diff --git a/h3_qpack_prefix.t b/h3_qpack_prefix.t
new file mode 100644
index 00000000..bd96e80d
--- /dev/null
+++ b/h3_qpack_prefix.t
@@ -0,0 +1,106 @@
+#!/usr/bin/perl
+
+# (C) Daniel Carlier
+
+# Tests for HTTP/3 QPACK field section prefix validation.
+
+###############################################################################
+
+use warnings;
+use strict;
+
+use Test::More;
+
+BEGIN { use FindBin; chdir($FindBin::Bin); }
+
+use lib 'lib';
+use Test::Nginx;
+use Test::Nginx::HTTP3;
+
+###############################################################################
+
+select STDERR; $| = 1;
+select STDOUT; $| = 1;
+
+my $t = Test::Nginx->new()->has(qw/http http_v3 cryptx/)
+	->has_daemon('openssl')->plan(2)
+	->write_file_expand('nginx.conf', <<'EOF');
+
+%%TEST_GLOBALS%%
+
+daemon off;
+
+events {
+}
+
+http {
+    %%TEST_GLOBALS_HTTP%%
+
+    ssl_certificate_key localhost.key;
+    ssl_certificate localhost.crt;
+
+    server {
+        listen       127.0.0.1:%%PORT_8980_UDP%% quic;
+        server_name  localhost;
+
+        location / {
+            return 200;
+        }
+    }
+}
+
+EOF
+
+$t->write_file('openssl.conf', <<EOF);
+[ req ]
+default_bits = 2048
+encrypt_key = no
+distinguished_name = req_distinguished_name
+[ req_distinguished_name ]
+EOF
+
+my $d = $t->testdir();
+
+foreach my $name ('localhost') {
+	system('openssl req -x509 -new '
+		. "-config $d/openssl.conf -subj /CN=$name/ "
+		. "-out $d/$name.crt -keyout $d/$name.key "
+		. ">>$d/openssl.out 2>&1") == 0
+		or die "Can't create certificate for $name: $!\n";
+}
+
+$t->run();
+
+###############################################################################
+
+my ($s, $sid, $frames, $frame);
+
+# baseline: normal request works
+
+$s = Test::Nginx::HTTP3->new();
+$sid = $s->new_stream({ path => '/' });
+$frames = $s->read(all => [{ sid => $sid, fin => 1 }]);
+
+($frame) = grep { $_->{type} eq "HEADERS" } @$frames;
+is($frame->{headers}->{':status'}, 200, 'normal request');
+
+# field section prefix with oversized delta_base (value 2^32)
+# followed by a post-base indexed field reference (index 0)
+# which forces the decoder to use base for a dynamic table lookup
+#
+# On 32-bit: triggers truncation check (pint.value > ngx_uint_t max)
+# On 64-bit: lookup at base + 0 = 2^32 fails dynamic table bounds check
+
+$sid = $s->new_stream({ headers => [
+	{ name => ':method', value => 'GET', mode => 0 },
+	{ name => ':scheme', value => 'http', mode => 0 },
+	{ name => ':path', value => '/', mode => 0 },
+	{ name => ':authority', value => 'localhost', mode => 4 }],
+	raw_prefix => pack("C*", 0x00, 0x7F, 0x81, 0xFF, 0xFF, 0xFF, 0x0F)
+		. pack("C", 0x10) });
+$frames = $s->read(all => [{ type => 'RESET_STREAM' }], wait => 2);
+
+($frame) = grep { $_->{type} eq "RESET_STREAM" } @$frames;
+ok($frame, 'oversized delta_base - stream reset');
+
+###############################################################################
diff --git a/lib/Test/Nginx/HTTP3.pm b/lib/Test/Nginx/HTTP3.pm
index e8840958..092b484b 100644
--- a/lib/Test/Nginx/HTTP3.pm
+++ b/lib/Test/Nginx/HTTP3.pm
@@ -452,11 +452,15 @@ sub build_new_stream {
 
 	# encoded field section prefix
 
-	my $table = $self->{dynamic_encode};
-	my $ric = $uri->{ric} ? $uri->{ric} : @$table ? @$table + 1 : 0;
-	my $base = $uri->{base} || 0;
-	$base = $base < 0 ? 0x80 + abs($base) - 1 : $base;
-	$input = pack("CC", $ric, $base) . $input;
+	if ($uri->{raw_prefix}) {
+		$input = $uri->{raw_prefix} . $input;
+	} else {
+		my $table = $self->{dynamic_encode};
+		my $ric = $uri->{ric} ? $uri->{ric} : @$table ? @$table + 1 : 0;
+		my $base = $uri->{base} || 0;
+		$base = $base < 0 ? 0x80 + abs($base) - 1 : $base;
+		$input = pack("CC", $ric, $base) . $input;
+	}
 
 	# set length, attach headers, body