# CVE-2026-11525: undici vulnerable to Set-Cookie SameSite attribute downgrade via permissive substring matching

> Attackers can downgrade cookie SameSite protections by exploiting permissive substring matching in undici's Set-Cookie parser, weakening CSRF defenses.

- **Severity:** Low 3.7 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/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:** 17 Jun 2026
- **Also known as:** AZL-91194, CLEANSTART-2026-BZ34895, CLEANSTART-2026-RS05836, DEBIAN-CVE-2026-11525, ECHO-e368-9a50-56c5, GHSA-g8m3-5g58-fq7m, MINI-3f9r-3g9q-4mmw, MINI-42q5-pfx4-q8g4, MINI-5fjh-pxxq-qc5j, MINI-5h42-v9mj-c43v, MINI-5xrq-9fm4-m6c9, MINI-7m2m-g42c-2h44, MINI-8q7x-m2rp-j2gv, MINI-93c5-v357-m8j4, MINI-g2vg-3m5m-hc2c, MINI-gj37-h88h-xr99, MINI-hfp4-h7wf-6xv4, MINI-jcw6-j8pm-g33g, MINI-jjqr-xqg3-8vr3, MINI-pq7c-33fx-8p3f, MINI-q2fh-gq2p-gcg2, MINI-wwxj-cpm4-gfwg, ROOT-APP-NPM-CVE-2026-11525, UBUNTU-CVE-2026-11525

## What is CVE-2026-11525?

CVE-2026-11525 is a low-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:
When undici parses a Set-Cookie header, it accepts any SameSite attribute value that contains Strict, Lax, or None as a substring, rather than the case-insensitive exact match specified by RFC 6265. Non-spec values are silently mapped to one of the three standard tokens. For example, SameSite=NoneOfYourBusiness is parsed as None (the most permissive setting), and SameSite=StrictLax is parsed as Lax (a downgrade from Strict).

Affected applications are those that consume Set-Cookie headers from server responses (for example via undici's fetch or proxy code paths) and then forward or rely on the parsed sameSite attribute. A malicious or non-compliant server can coerce the consumer's view of a cookie's SameSite policy to a weaker value, silently degrading the SameSite enforcement the cookie is supposed to provide.

This was introduced in undici 5.15.0 when the cookies feature was added.

Patches:
Upgrade to undici v6.26.0, v7.28.0 or v8.5.0.

Workarounds:
After parsing a Set-Cookie header, validate that the resulting sameSite attribute is one of 'Strict', 'Lax', or 'None' (exact, case-insensitive) before forwarding or relying on it.

Weakness: CWE-183 (Permissive List of Allowed Inputs)

## Which versions are affected by CVE-2026-11525?

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-2026-11525?

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

## Is CVE-2026-11525 being exploited?

There is no known exploitation of CVE-2026-11525 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-2026-11525 and fix it if it is affected.

What it is: CVE-2026-11525 is a low-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-2026-11525 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-2026-11525/ (Markdown: https://insights.sbomify.com/CVE-2026-11525/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-11525
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-11525
- OSV: https://osv.dev/vulnerability/CVE-2026-11525

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-11525/
Summaries are generated and may contain mistakes; the official description above is the record.