CVE-2026-3635
6.1 · MEDIUMOverview
CVE-2026-3635 is a medium-severity vulnerability affecting fastify fastify. It was published on March 23, 2026 and has a CVSS 3.1 base score of 6.1 (MEDIUM).
This vulnerability has a CVSS 3.1 base score of 6.1, rated MEDIUM. It requires local or adjacent network access to exploit. No authentication or special privileges are required for exploitation.
Technical Description
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.
Remediation
Check the references section for vendor advisories and patches from fastify. Update fastify to the latest patched version. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.
Affected Products
| Vendor | Product | Versions | Status |
|---|---|---|---|
| fastify | fastify | >= 0, < 5.8.3 | Affected |
Frequently Asked Questions
What is CVE-2026-3635?
CVE-2026-3635 is a medium-severity vulnerability affecting fastify fastify. It was published on March 23, 2026 and has a CVSS 3.1 base score of 6.1 (MEDIUM).
How severe is CVE-2026-3635?
This vulnerability has a CVSS 3.1 base score of 6.1, rated MEDIUM. It requires local or adjacent network access to exploit. No authentication or special privileges are required for exploitation.
How do I fix or remediate CVE-2026-3635?
Check the references section for vendor advisories and patches from fastify. Update fastify to the latest patched version. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.
How can CyberStrike help with CVE-2026-3635?
CyberStrike's AI-powered security agents can automatically detect CVE-2026-3635 across your infrastructure using autonomous pentesting, DAST scanning, and HackBrowser. The platform continuously monitors for known vulnerabilities and provides actionable remediation guidance prioritized by real-world exploitability.
How CyberStrike Helps
AI agents map your attack surface to find vulnerabilities like this one.
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