CVE-2023-27491Public exploitNetwork attack vector

Security-policy bypass via invalid pseudo-header forwarding

Published Apr 4, 2023 · Updated Feb 11, 2025

Improper input validation in Envoy allows remote attackers to bypass security policies via crafted HTTP/2 or HTTP/3 requests. Envoy accepts an invalid :method pseudo-header and uses its value to construct an HTTP/1 request line instead of rejecting the request. The proxy must translate the request to a noncompliant HTTP/1 upstream that accepts malformed request lines, enabling unintended upstream requests without reported service interruption.

CVSS severity5.4
Medium
EPSS probability0.87%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper input validation in Envoy allows remote attackers to bypass security policies via crafted HTTP/2 or HTTP/3 requests. Envoy accepts an invalid :method pseudo-header and uses its value to construct an HTTP/1 request line instead of rejecting the request. The proxy must translate the request to a noncompliant HTTP/1 upstream that accepts malformed request lines, enabling unintended upstream requests without reported service interruption.

The record

CVE
CVE-2023-27491
Published
Apr 4, 2023
Updated
Feb 11, 2025
Vendor
Envoy Proxy
Product
Envoy
Classifications
CWE-444, CWE-20, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 4, 2023CVE publishedPublication date reported by the CVE source.
  2. Feb 11, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

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

  1. Affected versionversion=< 1.22.9
  2. Affected versionversion=>= 1.23.0, < 1.23.6
  3. Affected versionversion=>= 1.24.0, < 1.24.4
  4. Affected versionversion=>= 1.25.0, < 1.25.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Envoy Proxy · Envoyversion=< 1.22.9; version=>= 1.23.0, < 1.23.6; version=>= 1.24.0, < 1.24.4; version=>= 1.25.0, < 1.25.3

Published CVSS scores

9.1NIST National Vulnerability DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
5.4GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N

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 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 exploitPublished

Weakness, pattern, technique

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
CWE-20Improper Input Validation
T1190Exploit Public-Facing Application

Public exploit references

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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