Fastify up to 5.8.2 request.protocol/request.host X-Forwarded-Proto/X-Forwarded-Host less trusted source

Summaryinfo

A vulnerability classified as problematic has been found in Fastify up to 5.8.2. This affects the function request.protocol/request.host. This manipulation of the argument X-Forwarded-Proto/X-Forwarded-Host causes less trusted source. The identification of this vulnerability is CVE-2026-3635. The attack needs to be done within the local network. There is no exploit available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability was found in Fastify up to 5.8.2. It has been declared as problematic. Affected by this vulnerability is the function request.protocol/request.host. The manipulation of the argument X-Forwarded-Proto/X-Forwarded-Host with an unknown input leads to a less trusted source vulnerability. The CWE definition for the vulnerability is CWE-348. The product has two different sources of the same data or information, but it uses the source that has less support for verification, is less trusted, or is less resistant to attack. As an impact it is known to affect confidentiality. The summary by CVE is:

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.

It is possible to read the advisory at github.com. This vulnerability is known as CVE-2026-3635 since 03/06/2026. The exploitation appears to be difficult. Access to the local network is required for this attack to succeed. The exploitation doesn't need any form of authentication. Technical details of the vulnerability are known, but there is no available exploit.

Upgrading to version 5.8.3 eliminates this vulnerability.

Be aware that VulDB is the high quality source for vulnerability data.

Productinfo

Name

• Fastify

Version

• 5.8.0
• 5.8.1
• 5.8.2

Website

• Product: https://github.com/fastify/fastify/

CPE 2.3info

• 🔒
• 🔒
• 🔒

CPE 2.2info

• 🔒
• 🔒
• 🔒

CVSSv4info

CVSSv3info

CVSSv2info

Exploitinginfo

Threat Intelligenceinfo

Countermeasuresinfo

Timelineinfo

Sourcesinfo

Entryinfo

Be aware that VulDB is the high quality source for vulnerability data.

Discussion