CVE-2026-76172 fast-uri CVE debrief

Who should care

Developers and administrators using the fast-uri library in their applications, particularly those that allowlist on the parsed host or treat references with no authority as safe to resolve against their base, should be aware of this vulnerability and take necessary actions to protect their systems.

Technical summary

The fast-uri library for Node.js has a vulnerability that allows an attacker to manipulate the scheme component of a URI, potentially leading to off-site redirects, server-side request forgery, or address-policy bypass. The issue is caused by a legacy decoding pass over the scheme component and a lack of re-escaping during serialization. The affected versions are 2.3.1 up to but not including 2.4.5, 3.0.0 up to but not including 3.1.6, and 4.0.0 up to but not including 4.1.3. The issue is fixed in 2.4.5, 3.1.6, and 4.1.3.

Defensive priority

Upgrade to a patched version of fast-uri to prevent potential off-site redirects, server-side request forgery, or address-policy bypass.

Recommended defensive actions

• Upgrade to a patched version of fast-uri (2.4.5, 3.1.6, or 4.1.3) to prevent potential off-site redirects, server-side request forgery, or address-policy bypass.
• Allowlist on the parsed host and treat references with no authority as potentially malicious.
• Monitor for and block suspicious network-path references.
• Review compensating controls for exposed systems while remediation is scheduled and verified.
• Check relevant monitoring, detection, and logs for exposed assets that need extra review.
• Track exceptions, retest remediated assets, and close the item only after evidence is documented.
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.

Evidence notes

The issue is caused by a legacy decoding pass over the scheme component in fast-uri, which can lead to an attacker-chosen authority. The affected versions are 2.3.1 up to but not including 2.4.5, 3.0.0 up to but not including 3.1.6, and 4.0.0 up to but not including 4.1.3. Users should verify their deployments and consider upgrading to a patched version.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.