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CVE-2026-9679 medium CVSS 5.9

Vulnerability in undici (CVE-2026-9679)

Summary

Vulnerability in undici (CVE-2026-9679). Data can be tampered with by attackers. Exploitable via parseSetCookie. Mitigation: upgrade to 8.5.0 or later.

Original description

The description as published by the source.

Impact

undici's cookie parser in parseSetCookie percent-decodes cookie values via qsUnescape, turning encoded sequences like %0D%0A, %00, %3B, and %3D into their literal byte equivalents. RFC 6265 §5.4 does not specify any decoding and browsers do not decode either.

Applications that parse a Set-Cookie header and then forward the parsed value into a response header (proxies, middleware, SSR frameworks) become vulnerable to HTTP response header injection: an attacker-controlled upstream can inject arbitrary Set-Cookie, Location, or Cache-Control headers into the application's downstream response, enabling session fixation, open redirect, or cache poisoning.

Affected applications are those that use undici's cookie parsing (parseSetCookie, parseCookie, getSetCookies) and forward the parsed cookie value into a response header.

This was introduced in undici 7.0.0 via #3789.

Patches

Upgrade to undici v6.27.0, v7.28.0 or v8.5.0.

Workarounds

If upgrade is not immediately possible, do not forward values returned by parseSetCookie/parseCookie/getSetCookies directly into response headers; sanitize the value first to strip or reject CR, LF, NUL, ;, and = bytes.

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-2026-9679 has been found in undici.
Attackers can target a specific entry point like parseSetCookie over the network to misuse the product.
Data can be tampered with by attackers. CVSS score: 5.9/10.
What to do: upgrade undici to 8.5.0 or later.
If unsure, ask your IT team or search "undici CVE-2026-9679" on the vendor's site.

CVE-2026-9679 (undici) — CWE-93 / CVSS v3 5.9
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: parseSetCookie / qsUnescape / Location / parseCookie
Patched: 8.5.0 — apply immediately
Workaround: If upgrade is not immediately possible, do not forward values returned by parseSetCookie/parseCookie/getSetCookies directly into response headers; sanitize the value first to strip or reject CR, LF, NUL, ;, and = bytes.
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-93) exists in undici. Attackers reach the vulnerable code path via parseSetCookie without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of undici. Attack surface: parseSetCookie / qsUnescape / Location / parseCookie.

🔥 Severity

Severity: Medium (CVSS 5.9/10). Data can be tampered with by attackers.

🔧 How to fix

Update to 8.5.0.

🛡️ Workaround

Workaround: If upgrade is not immediately possible, do not forward values returned by parseSetCookie/parseCookie/getSetCookies directly into response headers; sanitize the value first to strip or reject CR, LF, NUL, ;, and = bytes.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'undici' . 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 'undici' . | 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 undici 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 
    If upgrade is not immediately possible, do not forward values returned by `parseSetCookie`/`parseCookie`/`getSetCookies` directly into response headers; sanitize the value first to strip or reject CR, LF, NUL, `;`, and `=` bytes.

    パッチが適用されるまでの応急処置として、If upgrade is not immediately possible, do not forward values returned by parseSetCookie/parseCookie/getSetCookies directly into response headers; sanitize the value first to strip or reject CR, LF, NUL, ;, and = bytes. 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 undici to 8.5.0

    ステージング環境で 8.5.0 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 undici
[{"type":"SEMVER","events":[{"introduced":"8.0.0"},{"fixed":"8.5.0"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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