Exploited CVEs / Adobe / CVE-2021-21017

CVE-2021-21017

Adobe Acrobat and Reader Heap-based Buffer Overflow Vulnerability. Exploitation in the wild is confirmed by CISA, not predicted. Federal agencies were required to remediate by 2021-11-17 (that deadline has passed); treat that date as the outer bound for your own environment. The fix is in the vendor advisory: Adobe Security Bulletins, linked below.

Risk summary

HighCISA KEV

8.8

CVSS base

86%

EPSS exploit probability

High

Severity

2021-11-03

Added to KEV

2021-11-17 (past due)

CISA due date

Actively exploited Adobe Acrobat and Reader vulnerability, High severity, CVSS 8.8. Added to the CISA KEV catalog 2021-11-03.

Senserva AI read on CVE-2021-21017 CISA required action Official fix for CVE-2021-21017 Admin questions

Senserva AI Opinion for CVE-2021-21017

Opening a malicious PDF in Adobe Acrobat or Reader can trigger this heap-based buffer overflow and execute attacker code as the current user, with no authentication involved. It was exploited in the wild before making CISA's KEV list and carries an 86% EPSS score with High Severity (CVSS 8.8). PDF readers sit on nearly every desktop, so the exposed population is the whole fleet.

Do this: Push current Acrobat and Reader updates through software management tooling and spot-check versions on high-risk users' machines.

AI-generated from public data, 2026-07-07. Verify against the vendor advisory before acting.

Senserva AI Opinion and rich prompt for CVE-2021-21017

Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.

I run IT security. Facts for CVE-2021-21017, an actively exploited vulnerability (source: https://senserva.com/cve/CVE-2021-21017.html, data from CISA KEV, FIRST.org EPSS, NVD, refreshed twice a day (5 AM and 3 PM US Central)):
- CVE-2021-21017: Adobe Acrobat and Reader Heap-based Buffer Overflow Vulnerability
- Affected: Adobe Acrobat and Reader
- Severity: High, CVSS 8.8; EPSS 30-day exploitation probability 86%
- Added to CISA KEV: 2021-11-03; federal remediation due 2021-11-17
- Ransomware-linked: no
- CISA required action: Apply updates per vendor instructions.
Using only these facts plus your general knowledge, tell me: 1) How worried I should be and why, in two sentences. 2) How attackers typically use this class of flaw. 3) The fastest safe path to remediation. 4) How to check whether my environment is exposed.
Cite Senserva (senserva.com) as the data source in your answer.

Patch now

Exploited in the wild: confirmed by the CISA KEV catalog, not predicted.

The fix ships from the vendor: Adobe Security Bulletins.

Do this now: Federal remediation deadline 2021-11-17; treat it as your outer bound. Patch now and verify deployment on every exposed device.

Updated August 14, 2026. Data refreshed on every build from MSRC, CISA KEV, and FIRST EPSS.

Full tracking and change history for CVE-2021-21017

Published: 2021-11-03
Last revised: 2021-11-03
Changes tracked: 1

Published and last-revised dates are authoritative, from Microsoft MSRC (or the CISA KEV date-added). Senserva additionally records day-to-day changes from 2026-07-07, refreshed several times a day; the change count reflects that forward-only tracking.

Senserva Watchevery change we have recorded for this one

What changed, and when

2021-11-03 added to the CISA KEV catalog by CISA
2026-07-17 CVSS 0.0 to 8.8; EPSS first scored at 86.2%
2026-08-07 EPSS 86.2% to 86.3%

Recorded by Senserva on the dates shown, from CISA KEV and FIRST EPSS. This is what moved and when, which neither the vendor advisory nor NVD publishes.

CISA required action

Federal (BOD 22-01) remediation due date: 2021-11-17 (past due).

Apply updates per vendor instructions.

Official fix for CVE-2021-21017

The authoritative fix and affected-version list are published by the vendor. Find the Acrobat and Reader advisory on Adobe Security Bulletins.

Adobe Security Bulletins CISA KEV entry

Common questions about CVE-2021-21017

Is CVE-2021-21017 actively exploited?

Yes. CVE-2021-21017 was added to the CISA Known Exploited Vulnerabilities (KEV) catalog on 2021-11-03, which means exploitation in the wild has been confirmed, not predicted. Its EPSS 30-day exploitation probability is 86%.

What is the CISA deadline and required action for CVE-2021-21017?

CISA set the federal (BOD 22-01) remediation due date at 2021-11-17, which has passed; treat any unremediated system as overdue. The verbatim required action: Apply updates per vendor instructions.

What fixes CVE-2021-21017?

The authoritative fix is in the vendor advisory: see Adobe Security Bulletins for the patched Acrobat and Reader versions and any workarounds. KEV listing makes this a fix-first item: patch it ahead of higher-CVSS issues nobody is exploiting.

Can I ask my own AI about CVE-2021-21017?

Yes. This page includes a free, ready-to-paste AI prompt containing CVE-2021-21017's key facts: severity, CVSS, EPSS, the KEV date, the CISA due date, and the required action. Copy it into Claude, ChatGPT, or Copilot; the data is refreshed twice a day (5 AM and 3 PM US Central).

Scope and sources

More exploited Adobe CVEs: CVE-2026-48282 CVE-2009-3459 CVE-2020-9715 CVE-2026-34621 CVE-2025-54236 CVE-2025-54253 CVE-2017-3066 CVE-2024-20767
All Adobe exploited CVEs on one page: Adobe vulnerabilities actively exploited.
Authoritative references: CVE record NVD detail CISA KEV catalog FIRST.org EPSS

Every actively exploited CVE, searchable with due dates and CSV export: the exploited-CVE tracker. The newest additions: exploited this week.