Security readout for executives and security teams
Plain-English summary
This Adobe Acrobat Reader DC flaw can expose information from a victim system's memory when a user opens a malicious PDF. The main business risk is that leaked memory may help an attacker bypass defenses such as ASLR during a broader attack chain.
Executive priority
Treat this as a moderate endpoint patching priority. It is not listed as actively exploited in the provided evidence, but PDF-based user interaction is realistic and the flaw may support more serious attack chains.
Technical view
CVE-2021-21042 is a CWE-125 out-of-bounds read in Adobe Acrobat Reader DC. Affected builds include 2020.013.20074 and earlier, 2020.001.30018 and earlier, and 2017.011.30188 and earlier. Impact is confidentiality only, with CVSS 6.5 and required user interaction.
Likely exposure
Exposure is likely where older Acrobat Reader DC versions remain installed on user workstations, especially systems that routinely receive PDFs from external parties. Server exposure is not indicated by the provided sources.
Exploitation context
The provided sources do not show active exploitation or CISA KEV listing. Exploitation requires convincing a user to open a malicious file, and the likely value is information disclosure or defense bypass support in a larger exploit chain.
Researcher notes
The vulnerability is an out-of-bounds read with high confidentiality impact and no integrity or availability impact in the CVSS vector. The source bundle does not provide exploit details, fixed build numbers, or telemetry indicating exploitation in the wild.
Mitigation direction
- Update Acrobat Reader DC beyond the affected builds using Adobe guidance.
- Review Adobe APSB21-09 for exact fixed versions and deployment instructions.
- Prioritize endpoints that process external or untrusted PDFs.
- Use email and web controls to reduce delivery of suspicious PDF files.
Validation and detection
- Inventory Acrobat Reader DC versions across managed endpoints.
- Flag builds at or below the listed affected versions.
- Confirm patch status against Adobe APSB21-09.
- Review security telemetry for suspicious PDF open events if available.
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-125: Exact CWE lookup
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Open ATT&CK lookupCVE-2021-21042 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
- 6.5 (3.0)
- Known Exploited
- No
- Published
Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N
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:N/A:N2.83.6Primary CVE scoreVulnerability scoring details
Base CVSS 3.0 score
6.5MediumVector: CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N
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 Read
Out-of-bounds Read represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.