High exploit reference

CVE-2026-33805 — @fastify/reply-from vulnerable to connection header abuse enabling stripping of proxy-added headers

@fastify/reply-from v12.6.1 and earlier and @fastify/http-proxy v11.4.3 and earlier process the client's Connection header after the proxy has added its own headers via rewriteRequestHeaders. This allows attackers to retroactively strip proxy-added headers from upstream requests by listing them in the Connection header value. Any header added by the proxy for routing, access control, or security purposes can be selectively removed by a client. @fastify/http-proxy is also affected as it delegates to @fastify/reply-from. Upgrade to @fastify/reply-from v12.6.2 or @fastify/http-proxy v11.4.4 or later.

Published Apr 15, 2026 · Updated Oct 13, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP · CNA: openjs

Low

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.

How the score was calculated

EPSS 0.6% (30-day exploitation probability)+7
Technical severity (high)+6
CISA SSVC: exploitation "poc"+5
Automatable exploitation (SSVC)+3

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation

Not confirmed

Not in CISA KEV — an absence of record, not proof it is unexploited.

EPSS FIRST

0.56%

ahead of 45% of scored CVEs · Oct 14, 2026

Affects

@fastify/reply-from

@fastify/http-proxy and 3 more

CVSS base

8.6

High · 4 source(s)

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Exploitation

Proof of concept

none · proof-of-concept · active

Automatable

Yes

can an attacker script the kill-chain steps

Technical impact

Partial

partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

Documented mitigation — github.com (NIST National Vulnerability Database)

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

Identify exposed assets running the affected vendor/product/version combinations (@fastify/reply-from @fastify/http-proxy, @fastify/reply-from @fastify/reply-from, Fastify Fastify/http-proxy…).
The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
Apply the documented mitigations until a definitive fix is available.
Search available logs for exploit probes, errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Vendor	Product	Versions	Status	Source
@fastify/reply-from	@fastify/http-proxy	< 11.4.4	Vulnerable	CVE Program (cve.org) e CISA ADP
@fastify/reply-from	@fastify/reply-from	< 12.6.2	Vulnerable	CVE Program (cve.org) e CISA ADP
Fastify	Fastify/http-proxy	< 11.4.4	Vulnerable	NIST National Vulnerability Database
Fastify	Reply-from	< 12.6.2	Vulnerable	NIST National Vulnerability Database

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector

Network

Attack complexity

Low

Privileges required

None

User interaction

None

Scope

Changed

Confidentiality

None

Integrity

High

Availability

None

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Scored by
9.0	CVSS 4.0	Critical	—	—	ce714d77-add3-4f53-aff5-83d477b104bb via NIST National Vulnerability Database
9.0	CVSS 4.0	Critical	—	—	openjs via CVE Program (cve.org) e CISA ADP
8.6	CVSS 3.1	High	3.9	4.0	nvd@nist.gov via NIST National Vulnerability Database
7.4	CVSS 3.1	High	2.2	5.2	0b0ca135-0b70-47e7-9f44-1890c2a1c46c via NIST National Vulnerability Database

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:H/A:N (score shown at the top: nvd@nist.gov)

Scoring entities (NIST, CNA, ADP) score independently; each score is shown with its origin. For prioritization we use 8.6 (NVD (NIST)) as a conservative provisional reference — not a definitive official score.

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-644

Other CVEs with this weakness →

EPSS over time FIRST

Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 0.56%

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).

web/proxy/WAF logsnetwork IDS/IPS

Timeline

What happened to this CVE, newest first.

2026
Affected products added · @fastify/reply-from/@fastify/reply-from, @fastify/reply-from/@fastify/http-proxy
Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
SSVC assessment updated
Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
Record published
Oct 13, 2026
Fix documented
Oct 13, 2026 · NIST National Vulnerability Database
CVSS changed · 8.6
Oct 13, 2026 · NIST National Vulnerability Database
Source status changed · → Modified
Oct 13, 2026 · NIST National Vulnerability Database
Record created in this database
Oct 13, 2026 · NIST National Vulnerability Database
CVE published
Apr 15, 2026 · openjs

Source activity

Repeated readings are counted rather than listed.

1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

6 on the record

cna.openjsf.org/security-advisories.html
Vendor Advisory, NIST National Vulnerability Database
github.com/fastify/fastify-reply-from/security/advisories/GHSA-gwhp-pf74-vj37
Exploit, Mitigation, Vendor Advisory, NIST National Vulnerability Database
access.redhat.com/security/cve/CVE-2026-33805
NIST National Vulnerability Database
access.redhat.com/errata/RHSA-2026:10175
NIST National Vulnerability Database
bugzilla.redhat.com/show_bug.cgi
NIST National Vulnerability Database
security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-33805.json
NIST National Vulnerability Database

Elsewhere on this site

@fastify/reply-from
every CVE for this vendor

CWE-644
other pages naming this weakness

Related CVEs (same products)

High CVE-2026-107318 @fastify/reply-from vulnerable to improper certificate validation in built-in HTTPS transports 7d
High CVE-2026-85124 @fastify/http-proxy vulnerable to prefix escape via backslash dot-segments Sep 3, 2026
Critical CVE-2026-16117 @fastify/http-proxy vulnerable to prefix escape via URL-encoded characters Jul 18, 2026
Critical CVE-2026-16158 @fastify/reply-from vulnerable to cross-upstream request routing via URL cache key collision Jul 18, 2026
Critical CVE-2026-15631 @fastify/http-proxy vulnerable to prefix escape via WebSocket path traversal Jul 18, 2026
Medium CVE-2025-66415 fastify-reply-from bypass of reply forwarding Dec 1, 2025

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2026-33805 is "@fastify/reply-from vulnerable to connection header abuse enabling stripping of proxy-added headers", rated High. It affects @fastify/http-proxy (@fastify/reply-from); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 8.6 describes technical severity. EPSS 0.56% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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