Evidence used
- No CISA KEV confirmation is currently recorded.
- The selected CVSS metric records a network-reachable, unauthenticated path with no user interaction.
- EPSS is 53.46% for the current model date.
BlackTreeCVE IntelligenceFull vulnerability report · 2021
CVE-2021-23017High confidence
n/a Nginx Web Server, Nginx Plus — Off-by-one Error
Unknown · Nginx Web Server, Nginx Plus
7.7HighCVSS 3.1
Patch only the product branches with a verified fix
Fix availability varies by productHigh technical severity; prioritise exposed affected systems while verifying vendor guidance. Verified remediation exists for at least one product or source, but 2 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.
R
Open reassessment dashboard →Operational reassessment
Published severity in operational context
Published severityHigh→Operational priority:Critical, raised one band.upgradedsince 5 Oct 2026
Compensating controls
- Validate the affected product branch and deploy the verified fixed release.
- Restrict the affected network interface to trusted sources where business-safe.
- Monitor vendor guidance and exploitation sources for a material change.
Verification
- Confirm that the asset runs Unknown Nginx Web Server, Nginx Plus and falls inside the recorded affected range.
- Verify the installed build against the product-specific fixed version after deployment.
- Validate exposure, authentication requirements and compensating controls in the actual environment.
- Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Remediation availability differs by product scope
Verified remediation exists for at least one product or source, but 2 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.
- SICK PSIRT: 1 affected or under-investigation product state without a fixed product state in the same current advisory
- SICK PSIRT: 1 affected or under-investigation product state without a fixed product state in the same current advisory
Distribution package intelligence
7 package statesRelease-specific package status
Alpine, Debian 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 · main | nginx | Vendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected. | 1.20.1-r0 | Alpine Security Database ↗Source updated 3 Oct 2026 |
| Alpine v3.22v3.22 · main | nginx | Vendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected. | 1.20.1-r0 | Alpine Security Database ↗Source updated 3 Oct 2026 |
| Alpine v3.21v3.21 · main | nginx | Vendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected. | 1.20.1-r0 | Alpine Security Database ↗Source updated 3 Oct 2026 |
| Debian trixietrixie · source | nginx | Vendor fix publishedDebian records a fixed source-package version for this release. | 1.18.0-6.1 | Debian Security Tracker ↗Source updated 5 Oct 2026 |
| Debian bookwormbookworm · source | nginx | Vendor fix publishedDebian records a fixed source-package version for this release. | 1.18.0-6.1 | Debian Security Tracker ↗Source updated 5 Oct 2026 |
| Debian forkyforky · source | nginx | Vendor fix publishedDebian records a fixed source-package version for this release. | 1.18.0-6.1 | Debian Security Tracker ↗Source updated 5 Oct 2026 |
| Debian sidsid · source | nginx | Vendor fix publishedDebian records a fixed source-package version for this release. | 1.18.0-6.1 | Debian Security Tracker ↗Source updated 5 Oct 2026 |
Direct vendor intelligence
2 currentAuthoritative vendor CSAF and VEX advisories
Structured product status and remediation from the issuing vendor. Product-state explanations are always visible; large lists can be searched or downloaded.
SCA-2025-0011 · CSAF 2.0 · revision 1.0.0 · finalSICK PSIRTVulnerabilities affecting Endress+Hauser SSG-E210GC
The vendor explicitly identifies these products as affected by this CVE.
- Endress+Hauser SSG-E210GC all Firmware versions
- Summary
- A security issue in nginx resolver was identified, which might allow an attacker who is able to forge UDP packets from the DNS server to cause 1-byte memory overwrite, resulting in worker process crash or potential other impact.
- Remediation
- Please make sure that only trusted entities have access to the device. Furthermore, you should apply the following General Security Measures when operating the product to mitigate the associated security risk. The collected resources "SICK Operating Guidelines" and "ICS-CERT recommended practices on Industrial Security" could help to implement the general security practices.
SCA-2025-0009 · CSAF 2.0 · revision 1 · finalSICK PSIRTVulnerabilities affecting SICK TDC-E210GC
The vendor explicitly identifies these products as affected by this CVE.
- SICK TDC-E210GC all Firmware versions
- Summary
- A security issue in nginx resolver was identified, which might allow an attacker who is able to forge UDP packets from the DNS server to cause 1-byte memory overwrite, resulting in worker process crash or potential other impact.
- Remediation
- Please make sure that only trusted entities have access to the device. Furthermore, you should apply the following General Security Measures when operating the product to mitigate the associated security risk. The collected resources "SICK Operating Guidelines" and "ICS-CERT recommended practices on Industrial Security" could help to implement the general security practices.
High technical severity; prioritise exposed affected systems while verifying vendor guidance. Verified remediation exists for at least one product or source, but 2 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.
Fix availability varies by product01
What, why and how
A security issue in nginx resolver was identified, which might allow an attacker who is able to forge UDP packets from the DNS server to cause 1-byte memory overwrite, resulting in worker process crash or potential other impact.
What
A security issue in nginx resolver was identified, which might allow an attacker who is able to forge UDP packets from the DNS server to cause 1-byte memory overwrite, resulting in worker process crash or potential other impact.
Why
A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value.
How
An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may disrupt the affected service.
What
A security issue in nginx resolver was identified, which might allow an attacker who is able to forge UDP packets from the DNS server to cause 1-byte memory overwrite, resulting in worker process crash or potential other impact.
Why
A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value.
How
An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may disrupt the affected service.
02
Exploit reality and attack path
CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.
Observed exploitation No confirmed evidence
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.No CISA KEV match was present at the last successful refresh. This means no confirmation from that source, not proof of no exploitation.
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 → Off-by-one Error → disrupt the affected service
- Attack surface
- Network
- Privileges required
- None: unauthenticated exploitation is possible
- User interaction
- None
- Attack complexity
- High: exploitation depends on specific conditions
- 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-193 ↗
CWE-193: Off-by-one Error. A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value.
- 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:H/PR:N/UI:N/S:U/C:H/I:H/A:LCommon 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.ACHighAttack complexity: Successful exploitation depends on specific conditions outside the attacker's direct control.PRNonePrivileges required: The attacker does not need an account or existing privileges.UINoneUser interaction: No action by another user is required.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.ALowAvailability impact: A successful attack can cause a limited 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. - Fix availability varies by product
- Action
- Use the product-specific evidence above. Patch only products with a verified fixed release, and keep every affected or under-investigation state without a matching fix in the remediation queue.
- Workaround
- Please make sure that only trusted entities have access to the device. Furthermore, you should apply the following General Security Measures when operating the product to mitigate the associated security risk. The collected resources "SICK Operating Guidelines" and "ICS-CERT recommended practices on Industrial Security" could help to implement the general security practices.
04
Evidence and provenance
Published 1 Jun 2021 · Last source change 3 Aug 2024, 18:58 UTC · CWE-193 · Off-by-one Error
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-2021-10135
Product sourceVendor CSAF · SICK PSIRT
Remediation sourceVendor CSAF · SICK PSIRT
CWE sourceCNA
NVD statusNVD modified after enrichment
Missing structured fields: affected product. Missing data is not evidence of low risk; review the primary advisory.
Material change intelligence
View recent updates ↗What changed after publication
- Vendor guidanceAuthoritative vendor guidance changed: added remediation: sick.com/sca-2025-0011.json; removed remediation: endress.com.SICK PSIRT ↗
- Before
- remediation: endress.com
- After
- remediation: sick.com/sca-2025-0011.json
- Vendor guidanceAuthoritative vendor guidance changed: added remediation: sick.com/special_information_sick_operating_guidelines_cybersecurity_by_sick_en_im0106719.pdf; removed remediation: sick.com/psirt.SICK PSIRT ↗
- Before
- remediation: sick.com/psirt
- After
- remediation: sick.com/special_information_sick_operating_guidelines_cybersecurity_by_sick_en_im0106719.pdf
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.