Security readout for executives and security teams
Plain-English summary
This issue can crash Adobe Acrobat Reader DC when a user opens a malicious file. It does not indicate data theft or code execution in the provided sources, but it can interrupt work for the affected user and support nuisance or targeted disruption.
Executive priority
Treat this as a medium-priority endpoint hygiene issue. It is not presented as data compromise or remote takeover, but patching is still appropriate because PDF-based user interaction is common and disruption can affect business workflows.
Technical view
CVE-2021-39851 is a CWE-476 null pointer dereference in Acrobat Reader DC. The CVSS 3.1 score is 5.5 with local attack vector, no privileges required, required user interaction, unchanged scope, and high availability impact only.
Likely exposure
Exposure is primarily on endpoints running affected Acrobat Reader DC versions: 2021.005.20060 and earlier, 2020.004.30006 and earlier, or 2017.011.30199 and earlier. Risk is higher where users routinely receive or open PDF files from email, web downloads, or external parties.
Exploitation context
The provided sources say exploitation requires a victim to open a malicious file. The CVE is not listed as CISA KEV in the supplied bundle, and no cited source in the bundle states active exploitation.
Researcher notes
Evidence supports application denial-of-service only, in the current user context. The affected-version data is explicit, but the supplied bundle does not include exploit details, active exploitation evidence, or detailed mitigation text beyond the Adobe advisory reference.
Mitigation direction
- Check Adobe APSB21-55 for the vendor-supported update path.
- Update affected Acrobat Reader DC installations to non-affected supported versions.
- Prioritize users who handle external PDFs or email attachments.
- Use attachment filtering and user awareness to reduce malicious file exposure.
Validation and detection
- Inventory Acrobat Reader DC versions across managed endpoints.
- Flag versions at or below the affected release thresholds.
- Confirm remediation status against Adobe APSB21-55.
- Review endpoint reports for repeated Acrobat Reader crashes after PDF opens.
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-476: 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 lookupCVE-2021-39851 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
- Medium
- CVSS
- 5.5 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:N/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:N/I:N/A:H1.83.6Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
5.5MediumVector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:N/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.
NULL Pointer Dereference
NULL Pointer Dereference represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.