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
CVSS Score
6.5
EPSS Score
0.005
Published
2026-03-12
Undici is an HTTP/1.1 client for Node.js. Prior to 7.18.0 and 6.23.0, the number of links in the decompression chain is unbounded and the default maxHeaderSize allows a malicious server to insert thousands compression steps leading to high CPU usage and excessive memory allocation. This vulnerability is fixed in 7.18.0 and 6.23.0.
CVSS Score
5.9
EPSS Score
0.004
Published
2026-01-14
Undici is an HTTP/1.1 client, written from scratch for Node.js. Undici cleared Authorization and Proxy-Authorization headers for `fetch()`, but did not clear them for `undici.request()`. This vulnerability was patched in version(s) 5.28.4 and 6.11.1.
CVSS Score
3.9
EPSS Score
0.007
Published
2024-04-04
Undici is an HTTP/1.1 client, written from scratch for Node.js. An attacker can alter the `integrity` option passed to `fetch()`, allowing `fetch()` to accept requests as valid even if they have been tampered. This vulnerability was patched in version(s) 5.28.4 and 6.11.1.
CVSS Score
2.6
EPSS Score
0.008
Published
2024-04-04
Undici is an HTTP/1.1 client, written from scratch for Node.js. Undici already cleared Authorization headers on cross-origin redirects, but did not clear `Proxy-Authentication` headers. This issue has been patched in versions 5.28.3 and 6.6.1. Users are advised to upgrade. There are no known workarounds for this vulnerability.
CVSS Score
3.9
EPSS Score
0.008
Published
2024-02-16
Undici is an HTTP/1.1 client written from scratch for Node.js. Prior to version 5.26.2, Undici already cleared Authorization headers on cross-origin redirects, but did not clear `Cookie` headers. By design, `cookie` headers are forbidden request headers, disallowing them to be set in RequestInit.headers in browser environments. Since undici handles headers more liberally than the spec, there was a disconnect from the assumptions the spec made, and undici's implementation of fetch. As such this may lead to accidental leakage of cookie to a third-party site or a malicious attacker who can control the redirection target (ie. an open redirector) to leak the cookie to the third party site. This was patched in version 5.26.2. There are no known workarounds.
CVSS Score
3.9
EPSS Score
0.012
Published
2023-10-12
Undici is an HTTP/1.1 client for Node.js. Prior to version 5.19.1, the `Headers.set()` and `Headers.append()` methods are vulnerable to Regular Expression Denial of Service (ReDoS) attacks when untrusted values are passed into the functions. This is due to the inefficient regular expression used to normalize the values in the `headerValueNormalize()` utility function. This vulnerability was patched in v5.19.1. No known workarounds are available.
CVSS Score
7.5
EPSS Score
0.013
Published
2023-02-16
undici is an HTTP/1.1 client, written from scratch for Node.js.`=< undici@5.8.0` users are vulnerable to _CRLF Injection_ on headers when using unsanitized input as request headers, more specifically, inside the `content-type` header. Example: ``` import { request } from 'undici' const unsanitizedContentTypeInput = 'application/json\r\n\r\nGET /foo2 HTTP/1.1' await request('http://localhost:3000, { method: 'GET', headers: { 'content-type': unsanitizedContentTypeInput }, }) ``` The above snippet will perform two requests in a single `request` API call: 1) `http://localhost:3000/` 2) `http://localhost:3000/foo2` This issue was patched in Undici v5.8.1. Sanitize input when sending content-type headers using user input as a workaround.
CVSS Score
5.3
EPSS Score
0.015
Published
2022-08-15
undici is an HTTP/1.1 client, written from scratch for Node.js.`undici` is vulnerable to SSRF (Server-side Request Forgery) when an application takes in **user input** into the `path/pathname` option of `undici.request`. If a user specifies a URL such as `http://127.0.0.1` or `//127.0.0.1` ```js const undici = require("undici") undici.request({origin: "http://example.com", pathname: "//127.0.0.1"}) ``` Instead of processing the request as `http://example.org//127.0.0.1` (or `http://example.org/http://127.0.0.1` when `http://127.0.0.1 is used`), it actually processes the request as `http://127.0.0.1/` and sends it to `http://127.0.0.1`. If a developer passes in user input into `path` parameter of `undici.request`, it can result in an _SSRF_ as they will assume that the hostname cannot change, when in actual fact it can change because the specified path parameter is combined with the base URL. This issue was fixed in `undici@5.8.1`. The best workaround is to validate user input before passing it to the `undici.request` call.
CVSS Score
5.3
EPSS Score
0.017
Published
2022-08-12
Authorization headers are cleared on cross-origin redirect. However, cookie headers which are sensitive headers and are official headers found in the spec, remain uncleared. There are active users using cookie headers in undici. This may lead to accidental leakage of cookie to a 3rd-party site or a malicious attacker who can control the redirection target (ie. an open redirector) to leak the cookie to the 3rd party site. This was patched in v5.7.1. By default, this vulnerability is not exploitable. Do not enable redirections, i.e. `maxRedirections: 0` (the default).
CVSS Score
3.7
EPSS Score
0.007
Published
2022-07-21