Security readout for executives and security teams
Plain-English summary
This flaw can let a malicious PDF run code as the user who opens it in affected Adobe Acrobat Reader DC versions. The main business risk is endpoint compromise through document handling, especially where users receive PDFs from outside the organization.
Executive priority
Treat as a high-priority workstation patching issue, not a platform-wide emergency. The impact is serious because successful exploitation can run code, but the attack requires user interaction and no active exploitation evidence is provided.
Technical view
CVE-2021-39838 is a CWE-416 use-after-free in Acrobat Reader DC AcroForm buttonGetCaption processing. A crafted malicious file can trigger arbitrary code execution in the current user context. Exploitation requires user interaction: the victim must open the file.
Likely exposure
Exposure is likely on endpoints running Acrobat Reader DC 2021.005.20060 or earlier, 2020.004.30006 or earlier, or 2017.011.30199 or earlier. Risk is higher for users who routinely open external PDFs.
Exploitation context
The provided data does not show CISA KEV listing or active exploitation. Exploitation is not network-wormable from the cited record; it depends on persuading a user to open a malicious file.
Researcher notes
Key constraints are user-assisted execution and current-user privilege context. The source bundle identifies the vulnerable action and affected version ceilings but does not provide exploit details. Avoid assuming affected Adobe products beyond Acrobat Reader DC.
Mitigation direction
- Inventory Acrobat Reader DC versions across managed endpoints.
- Review Adobe APSB21-55 for supported update or remediation guidance.
- Prioritize updating systems at or below the listed affected versions.
- Limit handling of untrusted PDFs on unpatched systems.
- Use vendor-approved mitigations only where Adobe documents them.
Validation and detection
- Confirm installed Reader DC versions exceed affected version thresholds.
- Verify the Adobe advisory has been reviewed for applicable branches.
- Check software inventory for unmanaged or stale Reader installations.
- Confirm endpoint controls cover malicious PDF delivery paths.
- Document any exceptions with compensating controls and owner.
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-2021-39838 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.