Security readout for executives and security teams
Plain-English summary
This flaw lets a malicious PDF potentially run code as the person who opens it. The attacker does not need an account, but the victim must open a crafted file. Business risk is highest where affected Adobe Acrobat Reader versions remain on endpoints handling email attachments or downloaded PDFs.
Executive priority
Treat as a high-priority endpoint patching issue, not an internet-facing emergency. The business risk is user-driven compromise through malicious documents, especially in teams that frequently receive external PDFs.
Technical view
CVE-2021-21040 is a CWE-416 use-after-free in Adobe Acrobat Reader DC. Affected versions include 2020.013.20074 and earlier, 2020.001.30018 and earlier, and 2017.011.30188 and earlier. CVSS 3.1 is 7.8, with local attack vector, low complexity, no privileges required, and required user interaction.
Likely exposure
Exposure is likely on workstations with the listed Acrobat Reader DC versions or earlier installed. Systems used to open external PDFs from email, web downloads, document portals, or file shares have the most practical exposure.
Exploitation context
The source bundle does not show CISA KEV listing or other evidence of active exploitation. Exploitation requires user interaction: a victim must open a malicious file. Successful exploitation could execute code with the current user's privileges.
Researcher notes
Available evidence identifies the bug class, affected version thresholds, CVSS vector, and user-interaction requirement. The bundle does not provide exploit details, root-cause internals, or confirmed in-the-wild exploitation, so validation should focus on version exposure and vendor advisory alignment.
Mitigation direction
- Apply Adobe guidance from APSB21-09 for affected Acrobat Reader installations.
- Prioritize endpoints that process external PDFs or email attachments.
- Remove or upgrade Reader versions at or below the listed vulnerable versions.
- Limit opening PDFs from untrusted sources until remediation is complete.
Validation and detection
- Inventory installed Adobe Acrobat Reader versions across managed endpoints.
- Compare versions against 2020.013.20074, 2020.001.30018, and 2017.011.30188 thresholds.
- Confirm remediation status against Adobe APSB21-09 guidance.
- Review endpoint telemetry for suspicious PDF-open events if vulnerable versions were present.
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-2021-21040 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/apsb21-09.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.