CVE-2021-21335Network attack vector

Authentication bypass via accepted Basic Auth errors

Published Mar 8, 2021 · Updated Aug 3, 2024

Authentication bypass in SPNEGO HTTP Authentication Module for nginx before 1.1.1 allows remote attackers to access protected content. The ngx_http_auth_spnego_handler accepts NGX_ERROR from ngx_http_auth_spnego_basic as success because it rejects only NGX_DECLINED. Basic Authentication fallback must be enabled, and a malformed username then bypasses the credential check and permits unauthorized content access.

CVSS severity5.3
Medium
EPSS probability1.66%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Authentication bypass in SPNEGO HTTP Authentication Module for nginx before 1.1.1 allows remote attackers to access protected content. The ngx_http_auth_spnego_handler accepts NGX_ERROR from ngx_http_auth_spnego_basic as success because it rejects only NGX_DECLINED. Basic Authentication fallback must be enabled, and a malformed username then bypasses the credential check and permits unauthorized content access.

The record

CVE
CVE-2021-21335
Published
Mar 8, 2021
Updated
Aug 3, 2024
Vendor
Sean Timothy Noonan
Product
SPNEGO HTTP Authentication Module for nginx
Classifications
CWE-287, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 8, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record 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=< 1.1.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Sean Timothy Noonan · SPNEGO HTTP Authentication Module for nginxversion=< 1.1.1

Published CVSS scores

7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P
5.3GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

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 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-287Improper Authentication
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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