config/src/main/java/org/springframework/security/config/http/WebConfigUtils.java
static void validateHttpRedirect(String url, ParserContext pc, Object source) {
		if (!StringUtils.hasText(url) || UrlUtils.isValidRedirectUrl(url)
				|| url.startsWith("$") || url.startsWith("#")) {
			return;
		}
		pc.getReaderContext().warning(
				url + " is not a valid redirect URL (must start with '/' or http(s))",
				source);
	}
static void validateHttpRedirect(String url, ParserContext pc, Object source) {
		if (!StringUtils.hasText(url) || UrlUtils.isValidRedirectUrl(url)
				|| url.startsWith("$") || url.startsWith("#")) {
			return;
		}
		pc.getReaderContext().warning(
				url + " is not a valid redirect URL (must start with '/' or http(s))",
				source);
	}
Checks the value of an XML attribute which represents a redirect URL. If not empty
or starting with "$" (potential placeholder), or starting with "#" (potential
SpEL), "/" or "http" it will raise an error.