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CVE-2026-86818 medium CVSS 4.8

Vulnerability in fast-uri (CVE-2026-86818)

Summary

Vulnerability in fast-uri (CVE-2026-86818). Risk of unauthorized operations or information disclosure. Exploitable via mailto. Mitigation: upgrade to 4.1.5 or later.

Original description

The description as published by the source.

Impact

fast-uri's mailto scheme parser compares each query field name to the reserved names (to, subject, body) while the name is still percent-encoded, and only percent-decodes it when storing it as a generic header. On serialize, the decoded name is re-emitted, so a field name such as %74o (percent-encoded to) is not recognized as a recipient at parse time (parse().to shows only the legitimate recipient) but materializes as a literal to= field after serialize(), and reparsing then treats it as a recipient. The same technique smuggles subject and body through %73ubject and %62ody.

An application that parses an untrusted mailto URI, makes a display, allowlist, or logging decision on parse().to, then re-serializes the result and passes the serialized string to a mail client or an outbound send path can gain an attacker-chosen recipient, subject, or body that was not visible when the recipient list was checked. A scanner inspecting the raw input for an extra to= sees nothing, because the injected field appears only after fast-uri serializes.

Patches

Upgrade to fast-uri 4.1.5.

Workarounds

Percent-decode and compare mailto field names case-insensitively before trusting parse().to, or re-check the recipient list on the serialized output rather than only on the initial parse, 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-2026-86818 has been found in fast-uri.
Attackers can target a specific entry point like mailto 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-2026-86818" on the vendor's site.

CVE-2026-86818 (fast-uri) — CWE-172 / CVSS v3 4.8
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: mailto / subject / body
Patched: 4.1.5 — apply immediately
Workaround: Percent-decode and compare mailto field names case-insensitively before trusting parse().to, or re-check the recipient list on the serialized output rather than only on the initial parse, 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-172) exists in fast-uri. Attackers reach the vulnerable code path via mailto without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of fast-uri. Attack surface: mailto / subject / body.

🔥 Severity

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

🔧 How to fix

Update to 4.1.5.

🛡️ Workaround

Workaround: Percent-decode and compare mailto field names case-insensitively before trusting parse().to, or re-check the recipient list on the serialized output rather than only on the initial parse, 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 
    Percent-decode and compare mailto field names case-insensitively before trusting `parse().to`, or re-check the recipient list on the serialized output rather than only on the initial parse, until upgrading.

    パッチが適用されるまでの応急処置として、Percent-decode and compare mailto field names case-insensitively before trusting parse().to, or re-check the recipient list on the serialized output rather than only on the initial parse, 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"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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