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.