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Vulnerabilities

Vulnerability · CVE-2026-3635

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),…

Published Mar 23, 2026 · catalog updated Jun 17, 2026 · the affected product

Check if you run it

No confirmed exploitation is on record for CVE-2026-3635. EPSS is 0.18%. Act only if a listed version of the affected product is installed.

Gaps on this record

  • No plain product name is stored. The page cannot tell you which product to check.
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
Medium6.1
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.18%
Remediation
Version ranges on file
Affected products
cpe fastify/fastify · stated fastify

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.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • cpe fastify/fastifyversion unspecified
  • stated fastify<= 5.8.2

How to fix CVE-2026-3635

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2026-3635. EPSS is 0.18%. Act only if a listed version of the affected product is installed.

Which versions are affected?

Compare what you have installed with <= 5.8.2. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-348. EPSS 0.18%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

Timeline

Detection, updates, and resolution for this record.

  1. Mar 23, 2026 · NIST NVD

    Detected

    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

  2. Jun 17, 2026 · NIST NVD

    Updated

    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

Sources

  • CVE.orgVulnerability recordCVE-2026-3635

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2026-3635

    30-day exploitation probability 0.18%

    first.org
  • NIST NVDVulnerability recordCVE-2026-3635

    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 conne

    nvd.nist.gov

References

Freshness

Last verified in this catalog Oct 23, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.