Vulnerability in fast-uri (CVE-2026-16221)
Summary
Vulnerability in fast-uri (CVE-2026-16221). Data can be tampered with by attackers. Exploitable via URL. Mitigation: upgrade to 4.1.1 or later.
Original description
The description as published by the source.
Impact
fast-uri v4.1.0 and earlier do not treat a literal backslash (U+005C) as an authority delimiter. Node's native WHATWG URL (used by fetch(), undici, and Node's http/https clients) normalizes \ to / for special schemes (http, https, ws, wss, ftp, file), so the two parsers extract different hosts from the same input string.
For example, http://evil.com\@allowed.com is treated by fast-uri as host allowed.com with userinfo evil.com\, while Node's WHATWG URL parser and fetch() see host evil.com with path /@allowed.com.
Applications that use fast-uri to enforce host-based policy (allowlists, denylists, loopback/SSRF filtering, redirect validation, outbound proxy routing) before passing the same URL into Node's URL or fetch() consumers see a policy/use desync and can be steered to an unintended destination, including cloud metadata endpoints, loopback, or internal hosts.
Patches
Upgrade to fast-uri v4.1.1, v3.1.4, or v2.4.3.
Workarounds
None. Upgrade to the patched version.
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-2026-16221 has been found in fast-uri.
Attackers can target a specific entry point like URL over the network to misuse the product.
Data can be tampered with by attackers. CVSS score: 7.5/10.
What to do: upgrade fast-uri to 4.1.1 or later.
If unsure, ask your IT team or search "fast-uri CVE-2026-16221" on the vendor's site.
CVE-2026-16221 (fast-uri) — CWE-436 / CVSS v3 7.5
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: URL / undici / http / https
Patched: 4.1.1 — apply immediately
Workaround: None.
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.
A vulnerability (CWE-436) exists in fast-uri. Attackers reach the vulnerable code path via URL without authentication.
Check the vendor advisory for the affected versions of fast-uri. Attack surface: URL / undici / http / https.
Severity: High (CVSS 7.5/10). Data can be tampered with by attackers.
Update to 4.1.1.
Workaround: None.
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
-
1Identify 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。 -
2Apply temporary workaround mitigate
None.パッチが適用されるまでの応急処置として、None. を実施。回避策の副作用 (機能低下) を確認した上で。
-
3Apply patch patch
Upgrade fast-uri to 4.1.1ステージング環境で 4.1.1 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。
-
4Post-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
Fix status by distribution
Fixed packages published by each distribution for this CVE
- Debian DEBIAN-CVE-2026-16221 node-ajv
- Ubuntu UBUNTU-CVE-2026-16221 node-ajv
References
- ATLASSIAN JSWSERVER-26943
- patch cna.openjsf.org
- patch github.com
- web nvd.nist.gov
- web github.com
- web github.com
- web github.com
- web github.com
- web github.com
- web github.com
- web github.com