From 1006025d24f8fdd5b5ee8b3c15a714e6b24baaab Mon Sep 17 00:00:00 2001
From: Tim Cooper <tim.cooper@layeh.com>
Date: Tue, 12 Dec 2023 19:26:53 -0600
Subject: [PATCH] fix IPv6Prefix bug

---
 attribute.go      | 13 ++++++++-----
 attribute_test.go | 13 +++++++++++++
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/attribute.go b/attribute.go
index 473d230..366da72 100644
--- a/attribute.go
+++ b/attribute.go
@@ -467,18 +467,21 @@ func IPv6Prefix(a Attribute) (*net.IPNet, error) {
 	}
 
 	prefixLength := int(a[1])
-	if (len(a)-2)*8 < prefixLength {
+	if prefixLength > net.IPv6len*8 {
 		return nil, errors.New("invalid prefix length")
 	}
 
 	ip := make(net.IP, net.IPv6len)
 	copy(ip, a[2:])
 
-	// clear final non-mask bits
-	if i := uint(prefixLength % 8); i != 0 {
-		for ; i < 8; i++ {
-			ip[prefixLength/8] &^= 1 << (7 - i)
+	bit := uint(prefixLength % 8)
+	for octet := prefixLength / 8; octet < len(ip); octet++ {
+		for ; bit < 8; bit++ {
+			if ip[octet]&(1<<(7-bit)) != 0 {
+				return nil, errors.New("invalid prefix data")
+			}
 		}
+		bit = 0
 	}
 
 	return &net.IPNet{
diff --git a/attribute_test.go b/attribute_test.go
index 9935b4e..20655d0 100644
--- a/attribute_test.go
+++ b/attribute_test.go
@@ -130,6 +130,19 @@ func TestIPv6Prefix(t *testing.T) {
 	}
 }
 
+func TestIPv6Prefix_issue118(t *testing.T) {
+	ipNet, err := IPv6Prefix([]byte{0x00, 0x40, 0x20, 0x01, 0x15, 0x30, 0x10, 0x0e})
+	if err != nil {
+		t.Fatalf("unexpected error: %s", err)
+	}
+	if expected := net.ParseIP("2001:1530:100e::"); !ipNet.IP.Equal(expected) {
+		t.Fatalf("got %v, expected %v", ipNet.IP, expected)
+	}
+	if ones, size := ipNet.Mask.Size(); ones != 64 || size != 128 {
+		t.Fatalf("got %v:%v, expected 64, 128", ones, size)
+	}
+}
+
 func ipNetEquals(a, b *net.IPNet) bool {
 	if a == nil && b == nil {
 		return true