Vulnerability in fast-uri (CVE-2026-75899)
Summary
Vulnerability in fast-uri (CVE-2026-75899). 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 decodes a hostname's percent escapes twice in a single normalize() or resolve() call: once during parsing and again during authority recomposition. A nested percent-encoded host therefore survives the first decode and is turned into a live destination by the second, so normalize('http://%256c%256f%2563%2561%256c%2568%256f%2573%2574/') returns http://localhost/. Applications that normalize or resolve an untrusted URI before an SSRF check, redirect validation, or host allowlist can be steered to a different destination, including internal addresses such as loopback or a cloud metadata endpoint, than the encoded input appeared to contain. This is an incomplete-fix variant of CVE-2026-6322, whose encoded-authority-delimiter fix introduced the second decode.
Patches
Fixed in fast-uri 2.4.5, 3.1.6, and 4.1.3.
Workarounds
Reject untrusted URIs whose host component contains an encoded percent sign (%25) before passing them to normalize() or resolve().
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-2026-75899 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-2026-75899" on the vendor's site.
CVE-2026-75899 (fast-uri) — CWE-174 / CVSS v3 7.5
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Patched: 4.1.3 — apply immediately
Workaround: Reject untrusted URIs whose host component contains an encoded percent sign (%25) before passing them to normalize() or resolve().
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-174) exists in fast-uri.
Check the vendor advisory for the affected versions of fast-uri.
Severity: High (CVSS 7.5/10). Data can be tampered with by attackers.
Update to 4.1.3.
Workaround: Reject untrusted URIs whose host component contains an encoded percent sign (%25) before passing them to normalize() or resolve().
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
Reject untrusted URIs whose host component contains an encoded percent sign (`%25`) before passing them to `normalize()` or `resolve()`.パッチが適用されるまでの応急処置として、Reject untrusted URIs whose host component contains an encoded percent sign (
%25) before passing them tonormalize()orresolve(). 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 4.1.3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。
-
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
References
- ATLASSIAN JSWSERVER-26967
- 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