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CVE-2026-75975 high CVSS 7.5

Vulnerability in fast-uri (CVE-2026-75975)

Summary

Vulnerability in fast-uri (CVE-2026-75975). Data can be tampered with by attackers. Mitigation: upgrade to 4.1.3 or later.

Original description

The description as published by the source.

Impact

fast-uri does not validate the complete RFC 3986 grammar for bracketed IPv6 literals, so a malformed literal with invalid trailing text is silently truncated to a different valid IPv6 address with no error reported. For example, normalize('http://[::not-valid]/private') returns http://[::]/private, and [fc00::not-hex] and [fe80::not-hex] collapse to [fc00::] and [fe80::]. An application that normalizes an untrusted URL before an outbound request, redirect, or host-policy check can be routed to a local or private address such as loopback (::1), unique-local, or link-local. Because parse().error is unset for these inputs, checking it does not protect the consumer.

Patches

Upgrade to fast-uri 2.4.5, 3.1.6, or 4.1.3. Malformed IPv6 literals are now rejected with a host error instead of being normalized to a valid address.

Workarounds

Reject untrusted URLs whose host is a bracketed IPv6 literal before passing them to fast-uri, or route outbound requests against an explicit allowlist of addresses rather than trusting the normalized host.

Source: NIST National Vulnerability Database NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-75975 has been found in fast-uri.
Data can be tampered with by attackers. CVSS score: 7.5/10.
What to do: upgrade fast-uri to 4.1.3 or later.
If unsure, ask your IT team or search "fast-uri CVE-2026-75975" on the vendor's site.

CVE-2026-75975 (fast-uri) — CWE-20 / CVSS v3 7.5
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Patched: 4.1.3 — apply immediately
Workaround: Reject untrusted URLs whose host is a bracketed IPv6 literal before passing them to fast-uri, or route outbound requests against an explicit allowlist of addresses rather than trusting the normalized host.
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-20) exists in fast-uri.

📍 Affected scope

Check the vendor advisory for the affected versions of fast-uri.

🔥 Severity

Severity: High (CVSS 7.5/10). Data can be tampered with by attackers.

🔧 How to fix

Update to 4.1.3.

🛡️ Workaround

Workaround: Reject untrusted URLs whose host is a bracketed IPv6 literal before passing them to fast-uri, or route outbound requests against an explicit allowlist of addresses rather than trusting the normalized host.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'fast-uri' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'fast-uri' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で fast-uri を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply temporary workaround mitigate 
    Reject untrusted URLs whose host is a bracketed IPv6 literal before passing them to `fast-uri`, or route outbound requests against an explicit allowlist of addresses rather than trusting the normalized host.

    パッチが適用されるまでの応急処置として、Reject untrusted URLs whose host is a bracketed IPv6 literal before passing them to fast-uri, or route outbound requests against an explicit allowlist of addresses rather than trusting the normalized host. を実施。回避策の副作用 (機能低下) を確認した上で。

  3. 3
    Apply patch patch 
    Upgrade fast-uri to 4.1.3

    ステージング環境で 4.1.3 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  4. 4
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

npm fast-uri
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"4.1.3"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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