1 file changed, 4 insertions, 0 deletions
diff --git a/source/html.c b/source/html.c
index b6e62a6..d1e28f8 100644
--- a/source/html.c
+++ b/source/html.c
@@ -367,6 +367,10 @@ void html_header_arg_in_quotes(const char *txt)
 			esc = "\\n";
 		else if (c == '"')
 			esc = "\\\"";
+		// A header line carries no other control byte either, and a web
+		// server refuses the whole response over one.
+		else if (c < 0x20 || c == 0x7f)
+			esc = "?";
 		if (esc) {
 			html_raw(txt, p - txt);
 			html(esc);