CVE-2018-21246Network attack vector

Authentication bypass via mismatched SNI and Host

Published Jun 15, 2020 · Updated Aug 5, 2024

Improper authentication in Caddy before 0.10.13 allows remote attackers to bypass TLS client authentication using mismatched SNI and Host names. When protected and unprotected virtual hosts share a listener, Server.serveHTTP accepts the HTTP Host after selecting the TLS configuration from a different handshake SNI, without checking that both names match. The bypass requires such a shared-listener configuration and lets an unauthenticated request reach a host that requires a valid client certificate.

CVSS severity9.8
Critical
EPSS probability2.72%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper authentication in Caddy before 0.10.13 allows remote attackers to bypass TLS client authentication using mismatched SNI and Host names. When protected and unprotected virtual hosts share a listener, Server.serveHTTP accepts the HTTP Host after selecting the TLS configuration from a different handshake SNI, without checking that both names match. The bypass requires such a shared-listener configuration and lets an unauthenticated request reach a host that requires a valid client certificate.

The record

CVE
CVE-2018-21246
Published
Jun 15, 2020
Updated
Aug 5, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-287, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 15, 2020CVE publishedPublication date reported by the CVE source.
  2. Aug 5, 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?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P
9.8GitHub Advisory DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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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