# CVE-2026-1525: undici is vulnerable to Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

> Attackers can cause HTTP request smuggling or denial of service by exploiting inconsistent handling of duplicate Content-Length headers in undici.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 12 Mar 2026
- **Updated:** 12 Mar 2026
- **Also known as:** BELL-CVE-2026-1525, CLEANSTART-2026-CC67789, CLEANSTART-2026-NI54084, CLSA-2026-1779065182, DEBIAN-CVE-2026-1525, ECHO-7e19-b46c-b4e1, GHSA-2mjp-6q6p-2qxm, MINI-4xrw-4529-f744, MINI-643j-8cv9-9fv5, MINI-8ff6-wv6x-w9rv, MINI-9wjp-28qw-wcfj, MINI-fcrp-4h5m-cg8r, MINI-gr9p-m7v5-9rxv, MINI-hxmw-xmg6-pc3v, MINI-mm3x-mxr3-wr2q, MINI-ppp5-56vp-xp4m, MINI-q7m3-2969-xmrv, MINI-v453-gxrh-53hf, ROOT-APP-NPM-CVE-2026-1525, UBUNTU-CVE-2026-1525

## What is CVE-2026-1525?

CVE-2026-1525 is a medium-severity vulnerability in Nodejs Undici, published on 12 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Undici allows duplicate HTTP Content-Length headers when they are provided in an array with case-variant names (e.g., Content-Length and content-length). This produces malformed HTTP/1.1 requests with multiple conflicting Content-Length values on the wire.

Who is impacted:

  *  Applications using undici.request(), undici.Client, or similar low-level APIs with headers passed as flat arrays
  *  Applications that accept user-controlled header names without case-normalization

Potential consequences:

  *  Denial of Service: Strict HTTP parsers (proxies, servers) will reject requests with duplicate Content-Length headers (400 Bad Request)
  *  HTTP Request Smuggling: In deployments where an intermediary and backend interpret duplicate headers inconsistently (e.g., one uses the first value, the other uses the last), this can enable request smuggling attacks leading to ACL bypass, cache poisoning, or credential hijacking

Weakness: CWE-444 (Inconsistent Interpretation of HTTP Requests)

## Which versions are affected by CVE-2026-1525?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `undici` | npm | All versions | 6.24.0 |
| `undici` | npm | All versions | 6.24.0 |
| `undici` | npm | From 7.0.0 | 7.24.0 |
| `undici` | npm | From 7.0.0 | 7.24.0 |

Products: Nodejs Undici.

## How do I fix CVE-2026-1525?

Upgrade undici to a fixed version for your release line: 6.24.0 or 7.24.0.

## Is CVE-2026-1525 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-1525 exists.

## 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-2026-1525 and fix it if it is affected.

What it is: CVE-2026-1525 is a medium-severity vulnerability in Nodejs Undici, published on 12 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-1525 exists.

Affected packages and fixed versions:
- undici (npm): affected all versions; fixed in 6.24.0
- undici (npm): affected all versions; fixed in 6.24.0
- undici (npm): affected from 7.0.0; fixed in 7.24.0
- undici (npm): affected from 7.0.0; fixed in 7.24.0

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-2026-1525/ (Markdown: https://insights.sbomify.com/CVE-2026-1525/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-1525
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-1525
- OSV: https://osv.dev/vulnerability/CVE-2026-1525

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-1525/
Summaries are generated and may contain mistakes; the official description above is the record.