Security readout for executives and security teams
Plain-English summary
A malicious PDF or page could make affected Adobe Acrobat Reader DC run attacker-controlled code as the signed-in user. The victim must open a malicious file or visit a malicious page. The source bundle does not show CISA KEV listing or active exploitation evidence.
Executive priority
Treat as high priority for endpoint patching, especially in departments that handle external PDFs. The business risk is user-level code execution after a plausible phishing or web lure, but the provided evidence does not show confirmed active exploitation.
Technical view
CVE-2021-39840 is a CWE-416 use-after-free in Acrobat Reader DC AcroForm processing. Adobe lists affected Reader DC branches up to 2021.005.20060, 2020.004.30006, and 2017.011.30199. CVSS is 7.8 with user interaction required and high confidentiality, integrity, and availability impact.
Likely exposure
Organizations are exposed where users run affected Acrobat Reader DC versions and receive or open PDFs from email, web downloads, customer portals, or shared drives. Exposure is higher on unmanaged endpoints or systems with delayed Adobe patching.
Exploitation context
The bundle says exploitation requires user interaction: visiting a malicious page or opening a malicious file. It does not identify public exploitation, weaponized samples, or KEV status. Treat exploitation status as unconfirmed from the provided sources.
Researcher notes
Focus validation on version exposure and AcroForm PDF handling paths. The source bundle identifies the vulnerability class and affected Reader thresholds, but does not provide exploit details, indicators, or explicit fixed build numbers beyond the Adobe advisory reference.
Mitigation direction
- Inventory Acrobat Reader DC versions across endpoints.
- Review Adobe APSB21-55 for vendor-fixed Reader releases.
- Deploy Adobe-recommended updates to affected Reader DC branches.
- Prioritize users handling untrusted PDFs or external submissions.
- Use endpoint controls to limit untrusted PDF execution paths.
Validation and detection
- Confirm installed Reader versions are newer than affected branch thresholds.
- Verify patch deployment status in endpoint management tooling.
- Check PDF-handling systems for affected Reader DC usage.
- Review email and web controls for untrusted PDF delivery exposure.
- Document exceptions and track them to vendor guidance.
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-2021-39840 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.