How severe is CVE-2026-15631?

CVE-2026-15631 has a CVSS 3.x base score of 8.7, rated high severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.

Is CVE-2026-15631 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (41st percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-15631?

CVE-2026-15631 affects Fastify Fastify/http-proxy. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-15631?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2026-15631 published?

CVE-2026-15631 was published on 2026-07-18 and last updated on 2026-07-28.

CVE-2026-15631

CVE-2026-15631 is a high-severity vulnerability in Fastify Fastify/http-proxy with a CVSS 3.x base score of 8.7. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-22.

Key facts

Severity: High (CVSS 3.x base score 8.7)
EPSS exploit prediction: 1% (41st percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-22
Affected product: Fastify Fastify/http-proxy
Published: 2026-07-18
Last modified: 2026-07-28

Description

Impact: @fastify/http-proxy versions from 9.4.0 up to and including 11.5.0 fail to validate the resolved WebSocket destination path against the configured rewrite prefix. The WebSocket routing path in WebSocketProxy.findUpstream resolves the destination via the WHATWG URL constructor, which collapses dot segments, so a crafted upgrade request with path traversal sequences can escape the rewrite prefix and reach upstream endpoints that were not meant to be exposed by the proxy. This is a variant of CVE-2021-21322 in a code path that never went through the HTTP fix in fastify/reply-from. Exploitation requires a non-normalizing WebSocket client, since browsers and the ws package normalize the request path before sending, but raw HTTP clients or downstream proxies that forward the request target unchanged make the attack reachable in production topologies. Patches: upgrade to @fastify/http-proxy 11.6.0. Workarounds: none.

Frequently asked questions

What is CVE-2026-15631?: Impact: @fastify/http-proxy versions from 9.4.0 up to and including 11.5.0 fail to validate the resolved WebSocket destination path against the configured rewrite prefix. The WebSocket routing path in WebSocketProxy.findUpstream resolves the destination via the WHATWG URL constructor, which collapses dot segments, so a crafted upgrade request with path traversal sequences can escape the rewrite prefix and reach upstream endpoints that were not meant to be exposed by the proxy. This is a variant of CVE-2021-21322 in a code path that never went through the HTTP fix in fastify/reply-from. Exploitation requires a non-normalizing WebSocket client, since browsers and the ws package normalize the request path before sending, but raw HTTP clients or downstream proxies that forward the request target unchanged make the attack reachable in production topologies. Patches: upgrade to @fastify/http-proxy 11.6.0. Workarounds: none.
How severe is CVE-2026-15631?: CVE-2026-15631 has a CVSS 3.x base score of 8.7, rated high severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.
Is CVE-2026-15631 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (41st percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-15631?: CVE-2026-15631 affects Fastify Fastify/http-proxy. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-15631?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2026-15631 published?: CVE-2026-15631 was published on 2026-07-18 and last updated on 2026-07-28.

References

Affected products (1)

cpe:2.3:a:fastify:fastify\/http-proxy:*:*:*:*:*:node.js:*:*

More vulnerabilities in Fastify Fastify/http-proxy

CVE-2026-16117 — Critical (CVSS 10.0): Impact: @fastify/http-proxy versions up to and including 11.5.0 fail to rewrite the request prefix when the prefix…
CVE-2026-33805 — High (CVSS 8.6): @fastify/reply-from v12.6.1 and earlier and @fastify/http-proxy v11.4.3 and earlier process the client's Connection…
CVE-2026-85124 — High (CVSS 7.5): @fastify/http-proxy versions before 11.6.2 do not validate proxied HTTP request paths for backslash based dot-segments…

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