CVE-2021-21017
Adobe Acrobat and Reader Heap-based Buffer Overflow Vulnerability
Description
CVE-2021-21017 is a high-severity heap-based buffer overflow vulnerability in Adobe Acrobat and Reader. The flaw allows an unauthenticated attacker to achieve arbitrary code execution in the context of the current user by delivering a specially crafted PDF document. Exploitation requires user interaction, as the victim must open the malicious file. CISA has added CVE-2021-21017 to its Known Exploited Vulnerabilities catalog, confirming active exploitation in the wild. With an EPSS score of 90.64% (99.6th percentile), this Adobe Acrobat Reader vulnerability has a near-certain probability of exploitation activity.
KEV Information
CVSS Score
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:HOpen in CalculatorCWEs
Affected Products
| Vendor | Product | Version |
|---|---|---|
| adobe | acrobat | >= 17.0, <= 17.011.30188; >= 20.0, <= 20.001.30018 |
| adobe | acrobat dc | <= 20.013.20074 |
| adobe | acrobat reader | >= 17.0, <= 17.011.30188; >= 20.0, <= 20.001.300183 |
| adobe | acrobat reader dc | <= 20.013.20074 |
References
- https://helpx.adobe.com/security/products/acrobat/apsb21-09.html(Vendor Advisory)
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-21017(Third Party Advisory, US Government Resource)
Weakness Type
CWE-122: Heap-based Buffer Overflow
Heap-based Buffer Overflow occurs when a program writes more data to a heap-allocated buffer than its allocated size, corrupting adjacent heap structures. In Adobe Acrobat and Reader, this manifests when parsing specially crafted PDF content that causes the application to write beyond heap buffer boundaries, enabling attackers to corrupt memory and achieve code execution.
Learn more: CWE-122 — Heap-based Buffer Overflow
CWE-787: Out-of-bounds Write
Out-of-bounds Write occurs when software writes data to a memory location outside the intended boundaries of a buffer. In the context of this Adobe Acrobat vulnerability, the out-of-bounds write allows attackers to corrupt adjacent memory through malformed PDF content, potentially overwriting control flow information to redirect execution.
Learn more: CWE-787 — Out-of-bounds Write
Impact Analysis
CVE-2021-21017 carries a CVSS 3.1 score of 8.8 (HIGH), indicating a severe threat to organizations running unpatched Adobe Acrobat and Reader installations. The vulnerability is exploitable over the network with low attack complexity and requires no authentication, though user interaction is needed in that a victim must open a malicious PDF file. Successful exploitation results in high impact to confidentiality, integrity, and availability, allowing the attacker to execute arbitrary code with the privileges of the current user. The EPSS score of 90.64% (99.6th percentile) places this vulnerability among the most likely to be actively exploited, indicating widespread weaponization. Organizations with users who regularly open PDF documents from untrusted sources face significant risk of compromise through targeted phishing campaigns leveraging this vulnerability.
Exploit Maturity
CVE-2021-21017 is confirmed as actively exploited through its inclusion in the CISA Known Exploited Vulnerabilities catalog. The EPSS score of 90.64% (99.6th percentile) indicates near-certain exploitation activity, making this one of the most actively targeted vulnerabilities in the Adobe Acrobat and Reader product line. Federal agencies were required to remediate this vulnerability by 2021-11-17 per CISA's binding operational directive. The high EPSS score and confirmed active exploitation indicate that reliable exploit techniques exist, likely delivered through malicious PDF documents sent via phishing emails or hosted on compromised websites.
Remediation
- Apply vendor patches immediately as mandated by CISA KEV: Apply updates per vendor instructions. Update Adobe Acrobat DC and Reader DC to versions newer than 2020.013.20074, Acrobat 2020 and Reader 2020 past 2020.001.30018, and Acrobat 2017 and Reader 2017 past 2017.011.30188.
- Verify that all instances of affected Adobe products across the organization have been updated. Use software inventory tools to identify unpatched installations, including standalone and browser plugin versions of Acrobat Reader.
- Enable Protected Mode and Enhanced Security in Adobe Acrobat and Reader to limit the impact of exploitation. Configure the application to block JavaScript execution in PDFs and disable embedded file attachments from untrusted sources.
- Implement email gateway filtering to scan and sandbox PDF attachments before delivery to end users. Deploy endpoint detection and response (EDR) solutions to monitor for anomalous process behavior associated with PDF exploitation, such as Acrobat Reader spawning child processes.
- Educate users about the risks of opening PDF documents from unknown or untrusted sources, and establish organizational policies for handling external PDF files through sandboxed viewing environments.
Technical Details
CVE-2021-21017 is a heap-based buffer overflow vulnerability affecting Adobe Acrobat Reader DC versions 2020.013.20074 and earlier, 2020.001.30018 and earlier, and 2017.011.30188 and earlier. The vulnerability is classified under both CWE-122 (Heap-based Buffer Overflow) and CWE-787 (Out-of-bounds Write), indicating that specially crafted PDF content triggers an out-of-bounds write operation on a heap-allocated buffer during document parsing. The CVSS vector (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H) reflects that while the attack is network-delivered and requires no privileges, user interaction is needed to open the malicious document. Once triggered, the heap corruption can be leveraged through techniques such as heap spraying or heap feng shui to achieve reliable arbitrary code execution in the context of the current user, bypassing security boundaries within the application.
Frequently Asked Questions
Is CVE-2021-21017 being actively exploited?
Yes. CVE-2021-21017 is listed in the CISA Known Exploited Vulnerabilities catalog, confirming active exploitation in the wild. The EPSS score of 90.64% (99.6th percentile) indicates near-certain exploitation, making this one of the most targeted Adobe Acrobat and Reader vulnerabilities.
What products are affected by CVE-2021-21017?
CVE-2021-21017 affects Adobe Acrobat and Reader across multiple product tracks. Specifically: Adobe Acrobat DC and Acrobat Reader DC versions 20.013.20074 and earlier, Adobe Acrobat 2020 and Reader 2020 versions 20.001.30018 and earlier, and Adobe Acrobat 2017 and Reader 2017 versions 17.011.30188 and earlier.
How do I fix CVE-2021-21017?
Apply the security updates from Adobe Security Bulletin APSB21-09. Update all Adobe Acrobat and Reader installations to the latest available versions. Additionally, enable Protected Mode and Enhanced Security features, and implement email filtering for PDF attachments.
How severe is CVE-2021-21017?
CVE-2021-21017 has a CVSS 3.1 score of 8.8 (HIGH). With an EPSS score at the 99.6th percentile and confirmed active exploitation, this is a high-priority vulnerability that requires immediate remediation for any organization using Adobe Acrobat or Reader.
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