Security readout for executives and security teams
Plain-English summary
This flaw lets a malicious PDF trigger code execution in Adobe Acrobat Reader as the signed-in user. It is high risk because opening one crafted file is enough, but the sources do not show active exploitation or CISA KEV listing.
Executive priority
Treat as a high-priority endpoint patching item. It can lead to full user-context compromise from a malicious document, but urgency is lower than known-exploited vulnerabilities absent KEV or cited exploitation evidence.
Technical view
CVE-2021-21028 is a CWE-416 use-after-free in Acrobat Reader DC affecting 2020.013.20074 and earlier, 2020.001.30018 and earlier, and 2017.011.30188 and earlier. CVSS 3.0 is 8.8 with network attack vector, low complexity, no privileges required, and required user interaction.
Likely exposure
Exposure is most likely on endpoints running affected Acrobat Reader DC versions, especially where users open PDFs from email, web downloads, file shares, or customer submissions.
Exploitation context
Exploitation requires user interaction: a victim must open a malicious file. The bundle marks KEV as false and provides no cited evidence of active exploitation, so active exploitation should not be assumed.
Researcher notes
The source bundle identifies the bug class, affected version thresholds, CVSS vector, and user-interaction requirement. It does not include exploit details, fixed build numbers, or telemetry confirming exploitation; rely on Adobe APSB21-09 for exact remediation guidance.
Mitigation direction
- Inventory Acrobat Reader DC versions across endpoints.
- Review Adobe APSB21-09 and apply the vendor-recommended security update.
- Prioritize users who routinely handle external PDFs.
- Limit opening untrusted PDFs until affected systems are updated.
- Ensure affected users do not operate with unnecessary privileges.
Validation and detection
- Confirm installed Reader versions are not within the affected version ranges.
- Check patch management records against Adobe APSB21-09 guidance.
- Validate high-risk endpoint groups are included in update coverage.
- Review security telemetry for suspicious Reader activity around untrusted PDFs.
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-2021-21028 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
- 8.8 (3.0)
- Known Exploited
- No
- Published
Vector: CVSS:3.0/AV:N/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:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H2.85.9Primary CVE scoreVulnerability scoring details
Base CVSS 3.0 score
8.8HighVector: CVSS:3.0/AV:N/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.