# CVE-2026-9679: undici vulnerable to HTTP header injection via Set-Cookie percent-decoding

> Attackers can inject HTTP headers via undici's cookie parser, enabling session fixation or cache poisoning in applications forwarding parsed cookies.

- **Severity:** Medium 5.9 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 17 Jun 2026
- **Updated:** 23 Jun 2026
- **Also known as:** AZL-91188, CLEANSTART-2026-RT55969, CLEANSTART-2026-YY11286, DEBIAN-CVE-2026-9679, ECHO-d5e0-7732-0397, GHSA-p88m-4jfj-68fv, MINI-565f-pw9v-hmwp, MINI-67g2-6cpp-6wqg, MINI-85rc-h96g-rr5q, MINI-8wmv-h2x3-936g, MINI-f8xf-6m4f-gv29, MINI-fj6j-7582-33fw, MINI-fprg-hxrh-52m8, MINI-fw8j-pmpx-2x9m, MINI-hf4v-c9jw-fph6, MINI-rhqx-49x5-3hv8, MINI-rwr4-98rr-8jx8, MINI-v477-cm3p-2wvg, MINI-vg6g-wqhc-fhpm, MINI-vgmf-qp4v-vh4g, MINI-w358-3h52-pw65, MINI-x3w4-pw3j-r87c, ROOT-APP-NPM-CVE-2026-9679, UBUNTU-CVE-2026-9679

## What is CVE-2026-9679?

CVE-2026-9679 is a medium-severity vulnerability in Nodejs Undici, published on 17 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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 PR #3789.

Patches:
Upgrade to undici v6.26.0, 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.

Weakness: CWE-93 (Improper Neutralization of CRLF Sequences)

## Which versions are affected by CVE-2026-9679?

6 open source packages ship an affected version, including undici. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `undici` | npm | All versions | 6.27.0 |
| `undici` | npm | All versions | 6.27.0 |
| `undici` | npm | From 7.0.0 | 7.28.0 |
| `undici` | npm | From 7.0.0 | 7.28.0 |
| `undici` | npm | From 8.0.0 | 8.5.0 |
| `undici` | npm | From 8.0.0 | 8.5.0 |

Products: Nodejs Undici.

## How do I fix CVE-2026-9679?

Upgrade undici to a fixed version for your release line: 6.27.0, 7.28.0 or 8.5.0.

## Is CVE-2026-9679 being exploited?

There is no known exploitation of CVE-2026-9679 in the wild and no public exploit code.

## Other vulnerabilities in Nodejs Undici

- [CVE-2026-109855](https://insights.sbomify.com/CVE-2026-109855/): undici WebSocket client vulnerable to Denial of Service via failed HTTP/2 handshake (high 7.5)
- [CVE-2026-109869](https://insights.sbomify.com/CVE-2026-109869/): undici vulnerable to Denial of Service via unhandled error on excessive content-encodings (high 7.5)
- [CVE-2026-87018](https://insights.sbomify.com/CVE-2026-87018/): undici vulnerable to Denial of Service via TLS 1.2 certificate-changing renegotiation (high 7.5)
- [CVE-2026-19534](https://insights.sbomify.com/CVE-2026-19534/): undici vulnerable to Denial of Service via unrequested WebSocket subprotocol (high 7.5)
- [CVE-2026-12151](https://insights.sbomify.com/CVE-2026-12151/): undici WebSocket client vulnerable to denial of service via fragment count bypass (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-9679 and fix it if it is affected.

What it is: CVE-2026-9679 is a medium-severity vulnerability in Nodejs Undici, published on 17 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-9679 in the wild and no public exploit code.

Affected packages and fixed versions:
- undici (npm): affected all versions; fixed in 6.27.0
- undici (npm): affected all versions; fixed in 6.27.0
- undici (npm): affected from 7.0.0; fixed in 7.28.0
- undici (npm): affected from 7.0.0; fixed in 7.28.0
- undici (npm): affected from 8.0.0; fixed in 8.5.0
- undici (npm): affected from 8.0.0; fixed in 8.5.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-9679/ (Markdown: https://insights.sbomify.com/CVE-2026-9679/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-9679
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-9679
- OSV: https://osv.dev/vulnerability/CVE-2026-9679

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-9679/
Summaries are generated and may contain mistakes; the official description above is the record.