The undici WebSocket client is vulnerable to a denial-of-service attack via unbounded memory consumption during permessage-deflate decompression. When a WebSocket connection negotiates the permessage-deflate extension, the client decompresses incoming compressed frames without enforcing any limit on the decompressed data size. A malicious WebSocket server can send a small compressed frame (a "decompression bomb") that expands to an extremely large size in memory, causing the Node.js process to exhaust available memory and crash or become unresponsive.
The vulnerability exists in the PerMessageDeflate.decompress() method, which accumulates all decompressed chunks in memory and concatenates them into a single Buffer without checking whether the total size exceeds a safe threshold.
CVSS Score
7.5
EPSS Score
0.011
Published
2026-03-12
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
0.005
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 for Node.js. Starting with version 2.0.0 and prior to version 5.19.1, the undici library does not protect `host` HTTP header from CRLF injection vulnerabilities. This issue is patched in Undici v5.19.1. As a workaround, sanitize the `headers.host` string before passing to undici.
CVSS Score
6.5
EPSS Score
0.011
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
0.013
Published
2022-08-15