReader.exe -> cmd.exe
• FoxitReader.exe -> powershell.exe
• FoxitReader.exe -> wscript.exe
• FoxitReader.exe -> cscript.exe
• FoxitReader.exe -> mshta.exe
• FoxitReader.exe -> rundll32.exe
• FoxitReader.exe -> regsvr32.exe
• FoxitReader.exe -> certutil.exe
• FoxitReader.exe -> bitsadmin.exe
• FoxitPDFEditor.exe -> cmd.exe
• FoxitPDFEditor.exe -> powershell.exe
• FoxitPDFEditor.exe -> mshta.exe
• FoxitPDFEditor.exe -> rundll32.exe
• %APPDATA%\Microsoft\Windows\Start Menu\Programs\Startup\
• %TEMP%\
• %LOCALAPPDATA%\Temp\
• Windows Error Reporting
• Application Error
• Exception code: 0xc0000005

What your security team should do

• Prioritize patching or upgrading Foxit PDF Reader/Foxit PDF Editor on endpoints because the EPSS score is very high and exploitation requires only user interaction.
• Inventory all Foxit PDF installations, including legacy Foxit PhantomPDF/Foxit PDF Editor deployments, and remove unsupported versions.
• Harden email and web gateways to detonate or block suspicious PDFs, especially unsolicited attachments and PDFs with abnormal object structures.
• Create EDR/SIEM detections for Foxit processes spawning command shells, scripting engines, download utilities, archive tools, or unexpected network connections.
• Restrict child-process creation from PDF reader applications using EDR, application control, Microsoft Defender Attack Surface Reduction rules, or equivalent controls.
• Review recent endpoint telemetry for Foxit crashes followed by process creation, file writes to user-writable directories, or outbound network activity.
• Educate users to treat unexpected PDFs and document links as high risk, especially when received through email or messaging platforms.
• If suspicious activity is found, isolate affected endpoints, preserve the malicious PDF and memory/process telemetry, reset exposed credentials, and hunt for secondary payloads.