diff --git a/spring-web/src/main/java/org/springframework/web/util/UriComponentsBuilder.java b/spring-web/src/main/java/org/springframework/web/util/UriComponentsBuilder.java
index 1e0dc1d8d1cf..5e19d61b2523 100644
--- a/spring-web/src/main/java/org/springframework/web/util/UriComponentsBuilder.java
+++ b/spring-web/src/main/java/org/springframework/web/util/UriComponentsBuilder.java
@@ -357,9 +357,19 @@ public static InetSocketAddress parseForwardedFor(
 				String value = matcher.group(1).trim();
 				String host = value;
 				int portSeparatorIdx = value.lastIndexOf(':');
-				if (portSeparatorIdx > value.lastIndexOf(']')) {
+				int squareBracketIdx = value.lastIndexOf(']');
+				if (portSeparatorIdx > squareBracketIdx) {
+					if (squareBracketIdx == -1 && value.indexOf(':') != portSeparatorIdx) {
+						throw new IllegalArgumentException("Invalid IPv4 address: " + value);
+					}
 					host = value.substring(0, portSeparatorIdx);
-					port = Integer.parseInt(value.substring(portSeparatorIdx + 1));
+					try {
+						port = Integer.parseInt(value.substring(portSeparatorIdx + 1));
+					}
+					catch (NumberFormatException ex) {
+						throw new IllegalArgumentException(
+								"Failed to parse a port from \"forwarded\"-type header value: " + value);
+					}
 				}
 				return new InetSocketAddress(host, port);
 			}
@@ -886,7 +896,11 @@ private boolean isForwardedSslOn(HttpHeaders headers) {
 
 	private void adaptForwardedHost(String rawValue) {
 		int portSeparatorIdx = rawValue.lastIndexOf(':');
-		if (portSeparatorIdx > rawValue.lastIndexOf(']')) {
+		int squareBracketIdx = rawValue.lastIndexOf(']');
+		if (portSeparatorIdx > squareBracketIdx) {
+			if (squareBracketIdx == -1 && rawValue.indexOf(':') != portSeparatorIdx) {
+				throw new IllegalArgumentException("Invalid IPv4 address: " + rawValue);
+			}
 			host(rawValue.substring(0, portSeparatorIdx));
 			port(Integer.parseInt(rawValue.substring(portSeparatorIdx + 1)));
 		}
diff --git a/spring-web/src/test/java/org/springframework/web/filter/ForwardedHeaderFilterTests.java b/spring-web/src/test/java/org/springframework/web/filter/ForwardedHeaderFilterTests.java
index cd1f6bbad896..c6407c1de3df 100644
--- a/spring-web/src/test/java/org/springframework/web/filter/ForwardedHeaderFilterTests.java
+++ b/spring-web/src/test/java/org/springframework/web/filter/ForwardedHeaderFilterTests.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -36,6 +36,7 @@
 import org.springframework.web.testfixture.servlet.MockHttpServletResponse;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 import static org.mockito.Mockito.mock;
 
 /**
@@ -470,6 +471,13 @@ public void forwardedForMultipleIdentifiers() throws Exception {
 			assertThat(actual.getRemoteAddr()).isEqualTo(actual.getRemoteHost()).isEqualTo("203.0.113.195");
 			assertThat(actual.getRemotePort()).isEqualTo(MockHttpServletRequest.DEFAULT_SERVER_PORT);
 		}
+
+		@Test  // gh-26748
+		public void forwardedForInvalidIpV6Address() {
+			request.addHeader(FORWARDED, "for=\"2a02:918:175:ab60:45ee:c12c:dac1:808b\"");
+			assertThatIllegalArgumentException().isThrownBy(
+					ForwardedHeaderFilterTests.this::filterAndGetWrappedRequest);
+		}
 	}
 
 	@Nested
diff --git a/spring-web/src/test/java/org/springframework/web/util/UriComponentsBuilderTests.java b/spring-web/src/test/java/org/springframework/web/util/UriComponentsBuilderTests.java
index 13d9e61b7926..1db9b40628c5 100644
--- a/spring-web/src/test/java/org/springframework/web/util/UriComponentsBuilderTests.java
+++ b/spring-web/src/test/java/org/springframework/web/util/UriComponentsBuilderTests.java
@@ -453,6 +453,21 @@ void fromHttpRequestWithForwardedIPv6HostAndPort() {
 		assertThat(result.toString()).isEqualTo("http://[1abc:2abc:3abc::5ABC:6abc]:8080/mvc-showcase");
 	}
 
+	@Test  // gh-26748
+	void fromHttpRequestWithForwardedInvalidIPv6Address() {
+		MockHttpServletRequest request = new MockHttpServletRequest();
+		request.setScheme("http");
+		request.setServerName("localhost");
+		request.setServerPort(-1);
+		request.setRequestURI("/mvc-showcase");
+		request.addHeader("X-Forwarded-Host", "2a02:918:175:ab60:45ee:c12c:dac1:808b");
+
+		HttpRequest httpRequest = new ServletServerHttpRequest(request);
+
+		assertThatIllegalArgumentException().isThrownBy(() ->
+				UriComponentsBuilder.fromHttpRequest(httpRequest).build());
+	}
+
 	@Test
 	void fromHttpRequestWithForwardedHost() {
 		MockHttpServletRequest request = new MockHttpServletRequest();