diff --git a/.changeset/soft-webs-rush.md b/.changeset/soft-webs-rush.md
new file mode 100644
index 000000000..efc08c840
--- /dev/null
+++ b/.changeset/soft-webs-rush.md
@@ -0,0 +1,6 @@
+---
+"github.com/livekit/protocol": patch
+"@livekit/protocol": patch
+---
+
+SIP authentication: compare IP masks against remote address rather than From header.
diff --git a/sip/sip.go b/sip/sip.go
index f74653b4b..48f7a9522 100644
--- a/sip/sip.go
+++ b/sip/sip.go
@@ -428,19 +428,23 @@ func filterInvalidAddrMasks(masks []string) []string {
 	return out
 }
 
-func matchAddrMask(ip netip.Addr, mask string) bool {
-	if !strings.Contains(mask, "/") {
-		expIP, err := netip.ParseAddr(mask)
+func parseIPMask(mask string) (netip.Prefix, bool) {
+	addr, bits, hasBits := strings.Cut(mask, "/")
+	if strings.HasPrefix(addr, "[") && strings.HasSuffix(addr, "]") {
+		addr = addr[1 : len(addr)-1]
+	}
+	if !hasBits {
+		ip, err := netip.ParseAddr(addr)
 		if err != nil {
-			return false
+			return netip.Prefix{}, false
 		}
-		return ip == expIP
+		return netip.PrefixFrom(ip, ip.BitLen()), true
 	}
-	pref, err := netip.ParsePrefix(mask)
+	pref, err := netip.ParsePrefix(fmt.Sprintf("%s/%s", addr, bits))
 	if err != nil {
-		return false
+		return netip.Prefix{}, false
 	}
-	return pref.Contains(ip)
+	return pref, true
 }
 
 func matchAddrMasks(addr string, host string, masks []string) bool {
@@ -453,7 +457,12 @@ func matchAddrMasks(addr string, host string, masks []string) bool {
 		return true
 	}
 	for _, mask := range masks {
-		if mask == host || matchAddrMask(ip, mask) {
+		if pref, ok := parseIPMask(mask); ok {
+			// IP masks must only match the source IP, never the (spoofable) From host.
+			if pref.Contains(ip) {
+				return true
+			}
+		} else if mask == host {
 			return true
 		}
 	}
diff --git a/sip/sip_test.go b/sip/sip_test.go
index 57e3a8691..cef6ad69a 100644
--- a/sip/sip_test.go
+++ b/sip/sip_test.go
@@ -1163,27 +1163,37 @@ func TestEvaluateDispatchRule_RespectsRuleMediaTimeout(t *testing.T) {
 	require.Equal(t, 5*time.Minute, res.Media.MediaTimeout.AsDuration())
 }
 
-func TestMatchIP(t *testing.T) {
+func TestParseIPMask(t *testing.T) {
 	cases := []struct {
-		addr  string
-		mask  string
-		valid bool
-		exp   bool
+		mask string
+		exp  string // empty if not an IP mask
 	}{
-		{addr: "192.168.0.10", mask: "192.168.0.10", valid: true, exp: true},
-		{addr: "192.168.0.10", mask: "192.168.0.11", valid: true, exp: false},
-		{addr: "192.168.0.10", mask: "192.168.0.0/24", valid: true, exp: true},
-		{addr: "192.168.0.10", mask: "192.168.0.10/0", valid: true, exp: true},
-		{addr: "192.168.0.10", mask: "192.170.0.0/24", valid: true, exp: false},
+		{mask: "192.168.0.10", exp: "192.168.0.10/32"},
+		{mask: "192.168.0.0/24", exp: "192.168.0.0/24"},
+		{mask: "2001:db8::1", exp: "2001:db8::1/128"},
+		{mask: "[2001:db8::1]", exp: "2001:db8::1/128"},
+		{mask: "[2001:db8::]/32", exp: "2001:db8::/32"},
+		{mask: "[::]", exp: "::/128"},
+		{mask: "[::]/0", exp: "::/0"},
+
+		// Not IP masks: treated as hostnames.
+		{mask: "sip.example.com"},
+		{mask: "sip.example.com/24"},
+		{mask: "192.168.0.10/33"},
+		{mask: "192.168.0.10/"},
+		{mask: "[192.168.0.10"},
+		{mask: "[2001:db8::1]:5060"},
+		{mask: "[[2001:db8::1]]"},
 	}
 	for _, c := range cases {
 		t.Run(c.mask, func(t *testing.T) {
-			ip, err := netip.ParseAddr(c.addr)
-			require.NoError(t, err)
-			got := isValidMask(c.mask)
-			require.Equal(t, c.valid, got)
-			got = matchAddrMask(ip, c.mask)
-			require.Equal(t, c.exp, got)
+			pref, ok := parseIPMask(c.mask)
+			if c.exp == "" {
+				require.False(t, ok)
+				return
+			}
+			require.True(t, ok)
+			require.Equal(t, netip.MustParsePrefix(c.exp), pref)
 		})
 	}
 }
@@ -1281,6 +1291,59 @@ func TestMatchMasks(t *testing.T) {
 			},
 			exp: true,
 		},
+		{
+			name: "ip spoofed in from host",
+			addr: "203.0.113.5",
+			host: "192.168.0.10",
+			masks: []string{
+				"192.168.0.10",
+			},
+			exp: false,
+		},
+		{
+			name: "ip range spoofed in from host",
+			addr: "203.0.113.5",
+			host: "192.168.0.0/24",
+			masks: []string{
+				"192.168.0.0/24",
+			},
+			exp: false,
+		},
+		{
+			name: "ipv6 spoofed in from host",
+			addr: "203.0.113.5",
+			host: "[2001:db8::1]",
+			masks: []string{
+				"[2001:db8::1]",
+			},
+			exp: false,
+		},
+		{
+			name: "ipv6 bracketed",
+			addr: "2001:db8::1",
+			host: "sip.example.com",
+			masks: []string{
+				"[2001:db8::1]",
+			},
+			exp: true,
+		},
+		{
+			name: "ipv6 bracketed range",
+			addr: "2001:db8::1",
+			masks: []string{
+				"[2001:db8::]/32",
+			},
+			exp: true,
+		},
+		{
+			name: "ip matches source, from host differs",
+			addr: "192.168.0.10",
+			host: "sip.example.com",
+			masks: []string{
+				"192.168.0.10",
+			},
+			exp: true,
+		},
 	}
 	for _, c := range cases {
 		t.Run(c.name, func(t *testing.T) {