Security readout for executives and security teams
Plain-English summary
CVE-2021-21086 is a high-severity Adobe Acrobat Reader flaw. A malicious document could corrupt memory in the CoolType library and run code as the signed-in user if opened. The main business risk is compromise through a user opening a hostile file, commonly delivered by email or download.
Executive priority
Treat this as a high-priority endpoint patching issue, especially for staff exposed to external files. It is not documented here as actively exploited, but successful exploitation could give an attacker code execution as the current user.
Technical view
The issue is a CWE-787 out-of-bounds write in Adobe Acrobat Reader DC CoolType handling. Affected builds 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 and required user interaction.
Likely exposure
Organizations are exposed where affected Adobe Acrobat Reader versions remain installed on endpoints and users can open untrusted PDF or document content. Exposure is highest on workstations handling external email attachments, downloads, or submitted documents.
Exploitation context
The supplied sources say exploitation requires user interaction: the victim must open a malicious file. CISA KEV status is false in the bundle, and no supplied source confirms active exploitation.
Researcher notes
Evidence identifies CoolType out-of-bounds write leading to arbitrary code execution with user interaction. The bundle does not provide exploit details, proof-of-concept status, or fixed version numbers beyond the Adobe bulletin reference, so remediation should follow APSB21-09.
Mitigation direction
- Inventory Adobe Acrobat Reader installations and versions across endpoints.
- Check Adobe APSB21-09 for vendor-approved update guidance.
- Prioritize remediation for users handling external documents or email attachments.
- Limit use of affected Reader versions until vendor guidance is applied.
- Remove vulnerable Reader installations where the software is not required.
Validation and detection
- Compare installed Reader versions against the affected version thresholds in the CVE.
- Confirm patched or remediated systems no longer report affected builds.
- Review endpoint coverage for users who process external PDF files.
- Verify security teams track APSB21-09 as the authoritative vendor reference.
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-787: 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-2021-21086 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.
Out-of-bounds Write
Out-of-bounds Write represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.