Cyber Resilience

CVE-2026-16158

Fastify\/Reply-From 8.3.1 – 12.6.4

Published
18 July 2026
Modified
05 August 2026
Patch / advisory
View advisory
CVSS Score v3.1 8.7
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:N
EPSS Score 0.0023 14th percentile
Risk Priority 59 floored blend · peak EPSS

Summary

CVE-2026-16158 is a high-severity Confused Deputy (CWE-441) vulnerability in Fastify Fastify\/Reply-From. Its CVSS base score is 8.7 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Proxy (T1090); ranked at the 14th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified map to AC-16 (Security and Privacy Attributes) and AC-4 (Information Flow Enforcement) — see the control section below for these in your framework.

OWASP Top 10 for Web (2025)

A01:2025 Broken Access Control

EU & UK References

Vulnerability Data

Impact: @fastify/reply-from versions from 8.3.1 up to but not including 12.6.4 build the internal URL cache key by concatenating the destination and source path without a delimiter. Different destination and source pairs can therefore produce the same key while resolving…

more

to different upstream URLs. When getUpstream selects an upstream from request data, a URL cached for one upstream can be reused for a request intended for another upstream, causing cross-upstream data access and modification. The default configuration is affected. Setting disableCache to true prevents the behavior. Patches: upgrade to @fastify/reply-from 12.6.4. Workarounds: pass disableCache: true when registering the plugin.

CWE(s)
CWE-441

Related Threats

MITRE ATT&CK Enterprise Techniques

T1090 Proxy Command And Control
Adversaries may use a connection proxy to direct network traffic between systems or act as an intermediary for network communications to a command and control server to avoid direct connections to their infrastructure.
attack.mitre.org →
T1090.002 External Proxy Command And Control
Adversaries may use an external proxy to act as an intermediary for network communications to a command and control server to avoid direct connections to their infrastructure.
attack.mitre.org →
T1090.001 Internal Proxy Command And Control
Adversaries may use an internal proxy to direct command and control traffic between two or more systems in a compromised environment.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2023-31313Shared CWE-441
CVE-2026-0021Shared CWE-441
CVE-2026-36608Shared CWE-441
CVE-2026-0098Shared CWE-441
CVE-2026-45003Shared CWE-441
CVE-2025-11393Shared CWE-441
CVE-2025-48710Shared CWE-441
CVE-2025-66415Shared CWE-441
CVE-2026-39906Shared CWE-441
CVE-2026-24471Shared CWE-441

Affected Assets

fastify
fastify\/reply-from
8.3.1 — 12.6.4

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V3.2.1
  • V3.5.1
  • V3.5.3
  • V3.5.8

Mitigating Controls (NIST 800-53 r5) AI

AC-16 Security and Privacy Attributes good match

Associating and preserving security attributes such as original source identity prevents the loss of provenance that creates the confused deputy.

AC-4 Information Flow Enforcement good match

Information flow enforcement requires preserving and checking source attributes before forwarding requests outside the trust boundary.

AC-3 Access Enforcement partial match

Access enforcement directly stops a component from acting on forwarded requests without verifying original source authorizations.

SC-7 Boundary Protection partial match

Boundary protection limits and inspects external forwarding, reducing the ability of an intermediary to act as an unintended proxy.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.AA-04 mostly match
prevents

Proper conveyance and verification of original identity assertions directly prevents loss of request source when forwarding.

PR.AA-05 partial match
prevents

Enforcing least-privilege authorizations on forwarded requests reduces confused-deputy abuse even if source identity is lost.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.5.15 Access control partial match
prevents

Access-control rules can limit which upstream identities may cause the product to act on their behalf, reducing confused-deputy risk.

A.5.16 Identity management partial match
prevents

Proper identity management ensures the original requester identity is preserved and validated before the product forwards requests.

A.5.18 Access rights partial match
degrades

Explicit access-rights assignment can restrict the product’s ability to act as an unintended proxy for external actors.

A.8.2 Privileged access rights partial match
degrades

Privileged-access controls limit the rights the product may exercise on behalf of upstream callers, mitigating confused-deputy abuse.

A.8.20 Networks security partial match
degrades

Network-security controls can enforce source validation and prevent the product from blindly proxying traffic to external actors.

A.8.25 Secure development life cycle partial match
prevents

Secure-SDLC practices include design reviews that detect and eliminate confused-deputy proxy patterns before deployment.

References