Security readout for executives and security teams
Plain-English summary
A malicious PDF can abuse a memory error in Adobe Acrobat Reader DC to run code as the person who opens it. The main business risk is compromise of employee workstations through document delivery, especially where staff regularly handle external PDFs.
Executive priority
Treat this as a high-priority workstation patching issue, not an internet-facing server emergency. The attack still needs user interaction, but successful exploitation can give an attacker the same access as the targeted user.
Technical view
CVE-2021-39837 is a CWE-416 use-after-free in AcroForm deleteItemAt processing. A crafted file can trigger arbitrary code execution in the current user context. The CVSS 3.0 score is 7.8, with low attack complexity but required user interaction.
Likely exposure
Exposure is likely on endpoints running Acrobat Reader DC 2021.005.20060 and earlier, 2020.004.30006 and earlier, or 2017.011.30199 and earlier. Risk is higher for users who open PDFs from email, web downloads, partners, or public submission workflows.
Exploitation context
The provided sources do not show CISA KEV listing or active exploitation. Exploitation requires a victim to open a malicious file, so phishing, document exchange, and drive-by download scenarios are the relevant defensive context.
Researcher notes
Focus validation on Reader DC version branches and PDF handling workflows. The source bundle identifies arbitrary code execution but does not provide exploit status, indicators, or detailed fixed-version data beyond the Adobe advisory reference.
Mitigation direction
- Review Adobe APSB21-55 and apply the applicable Acrobat Reader update.
- Prioritize patching users who process external or unsolicited PDFs.
- Reduce exposure by limiting automatic opening of downloaded PDF files.
- Use endpoint controls to detect suspicious child processes from Reader.
- Warn users against opening unexpected PDF attachments or links.
Validation and detection
- Inventory Acrobat Reader versions across managed endpoints.
- Flag versions at or below the affected release numbers.
- Confirm remediated hosts match Adobe APSB21-55 guidance.
- Review email and endpoint telemetry for suspicious PDF delivery.
- Check whether Reader launched unusual child processes.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-416: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupExecution behavior lookup
The CVE wording references code or command execution, so execution technique review may help defensive triage. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2021-39837 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- High
- CVSS
- 7.8 (3.0)
- Known Exploited
- No
- Published
Vector: CVSS:3.0/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.0/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H1.85.9Primary CVE scoreVulnerability scoring details
Base CVSS 3.0 score
7.8HighVector: CVSS:3.0/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
Source materials
- CVE List V5 sourceCVE List V5
- https://helpx.adobe.com/security/products/acrobat/apsb21-55.htmlCVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Use After Free
Use After Free represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.