Adobe Acrobat and Reader Heap-based Buffer Overflow
CVE-2021-21017 is a medium-severity out-of-bounds write vulnerability in Acrobat Reader from Adobe, with a CVSS 2.0 base score of 6.8 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. Affected Acrobat Reader versions: 2020.013.20074 and earlier; 2020.001.30018 and earlier; 2017.011.30188 and earlier. The record does not name a fixed version, so check the vendor advisory. CISA added it to the Known Exploited Vulnerabilities catalogue on November 3, 2021, which means attacks have been observed in the wild.
Attackers are already exploiting this vulnerability, according to CISA.
What to do
- Check which version of Acrobat Reader you run against the affected versions below, and install the vendor's latest release.
- CISA's required action: Apply updates per vendor instructions.
- Read the advisory on helpx.adobe.com for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Adobe | Acrobat Reader | 2020.013.20074 and earlier; 2020.001.30018 and earlier; 2017.011.30188 and earlier | Not stated |
| Adobe | Acrobat | 17.0 through 17.011.30188; 20.0 through 20.001.30018 | Not stated |
| Adobe | Acrobat Dc | 20.013.20074 and earlier | Not stated |
| Adobe | Acrobat Reader Dc | 20.013.20074 and earlier | Not stated |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Access vector
- NetworkCan be exploited remotely over a network.
- Access complexity
- MediumSome conditions must be met.
- Authentication
- NoneNo login is needed.
- Confidentiality
- PartialSome data can be read.
- Integrity
- PartialSome data can be modified.
- Availability
- PartialAvailability is reduced.
Vector: AV:N/AC:M/Au:N/C:P/I:P/A:P. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 8.8 | High | CNA (adobe.com) |
| CVSS 2.0 | 6.8 | Medium | NVD |
Official description
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.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Out-of-bounds Write
The product writes data past the end, or before the beginning, of the intended buffer.
How developers prevent this class of flaw
- Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, many languages that perform their own memory management, such as Java and Perl, are not subject to buffer overflows. Other languages, such as Ada and C#, typically provide overflow protection, but the protection can be disabled by the programmer. Be wary that a language's interface to native code may still be subject to overflows, even if the language itself is theoretically safe.
- Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. Examples include the Safe C String Library (SafeStr) by Messier and Viega, and the Strsafe.h library from Microsoft. These libraries provide safer versions of overflow-prone string-handling functions.
- Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking. D3-SFCV (Stack Frame Canary Validation) from D3FEND discusses canary-based detection in detail.
Timeline
- CVE record published.
- Added to CISA's Known Exploited Vulnerabilities catalogue.
- Deadline for US federal agencies to remediate, set by CISA.
- CISA published its SSVC assessment (exploitation: active, automatable: no, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Third-party and government advisories
- cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-21017Third Party Advisory, US Government Resource
Other vulnerabilities in Acrobat Reader
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Recent Out-of-bounds Write vulnerabilities
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Google Chromium V8 Heap Buffer Overflow Exploited
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Google Chromium V8 Type Confusion Exploited
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