CVE-2026-20171Network attack vector

BGP session disruption via transitive attribute parsing

Published May 20, 2026 · Updated May 20, 2026

Incorrect BGP attribute parsing in Cisco NX-OS on affected Nexus switches allows remote attackers to interrupt BGP sessions. The enforce-first-as feature incorrectly parses a crafted transitive ATTR_SET attribute propagated through an established BGP peer session. Exposure requires BGP on a standalone Nexus 3000 or 9000 switch; the default-enabled feature drops the forwarding peer session and causes repeated flaps.

CVSS severity6.8
Medium
EPSS probability0.47%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Incorrect BGP attribute parsing in Cisco NX-OS on affected Nexus switches allows remote attackers to interrupt BGP sessions. The enforce-first-as feature incorrectly parses a crafted transitive ATTR_SET attribute propagated through an established BGP peer session. Exposure requires BGP on a standalone Nexus 3000 or 9000 switch; the default-enabled feature drops the forwarding peer session and causes repeated flaps.

The record

CVE
CVE-2026-20171
Published
May 20, 2026
Updated
May 20, 2026
Vendor
ClamAV
Product
Cisco NX-OS
Classifications
CWE-670, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 20, 2026CVE publishedPublication date reported by the CVE source.
  2. May 20, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=10.2(1)
  2. Affected versionversion=10.2(1q)
  3. Affected versionversion=10.2(2)
  4. Affected versionversion=10.2(2a)
  5. Affected versionversion=10.2(3)
  6. Affected versionversion=10.2(3t)
  7. Affected versionversion=10.2(3v)
  8. Affected versionversion=10.2(4)
  9. Affected versionversion=10.2(5)
  10. Affected versionversion=10.2(6)
  11. Affected versionversion=10.2(7)
  12. Affected versionversion=10.2(8)
  13. Affected versionversion=10.2(9)
  14. Affected versionversion=10.3(1)
  15. Affected versionversion=10.3(2)
  16. Affected versionversion=10.3(3)
  17. Affected versionversion=10.3(3o)
  18. Affected versionversion=10.3(3p)
  19. Affected versionversion=10.3(3q)
  20. Affected versionversion=10.3(3r)
  21. Affected versionversion=10.3(3w)
  22. Affected versionversion=10.3(3x)
  23. Affected versionversion=10.3(4)
  24. Affected versionversion=10.3(4a)
  25. Affected versionversion=10.3(4g)
  26. Affected versionversion=10.3(4h)
  27. Affected versionversion=10.3(5)
  28. Affected versionversion=10.3(6)
  29. Affected versionversion=10.3(7)
  30. Affected versionversion=10.3(8)
  31. Affected versionversion=10.3(99w)
  32. Affected versionversion=10.3(99x)
  33. Affected versionversion=10.4(1)
  34. Affected versionversion=10.4(2)
  35. Affected versionversion=10.4(3)
  36. Affected versionversion=10.4(4)
  37. Affected versionversion=10.4(4g)
  38. Affected versionversion=10.4(5)
  39. Affected versionversion=10.4(6)
  40. Affected versionversion=10.5(1)
  41. Affected versionversion=10.5(2)
  42. Affected versionversion=10.5(3)
  43. Affected versionversion=10.5(3e)
  44. Affected versionversion=10.5(3o)
  45. Affected versionversion=10.5(3p)
  46. Affected versionversion=10.5(3s)
  47. Affected versionversion=10.5(3t)
  48. Affected versionversion=10.6(1)
  49. Affected versionversion=10.6(1s)

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

ClamAV · Cisco NX-OSversion=10.2(1); version=10.2(1q); version=10.2(2); version=10.2(2a); version=10.2(3); version=10.2(3t); version=10.2(3v); version=10.2(4); version=10.2(5); version=10.2(6); version=10.2(7); version=10.2(8); version=10.2(9); version=10.3(1); version=10.3(2); version=10.3(3); version=10.3(3o); version=10.3(3p); version=10.3(3q); version=10.3(3r); version=10.3(3w); version=10.3(3x); version=10.3(4); version=10.3(4a); version=10.3(4g); version=10.3(4h); version=10.3(5); version=10.3(6); version=10.3(7); version=10.3(8); version=10.3(99w); version=10.3(99x); version=10.4(1); version=10.4(2); version=10.4(3); version=10.4(4); version=10.4(4g); version=10.4(5); version=10.4(6); version=10.5(1); version=10.5(2); version=10.5(3); version=10.5(3e); version=10.5(3o); version=10.5(3p); version=10.5(3s); version=10.5(3t); version=10.6(1); version=10.6(1s)

Published CVSS scores

6.8CiscoCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:H/E:X/RL:X/RC:X

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-670Always-Incorrect Control Flow Implementation
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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