Apache HTTPD 2.4 AH02429 error with phantom response header

Question

I've an Apache HTTPD 2.4.37 which, since this morning, is responding with 500 and [Mon Jan 24 12:27:03.132322 2022] [http:error] [pid 3650579:tid 140496433313536] [client 10.42.0.47:53214] AH02429: Response header name '[Mon Jan 24 12' contains invalid characters, aborting request while trying to render a Perl application.

If I try to call the website with curl -v I cannot see such "header" in the response headers.
Morevoer, if I copy the conf.modules.d folder from an Apache HTTPD 2.4.6 version it then works as expected.

Answer 1

After some backtracking, it seems like that a request header I'm setting it's breaking the request when this is empty. I was following https://httpd.apache.org/docs/2.4/env.html#fixheader to propagate an "invalid" (for Apache HTTPD) header and the regex used there matches even if the value of the header is empty (ie the header is not part of the request at all).

In such a case, for some reason the request gets broken.