Security readout for executives and security teams
Plain-English summary
A malicious PDF-like file could make vulnerable Adobe Acrobat Reader DC run code as the logged-in user. The victim must open the file, so phishing and document-based delivery are the main business concern. The source bundle does not show confirmed active exploitation.
Executive priority
Prioritize remediation for user workstations that routinely receive external documents. The issue has high confidentiality, integrity, and availability impact, but urgency is moderated by required user interaction and no KEV evidence in the provided sources.
Technical view
CVE-2021-39839 is a CWE-416 use-after-free in Acrobat Reader DC AcroForm getItem processing. Affected listed versions are 2021.005.20060 and earlier, 2020.004.30006 and earlier, and 2017.011.30199 and earlier. Successful exploitation may allow arbitrary code execution in the current user context. CVSS 3.0 score is 7.8.
Likely exposure
Exposure is most likely on workstations where users open external PDFs with affected Acrobat Reader DC versions. Systems with strict PDF sandboxing, patched Reader versions, or no Adobe Reader use have lower exposure, but the bundle does not provide environment-specific proof.
Exploitation context
The CVE requires user interaction: a victim must open a malicious file. The source bundle marks KEV as false and provides no cited evidence of active exploitation. Treat it as a high-impact document handling risk, not as confirmed exploited-in-the-wild.
Researcher notes
Useful validation centers on version exposure and document handling paths. The bundle does not include proof-of-concept details, exploit telemetry, or fixed-version mapping. Do not infer exploitation beyond malicious-file user interaction and arbitrary code execution in the current user context.
Mitigation direction
- Identify Adobe Acrobat Reader DC installations and versions across endpoints.
- Prioritize updating versions at or below the affected release lines.
- Check Adobe APSB21-55 for vendor-approved fixed versions and deployment guidance.
- Limit opening untrusted PDFs until affected clients are updated.
- Use endpoint controls to reduce impact of document-launched code execution.
Validation and detection
- Inventory Acrobat Reader DC versions against the affected version thresholds.
- Confirm patched endpoints no longer report affected Reader versions.
- Review email and web controls for untrusted PDF handling coverage.
- Check endpoint telemetry for suspicious Reader child processes or crashes.
- Document any exceptions with owner, compensating control, and remediation date.
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-2021-39839 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.