diff --git a/core/src/main/java/org/springframework/security/util/matcher/IpInetAddressMatcher.java b/core/src/main/java/org/springframework/security/util/matcher/IpInetAddressMatcher.java
index 7f7e338ea2e..f3c885a64f1 100644
--- a/core/src/main/java/org/springframework/security/util/matcher/IpInetAddressMatcher.java
+++ b/core/src/main/java/org/springframework/security/util/matcher/IpInetAddressMatcher.java
@@ -94,6 +94,13 @@ public boolean matches(@Nullable InetAddress toCheck) {
 		}
 		byte[] remAddr = toCheck.getAddress();
 		byte[] reqAddr = this.requiredAddress.getAddress();
+		if (remAddr.length != reqAddr.length) {
+			// Different address families (IPv4 vs IPv6): never match, per the class-level
+			// contract. Without this check, a mask wide enough to exceed the shorter
+			// array's length throws ArrayIndexOutOfBoundsException instead of returning
+			// false.
+			return false;
+		}
 		int nMaskFullBytes = this.nMaskBits / 8;
 		byte finalByte = (byte) (0xFF00 >> (this.nMaskBits & 0x07));
 		for (int i = 0; i < nMaskFullBytes; i++) {
diff --git a/core/src/test/java/org/springframework/security/util/matcher/IpInetAddressMatcherTests.java b/core/src/test/java/org/springframework/security/util/matcher/IpInetAddressMatcherTests.java
index ecac4658249..25e9966db15 100644
--- a/core/src/test/java/org/springframework/security/util/matcher/IpInetAddressMatcherTests.java
+++ b/core/src/test/java/org/springframework/security/util/matcher/IpInetAddressMatcherTests.java
@@ -111,6 +111,21 @@ void matchesWhenIpv6AndIpv4AddressThenReturnsFalse() throws Exception {
 		assertThat(matcher.matches(InetAddress.getByName("192.168.1.1"))).isFalse();
 	}
 
+	@Test
+	void matchesWhenIpv6CidrAndIpv4AddressThenReturnsFalse() throws Exception {
+		// The mask spans more full bytes (8) than the IPv4 address has (4). Without a
+		// family/length check, walking the byte arrays runs past the shorter one and
+		// throws ArrayIndexOutOfBoundsException instead of returning false.
+		IpInetAddressMatcher matcher = new IpInetAddressMatcher("2001:db8::/64");
+		assertThat(matcher.matches(InetAddress.getByName("32.1.13.184"))).isFalse();
+	}
+
+	@Test
+	void matchesWhenIpv4CidrAndIpv6AddressThenReturnsFalse() throws Exception {
+		IpInetAddressMatcher matcher = new IpInetAddressMatcher("192.168.1.0/24");
+		assertThat(matcher.matches(InetAddress.getByName("2001:db8::"))).isFalse();
+	}
+
 	@Test
 	void matchesWhenStringIpv4MatchThenReturnsTrue() {
 		IpInetAddressMatcher matcher = new IpInetAddressMatcher("192.168.1.1");