Expose the client cert DN (as TLS_PEER_SUBJECT) to fastcgi responders,
so that TLS client certs can be used for authorisation (not just for
authentication).

This is a scaled-down version of a patch I proposed some years ago[1].
That version also passed through the {issuer,serial} tuple (that
identifies any certificate uniquely), which made it rather more
intrusive ... and I assume that's why it wasn't accepted at the time.

All the recent activity in httpd reminded me that I should really
get around to having another go at getting it in the tree.

Today's version passes through only the DN, which I'm hoping makes it
unobtrusive enough to consider.  That's the bare minimum to make TLS
clients certs usable by fastcgi responders for authorisation purposes
and will suffice in most scenarios, as it identifies the entity to whom
the certificate was issued.

It does *not* differentiate between multiple valid client certs issued
to the same entity (whereas the patch in [1] did) ... but in the real
world the proportion of authorisation processes that genuinely need to
care about such things is fairly low (identifying the entity is usually
enough to make an authorisation decision).

Thoughts?

[1] https://marc.info/?l=openbsd-tech&m=153112641425436&w=2

Index: usr.sbin/httpd/httpd.conf.5
===================================================================
RCS file: /cvs/src/usr.sbin/httpd/httpd.conf.5,v
diff -u -p -r1.129 httpd.conf.5
--- usr.sbin/httpd/httpd.conf.5	18 Jan 2026 16:38:02 -0000	1.129
+++ usr.sbin/httpd/httpd.conf.5	28 Feb 2026 12:52:15 -0000
@@ -453,6 +453,11 @@ The revision of the HTTP specification u
 .It Ic SERVER_SOFTWARE
 The server software name of
 .Xr httpd 8 .
+.It Ic TLS_PEER_SUBJECT
+The subject
+.Pq distinguished name
+of the TLS client certficate
+.Pq omitted when TLS client verification is not in use .
 .It Ic TLS_PEER_VERIFY
 A variable that is set to a comma separated list of TLS client verification
 features in use
Index: usr.sbin/httpd/server_fcgi.c
===================================================================
RCS file: /cvs/src/usr.sbin/httpd/server_fcgi.c,v
diff -u -p -r1.99 server_fcgi.c
--- usr.sbin/httpd/server_fcgi.c	2 Jan 2026 08:45:16 -0000	1.99
+++ usr.sbin/httpd/server_fcgi.c	28 Feb 2026 12:52:15 -0000
@@ -34,6 +34,8 @@
 #include <event.h>
 #include <unistd.h>
 
+#include <tls.h>
+
 #include "httpd.h"
 #include "http.h"
 
@@ -269,6 +271,12 @@ server_fcgi(struct httpd *env, struct cl
 		if (srv_conf->tls_flags != 0 && fcgi_add_param(&param,
 		    "TLS_PEER_VERIFY", printb_flags(srv_conf->tls_flags,
 		    TLSFLAG_BITS), clt) == -1) {
+			errstr = "failed to encode param";
+			goto fail;
+		}
+		if (tls_peer_cert_provided(clt->clt_tls_ctx) &&
+		    fcgi_add_param(&param, "TLS_PEER_SUBJECT",
+		    tls_peer_cert_subject(clt->clt_tls_ctx), clt) == -1) {
 			errstr = "failed to encode param";
 			goto fail;
 		}