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CVE-2026-86472 medium CVSS 4.8

Vulnerability in fast-uri (CVE-2026-86472)

Summary

Vulnerability in fast-uri (CVE-2026-86472). Risk of unauthorized operations or information disclosure. Exploitable via parse. Mitigation: upgrade to 4.1.5 or later.

Original description

The description as published by the source.

Impact

fast-uri folds the host to lowercase before it percent-decodes the host, so a percent-encoded uppercase unreserved octet such as %41 decodes to a literal A that is never folded. For a scheme-relative reference (//host) there is no scheme, so the host canonicalization that repairs this on a scheme-bearing URL does not run. As a result parse, normalize, and equal disagree on the same host: parse("//%41.com").host returns "A.com" while parse("//a.com").host and parse("//A.com").host return "a.com", and equal("//%41.com", "//a.com") is false even though equal("//A.com", "//a.com") is true. An application that makes a case-sensitive host decision on fast-uri output for a scheme-relative reference, for example a host allowlist or denylist that compares parse(url).host or uses fast-uri.equal, can be steered past the check with a percent-encoded uppercase octet. Because a hostname is case-insensitive in DNS and HTTP routing, the evading spelling reaches the same host the check meant to gate, so the effect is check evasion rather than reaching a different registrable host.

Patches

Upgrade to fast-uri 4.1.5, 3.1.8, or 2.4.7.

Workarounds

Compare hosts case-insensitively (lowercase the parsed host before any allowlist or denylist decision), or avoid making case-sensitive host decisions on scheme-relative input until upgrading.

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-2026-86472 has been found in fast-uri.
Attackers can target a specific entry point like parse over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 4.8/10.
What to do: upgrade fast-uri to 4.1.5 or later.
If unsure, ask your IT team or search "fast-uri CVE-2026-86472" on the vendor's site.

CVE-2026-86472 (fast-uri) — CWE-178 / CVSS v3 4.8
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: parse / normalize / equal / false
Patched: 4.1.5 — apply immediately
Workaround: Compare hosts case-insensitively (lowercase the parsed host before any allowlist or denylist decision), or avoid making case-sensitive host decisions on scheme-relative input until upgrading.
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-178) exists in fast-uri. Attackers reach the vulnerable code path via parse without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of fast-uri. Attack surface: parse / normalize / equal / false.

🔥 Severity

Severity: Medium (CVSS 4.8/10). Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 4.1.5.

🛡️ Workaround

Workaround: Compare hosts case-insensitively (lowercase the parsed host before any allowlist or denylist decision), or avoid making case-sensitive host decisions on scheme-relative input until upgrading.

🔍 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 
    Compare hosts case-insensitively (lowercase the parsed host before any allowlist or denylist decision), or avoid making case-sensitive host decisions on scheme-relative input until upgrading.

    パッチが適用されるまでの応急処置として、Compare hosts case-insensitively (lowercase the parsed host before any allowlist or denylist decision), or avoid making case-sensitive host decisions on scheme-relative input until upgrading. 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 4.1.5

    ステージング環境で 4.1.5 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":"4.1.5"}]}]

References

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