Evidence used
- CISA confirms exploitation in the wild.
- EPSS is 51.85% for the current model date.
BlackTreeCVE IntelligenceFull vulnerability report · 2021
CVE-2021-28550High confidence
Adobe Acrobat and Reader Use-After-Free Vulnerability
Adobe · Acrobat and Reader
Official source article: Adobe APSB21-29 ↗. Check the applicable product and release in the original source.
8.8HighCVSS 3.1
Mitigate now
Mitigation availableCISA confirms exploitation in the wild and lists 2021-11-17 as the remediation due date.
R
Open reassessment dashboard →Operational reassessment
Published severity in operational context
Published severityHigh→Operational priority:Critical, raised one band.upgradedsince 3 Nov 2021
Compensating controls
- Apply the linked authoritative mitigation while planning the permanent fix.
- Restrict the affected network interface to trusted sources where business-safe.
- Increase monitoring for the attack path and post-exploitation behaviour described in the report.
Verification
- Confirm that the asset runs Adobe Acrobat and Reader and falls inside the recorded affected range.
- Recheck the vendor advisory before scheduling a change because no verified fixed version is currently retained.
- Validate exposure, authentication requirements and compensating controls in the actual environment.
- Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Distribution package intelligence
3 package statesRelease-specific package status
Alpine findings are scoped to the named distribution, release and source package. An absent finding does not mean a package is unaffected.
Repository candidate not checked
A published vendor fix does not prove that a matching update is enabled and installable on a particular asset. Confirm the local package candidate before scheduling remediation.
| Distribution release | Source package | Vendor state | Fixed version | Evidence |
|---|---|---|---|---|
| Alpine v3.23v3.23 · community | tor | Vendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected. | 0.4.6.5-r0 | Alpine Security Database ↗Source updated 11 Sep 2026 |
| Alpine v3.22v3.22 · community | tor | Vendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected. | 0.4.6.5-r0 | Alpine Security Database ↗Source updated 11 Sep 2026 |
| Alpine v3.21v3.21 · community | tor | Vendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected. | 0.4.6.5-r0 | Alpine Security Database ↗Source updated 19 Aug 2026 |
CISA confirms exploitation in the wild and lists 2021-11-17 as the remediation due date.
Mitigation available01
What, why and how
Adobe Acrobat and Reader contains a use-after-free vulnerability that could allow an unauthenticated attacker to achieve code execution in the context of the current user.
What
Adobe Acrobat and Reader contains a use-after-free vulnerability that could allow an unauthenticated attacker to achieve code execution in the context of the current user.
Why
The program can continue using memory after it has been released, producing unsafe and attacker-influenceable behaviour.
How
An attacker operating through a network path may attempt exploitation without authentication after a user interaction. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.
What
Adobe Acrobat and Reader contains a use-after-free vulnerability that could allow an unauthenticated attacker to achieve code execution in the context of the current user.
Why
The program can continue using memory after it has been released, producing unsafe and attacker-influenceable behaviour.
How
An attacker operating through a network path may attempt exploitation without authentication after a user interaction. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.
02
Exploit reality and attack path
CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.
Observed exploitation Confirmed in the wild
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.CISA added this CVE to its Known Exploited Vulnerabilities catalogue on 2021-11-03.
Public PoC / exploit material None recorded
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.No exploit-tagged reference or CISA SSVC proof-of-concept state is currently recorded. Research may still exist outside the structured feeds.
- Likely attack path
- a network path → Use After Free → cause the confidentiality, integrity or availability impact described by the vendor
- Attack surface
- Network
- Privileges required
- None: unauthenticated exploitation is possible
- User interaction
- Required interaction required
- Attack complexity
- Low: no specialised conditions are recorded
- Security boundary
- Unchanged: impact remains within the vulnerable component's security authority
- Weakness
?
CWE means Common Weakness Enumeration: a standard category for the underlying weakness. - CWE-416 ↗
CWE-416: Use After Free. The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.
- CVSS vector
?
CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity. CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:HCommon Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.
AVNetworkAttack vector: The vulnerable component can be reached over a network.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.PRNonePrivileges required: The attacker does not need an account or existing privileges.UIRequiredUser interaction: Another user must perform an action for exploitation to succeed.SUnchangedScope: The security impact remains within the vulnerable component's authority.CHighConfidentiality impact: A successful attack can cause a major loss.IHighIntegrity impact: A successful attack can cause a major loss.AHighAvailability impact: A successful attack can cause a major loss.- Post-exploitation / living off the land
- No specific living-off-the-land technique is confirmed in the structured sources. Monitor normal administration tools for activity inconsistent with the affected service's baseline.
03
Patch and workaround
Operational remediation based on structured source evidence.
- Status
?
Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change. - Mitigation available
- Action
- The Cyber Centre encourages users and administrators to review the provided web link and apply the necessary updates.
- Workaround
- No verified workaround is recorded. If business-safe, reduce exposure to the affected interface and allow only trusted sources until authoritative guidance is available.
04
Evidence and provenance
Published 2 Sept 2021 · Last source change 21 Oct 2025, 23:25 UTC · CWE-416 · Use After Free
CVE recordCVE.org · 5.1
CVSS sourceNIST NVD
EPSS source FIRST · tracked since 2026-08-14
?
The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.European sourceENISA EUVD · EUVD-2021-15226
Product sourceCISA KEV
Remediation sourceCVE/CNA references
CWE sourceCNA
NVD statusNVD enriched
Core structured fields are present and their contributing authorities are shown above.
Material change intelligence
View recent updates ↗What changed after publication
- Vendor guidanceAuthoritative vendor guidance changed: removed remediation: cyber.gc.ca/adobe-security-advisory-40.helpx.adobe.com ↗
- Before
- remediation: cyber.gc.ca/adobe-security-advisory-40 · vendor advisory: helpx.adobe.com/apsb21-29.html
- After
- vendor advisory: helpx.adobe.com/apsb21-29.html
- Remediation statusRemediation status changed from Mitigation available to Awaiting fix.helpx.adobe.com ↗
- Before
- Mitigation available
- After
- Awaiting fix
- Affected versionsThe structured affected or fixed version information changed.CNA ↗
- Before
- unspecified ≤ 2020.001.30020; unspecified ≤ 2021.001.20150; unspecified ≤ 2017.011.30194; unspecified ≤ None · Fixed: No fixed version is explicitly recorded in the structured CVE data.
- After
- Acrobat Reader: unspecified ≤ 2020.001.30020, unspecified ≤ 2021.001.20150, unspecified ≤ 2017.011.30194, unspecified ≤ None · Fixed: No fixed version is explicitly recorded in the structured CVE data.
- Remediation statusRemediation status changed from Awaiting fix to Mitigation available.Canadian Centre for Cyber Security ↗
- Before
- Awaiting fix
- After
- Mitigation available
CVE published
Added to CISA KEV
Remediation guidance recorded
Technical terms and abbreviations used in this report
- CVE
- Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
- CVSS
- Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
- EPSS
- Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
- CWE
- Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
- CNA
- CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
- CISA ADP
- Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
- NVD
- National Vulnerability Database: NIST's enrichment service for CVE records.
- CERT / CSIRT
- A computer security incident response team that publishes warnings or coordinates incident response.
- PoC
- Proof of concept: public material that demonstrates or helps reproduce exploitation.
- CSAF
- Common Security Advisory Framework: a machine-readable format for security advisories.
- LoTL
- Living off the land: abuse of legitimate tools or system functions during an attack.