CVE-2026-33805Public exploitNetwork attack vector

Access-control bypass via late Connection header processing

Published Apr 15, 2026 · Updated Apr 15, 2026

Header-order flaws in @fastify/reply-from before 12.6.2 and @fastify/http-proxy before 11.4.4 allow remote attackers to strip headers. The HTTP/1.1 and undici handlers read the client-controlled Connection value after rewriteRequestHeaders adds proxy headers, then delete every named header. Exploitation requires a reachable proxy route and security-relevant injected headers; removal can bypass upstream routing or access-control decisions.

CVSS severity9.0
Critical
EPSS probability0.44%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Header-order flaws in @fastify/reply-from before 12.6.2 and @fastify/http-proxy before 11.4.4 allow remote attackers to strip headers. The HTTP/1.1 and undici handlers read the client-controlled Connection value after rewriteRequestHeaders adds proxy headers, then delete every named header. Exploitation requires a reachable proxy route and security-relevant injected headers; removal can bypass upstream routing or access-control decisions.

The record

CVE
CVE-2026-33805
Published
Apr 15, 2026
Updated
Apr 15, 2026
Vendor
Fastify
Product
@fastify/http-proxy
Classifications
CWE-644, CWE-444, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 15, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 15, 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=0 <11.4.4

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Fastify · @fastify/http-proxyversion=0 <11.4.4
Fastify · @fastify/reply-fromversion=0 <12.6.2

Published CVSS scores

9.0OpenJS FoundationCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:L/SI:H/SA:N
7.4Red HatCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
8.6NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:H/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 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 exploitPublished

Weakness, pattern, technique

CWE-644Improper Neutralization of HTTP Headers for Scripting Syntax
CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
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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