CVE-2021-21017
CVE-2021-21017 is a security flaw in Adobe Acrobat and Reader, Adobe’s software for opening and editing PDFs. Attackers can run their own code on Adobe Acrobat and Reader from anywhere, without logging in. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 3 Nov 2021.[1]
Adobe Acrobat and Reader is in things households have, such as phones, computers, browsers and everyday apps, so the fix matters at home as much as at work. Adobe has published a fix or advice.
Description
CISA calls it a heap-based buffer overflow in Adobe Acrobat and Reader. In its words: “Acrobat Acrobat and Reader contain a heap-based buffer overflow vulnerability that could allow an unauthenticated attacker to achieve code execution in the context of the current user.”[1]
The CVE record adds: “Acrobat Reader DC versions versions 2020.013.20074 (and earlier), 2020.001.30018 (and earlier) and 2017.011.30188 (and earlier) are affected by a heap-based buffer overflow vulnerability. An unauthenticated attacker could leverage this vulnerability to achieve arbitrary code execution in the context of the current user. Exploitation of this issue requires user interaction in that a victim must open a malicious file.”[2]
The underlying weakness is heap buffer overflow (CWE-122), in MITRE’s list of common software weaknesses.
It scores 8.8 out of 10 on the Common Vulnerability Scoring System, which rates it high, scored by the organisation that published the CVE using CVSS 3.1.[2]
Affected versions
NVD’s record doesn’t list version ranges for it; Adobe’s advisory has them[3]. CISA names the product as Adobe Acrobat and Reader.
Every device with Adobe Acrobat and Reader needs the update.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 3 Nov 2021, 265 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 17 Nov 2021 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 86%. That’s in the top 1% of the flaws it scores.[4]
CISA assesses it as not automatable: each attack takes some work.
CISA doesn’t say who was attacked, or how.
Fix
Adobe has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions.”[1]
- Update Adobe Acrobat and Reader now.Install the latest update from Adobe on every device that has it, and restart when asked. Then turn on automatic updates, so the next fix arrives on its own.
- If there’s no update, stop using it.CISA’s advice when a flaw being used in attacks has no fix is to stop using the product until it does.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 8.8.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 86%.[4] |
See also
Other Adobe Acrobat and Reader flaws
- CVE-2009-3459: heap-based buffer overflow ·
- CVE-2026-34621: prototype pollution ·
- CVE-2023-21608: use-after-free ·
- CVE-2023-26369: out-of-bounds write ·
- CVE-2018-4990: double free ·
- CVE-2010-2883: stack-based buffer overflow ·
Other Adobe flaws
- CVE-2026-48356, Adobe Commerce and Magento: unrestricted upload of file with dangerous type ·
- CVE-2026-71362, Adobe Commerce and Magento: incorrect authorization ·
- CVE-2026-75650, Adobe Commerce and Magento: improper neutralization of special elements used in a template engine ·
- CVE-2026-48282, Adobe ColdFusion: path traversal ·
- CVE-2020-9715, Adobe Acrobat: use-after-free ·
Other heap buffer overflow flaws
- CVE-2026-94127, F5 BIG-IP APM: heap-based buffer overflow ·
- CVE-2025-25249, Fortinet products: heap-based buffer overflow ·
- CVE-2026-85880, Microsoft Windows: heap-based buffer overflow ·
- CVE-2025-32706, Microsoft Windows: common log file system (CLFS) driver heap-based buffer overflow ·
- CVE-2025-24993, Microsoft Windows: NTFS heap-based buffer overflow ·
Added the same day
- CVE-2021-42258, BQE BillQuick Web Suite: SQL injection ·
- CVE-2021-42013, Apache HTTP Server: path traversal ·
- CVE-2021-41773, Apache HTTP Server: path traversal ·
- CVE-2021-40539, Zoho ManageEngine: aDSelfService plus authentication bypass ·
- CVE-2021-40444, Microsoft MSHTML: remote code execution ·
Every Adobe flaw on CISA’s list · Everything added in November 2021
References
- ^ Cybersecurity and Infrastructure Security Agency. “Adobe Acrobat and Reader Heap-based Buffer Overflow Vulnerability”. Known Exploited Vulnerabilities Catalog, added 3 Nov 2021.
- ^ National Institute of Standards and Technology. “CVE-2021-21017 Detail”. National Vulnerability Database, published 11 Feb 2021.
- ^ Adobe. Advisories and fixes: helpx.adobe.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.863, percentile 99.7, 31 Oct 2026.
- CVE Program. “CVE-2021-21017”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.