Security readout for executives and security teams
Plain-English summary
This flaw lets a malicious PDF crash or corrupt Adobe Acrobat Reader DC in a way that may run attacker code as the logged-in user. It is not a remote network worm risk, but it matters wherever employees open PDFs from email, browsers, or shared drives.
Executive priority
Treat as a high-priority endpoint patching issue, especially for PDF-heavy business groups. It is serious because code execution is possible, but urgency is lower than internet-facing or confirmed exploited vulnerabilities.
Technical view
CVE-2021-39863 is a CWE-122 heap-based buffer overflow in Acrobat Reader DC PDF parsing. A crafted PDF can trigger arbitrary code execution with the current user's privileges. CVSS 3.1 is 7.8 high: local attack vector, low complexity, no privileges required, and required user interaction.
Likely exposure
Exposure is likely on workstations with Acrobat Reader DC 2021.005.20060 or earlier, 2020.004.30006 or earlier, or 2017.011.30199 or earlier. Risk is concentrated in users who handle external PDFs.
Exploitation context
The source bundle does not identify active exploitation, and KEV status is false. Exploitation requires convincing a user to open a malicious PDF; an unauthenticated attacker does not need an account on the target system.
Researcher notes
Evidence is limited to CVE data and Adobe's advisory reference. Do not assume impact beyond Acrobat Reader DC or claim exploitation in the wild from this bundle. Validation should focus on version detection and vendor advisory alignment.
Mitigation direction
- Check Adobe APSB21-55 and apply the vendor-recommended Acrobat Reader updates.
- Inventory endpoints for the affected Acrobat Reader DC version ranges.
- Prioritize users who frequently receive external PDFs.
- Limit opening unsolicited PDFs until affected clients are updated.
- Use least-privilege user accounts to reduce post-compromise impact.
Validation and detection
- Confirm installed Acrobat Reader DC versions against the affected ranges.
- Verify endpoints received the Adobe update referenced in APSB21-55.
- Review software inventory for unsupported Reader tracks.
- Check email and endpoint telemetry for suspicious PDF delivery attempts.
- Confirm PDF handling controls are active for high-risk users.
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
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CWE-122: Exact CWE lookup
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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-2021-39863 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 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/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:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H1.85.9Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
7.8HighVector: CVSS:3.1/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.
Heap-based Buffer Overflow
Heap-based Buffer Overflow represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.