CVE-2021-1590: Cisco NX-OS Software system login block-for Denial of Service Vulnerability
Noroxi assessment
Priority: Monitor (action score 21/100)
- The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
- Probability of exploitation in the next 30 days: 1.3% (FIRST EPSS); higher than 71.7% of all records.
- Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
- According to CISA the attack is automatable: targets can be found by mass scanning.
- According to the record, no fixed version is available yet.
- Mentioned 1 time in Turkish national cyber security advisories (siberguvenlik.gov.tr).
Low priority for now. Update when a fix is released; follow the vendor's advisory.
Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it
Primary sources: NVD · CVE.org · FIRST EPSS
A vulnerability in the implementation of the system login block-for command for Cisco NX-OS Software could allow an unauthenticated, remote attacker to cause a login process to unexpectedly restart, causing a denial of service (DoS) condition. This vulnerability is due to a logic error in the implementation of the system login block-for command when an attack is detected and acted upon. An attacker could exploit this vulnerability by performing a brute-force login attack on an affected device. A successful exploit could allow the attacker to cause a login process to reload, which could result in a delay during authentication to the affected device.
- Published
- Aug 25, 2021
- Updated
- Jun 16, 2026
- EPSS
- 1.3% · 71th percentile
- CWE
- CWE-787
Report tools
Action score
21
Monitor
Low priority for now.
- CVSS
- 21 / 40 · 5.3 / 10
- CISA KEV
- 0 / 30 · Not listed
- EPSS
- 0 / 30 · 1.3%
CISA SSVC decision
- Exploitation
- none
- Automatable
- yes
- Technical impact
- partial
Vulnrichment: CISA's decision-tree inputs.
CNA vs NVD score
- NVD
- 5.3
- CNA · cisco
- 5.3
- agree
The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.
Get told when this record changes
- Not in KEV: no record of exploitation in the wild
- No public exploit code on record
- No fixed version yet
If any of these changes, we'll tell you in the daily email digest.
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Noroxi analysis
What does it mean in your environment?
The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.
We use this product, ask for helpAffected systems
Is your version affected?
Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.
| Vendor | Product | CPE |
|---|---|---|
| cisco | nx-os | cpe:2.3:o:cisco:nx-os |
| cisco | nexus 3000 | cpe:2.3:h:cisco:nexus_3000 |
| cisco | nexus 3048 | cpe:2.3:h:cisco:nexus_3048 |
| cisco | nexus 31108pc-v | cpe:2.3:h:cisco:nexus_31108pc-v |
| cisco | nexus 31108tc-v | cpe:2.3:h:cisco:nexus_31108tc-v |
| cisco | nexus 31128pq | cpe:2.3:h:cisco:nexus_31128pq |
| cisco | nexus 3132c-z | cpe:2.3:h:cisco:nexus_3132c-z |
| cisco | nexus 3132q-v | cpe:2.3:h:cisco:nexus_3132q-v |
| cisco | nexus 3132q-x\/3132q-xl | cpe:2.3:h:cisco:nexus_3132q-x\/3132q-xl |
| cisco | nexus 3164q | cpe:2.3:h:cisco:nexus_3164q |
| cisco | nexus 3172pq\/pq-xl | cpe:2.3:h:cisco:nexus_3172pq\/pq-xl |
| cisco | nexus 3172tq-xl | cpe:2.3:h:cisco:nexus_3172tq-xl |
and 91 more
Affected versions
NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.
- cisco nx-os7.0(3)i4(0.116)
- cisco nx-os7.3(7)n1(1b)
- cisco unified computing systembefore 4.0\(4m\)
- cisco unified computing system4.1 and later · before 4.1\(3d\)
Versions reported by the vendor
Affected version ranges reported by the assigning authority (cisco). Independent of NVD's CPE analysis and usually ahead of it.
Cisco Cisco NX-OS Software
- n/aaffected
Same product
cisco: all recordsOther highest-scoring records for the same primary product.
- CVE-2024-20399Cisco NX-OS Software CLI Command Injection VulnerabilityKEV57Plan
- CVE-2018-0301A vulnerability in the NX-API feature of Cisco NX-OS Software could allow an unauthenticated, remote attacker to craft a packet to the manag44Plan
- CVE-2016-1453Buffer overflow in the Overlay Transport Virtualization (OTV) GRE feature in Cisco NX-OS 5.0 through 7.3 on Nexus 7000 and 7700 devices allo41Plan
Published before and after this one in the same product
- ← EarlierCVE-2021-1587Aug 25, 2021 · Cisco NX-OS Software VXLAN OAM (NGOAM) Denial of Service Vulnerability35Monitor
- ← EarlierCVE-2021-1588Aug 25, 2021 · Cisco NX-OS Software MPLS OAM Denial of Service Vulnerability34Monitor
- Later →CVE-2021-1591Aug 25, 2021 · Cisco Nexus 9500 Series Switches Access Control List Bypass Vulnerability21Monitor
Remediation
Which version to upgrade to
Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.
No fix version is recorded for this entry. Check the references for vendor advisories.
Exploit status
No known public exploit
No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.
Research context
For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.
Timeline
From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.
No dated events beyond publication.
No history yet (it accumulates with the daily EPSS job).
Patch and commit links
Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.
No commit or PR link among the references.
Credits
All researchersFinders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.
No credits in the CNA record.
Variant candidates
Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.
No nightly-computed relations.
Chain candidates
An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.
—
Bug bounty scope
No known public program.
Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).
National notice (Türkiye Cybersecurity Directorate / USOM)
Official security notices citing this record; remediation advice is on the agency's page.
- TR-21-0678 · Aug 26, 2021(Cisco Güvenlik Zafiyeti)
Technical details
Attack conditions
- Anyone who can reach it over the internet can trigger it.
- No account or password is required.
- No user action is required.
- No special conditions are required; it is repeatable.
If successful
- Confidentiality
- none
- Integrity
- none
- Availability
- low · the service can be disrupted
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality impact
- None
- Integrity impact
- None
- Availability impact
- Low
Weakness class (CWE)
CWE-787 · Out-of-bounds WriteCVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
nvd-primary
Attack context
MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.
MITRE has no CAPEC/ATT&CK mapping for this CWE.
Change log
No changes recorded on tracked fields yet. Score, KEV, exploit maturity and fix status changes appear here.
References
Vendor advisories and official records. Exploit/PoC links are deliberately left out.