Decode percent-encoded values

Closes gh-19136

Signed-off-by: Josh Cummings <3627351+jzheaux@users.noreply.github.com>

web/src/main/java/org/springframework/security/web/FormPostRedirectStrategy.java

@@ -17,6 +17,7 @@
 package org.springframework.security.web;
 
 import java.io.IOException;
+import java.nio.charset.StandardCharsets;
 import java.util.Base64;
 import java.util.List;
 import java.util.Map.Entry;
@@ -30,6 +31,7 @@ import org.springframework.security.crypto.keygen.Base64StringKeyGenerator;
 import org.springframework.security.crypto.keygen.StringKeyGenerator;
 import org.springframework.web.util.HtmlUtils;
 import org.springframework.web.util.UriComponentsBuilder;
+import org.springframework.web.util.UriUtils;
 
 /**
  * Redirect using an auto-submitting HTML form using the POST method. All query params
@@ -83,8 +85,12 @@ public final class FormPostRedirectStrategy implements RedirectStrategy {
 
 		final StringBuilder hiddenInputsHtmlBuilder = new StringBuilder();
 		for (final Entry<String, List<String>> entry : uriComponentsBuilder.build().getQueryParams().entrySet()) {
-			final String name = entry.getKey();
+			final String name = UriUtils.decode(entry.getKey(), StandardCharsets.UTF_8);
-			for (final String value : entry.getValue()) {
+			for (final String raw : entry.getValue()) {
+				if (raw == null) {
+					continue;
+				}
+				final String value = UriUtils.decode(raw, StandardCharsets.UTF_8);
 				// @formatter:off
 				final String hiddenInput = HIDDEN_INPUT_TEMPLATE
 					.replace("{{name}}", HtmlUtils.htmlEscape(name))

web/src/test/java/org/springframework/security/web/FormPostRedirectStrategyTests.java

@@ -101,6 +101,19 @@ public class FormPostRedirectStrategyTests {
 		assertThat(this.response).satisfies(hasScriptSrcNonce());
 	}
 
+	// gh-19136
+
+	@Test
+	public void absoluteUrlWithPercentEncodedQueryParamsRedirect() throws IOException {
+		this.redirectStrategy.sendRedirect(this.request, this.response, "https://example.com/cb?payload=a%2Bb%2Fc%3D");
+		assertThat(this.response.getStatus()).isEqualTo(HttpStatus.OK.value());
+		assertThat(this.response.getContentType()).isEqualTo(MediaType.TEXT_HTML_VALUE);
+		assertThat(this.response.getContentAsString()).contains("action=\"https://example.com/cb\"");
+		assertThat(this.response.getContentAsString())
+			.contains("<input name=\"payload\" type=\"hidden\" value=\"a+b/c=\" />");
+		assertThat(this.response).satisfies(hasScriptSrcNonce());
+	}
+
 	private ThrowingConsumer<MockHttpServletResponse> hasScriptSrcNonce() {
 		return (response) -> {
 			final String policyDirective = response.getHeader("Content-Security-Policy");