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FastifyApplication

CVE-2026-3635

MEDIUM · 6.1 CVSS v3.1 Published 2026-03-23
Fix available
A fix is available. Upgrade to 5.8.3 or later.
See remediation →
66/100
Remediation priority · Elevated
No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Summary When trustProxy is configured with a restrictive trust function (e.g., a specific IP like trustProxy: '10.0.0.1', a subnet, a hop count, or a custom function), the request.protocol and request.host getters read X-Forwarded-Proto and X-Forwarded-Host headers from any connection — including connections from untrusted IPs. This allows an attacker connecting directly to Fastify (bypassing the proxy) to spoof both the protocol and host seen by the application. Affected Versions fastify <= 5.8.2 Impact Applications using request.protocol or request.host for security decisions (HTTPS enforcement, secure cookie flags, CSRF origin checks, URL construction, host-based routing) are affected when trustProxy is configured with a restrictive trust function. When trustProxy: true (trust everything), both host and protocol trust all forwarded headers — this is expected behavior. The vulnerability only manifests with restrictive trust configurations.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

In Fastify versions <= 5.8.2, when trustProxy is configured with a restrictive trust function (specific IP, subnet, hop count, or custom function), the request.protocol and request.host getters incorrectly accept X-Forwarded-Proto and X-Forwarded-Host headers from ANY connection, including direct connections from untrusted IPs. This allows attackers bypassing the proxy to spoof protocol and host values used for security decisions.

MitigationReview all code using request.protocol and request.host for security purposes (HTTPS enforcement, secure cookies, CSRF checks, URL construction, host-based routing) and implement application-level validation; wait for Fastify patch and upgrade when available.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
FastifyApplication
Affected:< 5.8.3

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Adjacent
Complexity
High
Privileges
None
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check installed Fastify version
    Run 'npm list fastify' or check package.json to find the installed version number
    Affected if The installed version is 5.8.2 or lower (any version < 5.8.3)
  2. Verify trustProxy configuration
    Search your Fastify server initialization code for 'trustProxy' option and inspect its value
    Affected if trustProxy is configured with any value other than true (including specific IP addresses, subnet CIDRs, numeric hop counts, or a custom function)
  3. Identify code using request.protocol
    Search codebase for 'request.protocol' or 'req.protocol' usage in route handlers and middleware
    Affected if Code reads request.protocol for security decisions such as HTTPS enforcement, secure cookie settings, or conditional logic based on connection protocol
  4. Identify code using request.host
    Search codebase for 'request.host', 'request.hostname', 'req.host', or 'req.hostname' usage in route handlers and middleware
    Affected if Code reads request.host or request.hostname for security decisions such as CSRF validation, URL construction, or host-based access control

You are affected if Fastify version is < 5.8.3 AND trustProxy is configured AND your code uses request.protocol or request.host for security-sensitive operations.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 5.8.3 or later
Fixed in 5.8.3
Interim mitigation

Review all code using request.protocol and request.host for security purposes (HTTPS enforcement, secure cookies, CSRF checks, URL construction, host-based routing) and implement application-level validation; wait for Fastify patch and upgrade when available.

Recommended fix High confidence

[email protected]

  1. Run `npm install [email protected]` to upgrade to the patched version
  2. Alternatively, use `npm install fastify@latest` to get the latest stable release which includes the fix
  3. After upgrading, verify your application works correctly with `npm test` or your existing test suite
  4. Deploy the updated version to your production environment

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Fastify Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA2.0 h
20.0 hours of engineering $3,500
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $5,600.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-3635 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data