Fix SSRF (Server Side Request Forgery) in Vert.x
SSRF in Vert.x occurs when an attacker manipulates the server into making unauthorized requests to internal resources or third-party systems. This typically happens when unvalidated user input is passed into the Vert.x WebClient or HttpClient. Exploitation can lead to internal port scanning, access to cloud metadata services (like AWS IMDS), or bypassing firewalls.
The Vulnerable Pattern
WebClient client = WebClient.create(vertx);router.get(“/proxy”).handler(ctx -> { // DANGER: User provides the full URL directly String targetUrl = ctx.request().getParam(“url”);
client.getAbs(targetUrl) .send() .onSuccess(res -> ctx.response().end(res.bodyAsString())) .onFailure(err -> ctx.fail(500)); });
The Secure Implementation
To kill SSRF in Vert.x, you must implement a multi-layered defense. First, parse the input using java.net.URI to extract the host; never use regex on raw strings. Second, apply a strict allowlist of domains—do not use denylists as they are easily bypassed with decimal IPs or local aliases. Third, restrict protocols to HTTPS to prevent file:// or gopher:// smuggling. For high-security environments, configure the Vert.x WebClient with a custom AddressResolverOptions to block resolution of private IP ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) and cloud metadata IPs (169.254.169.254), preventing DNS rebinding attacks.
private static final SetALLOWED_DOMAINS = Set.of("api.trusted.com", "static.trusted.com"); router.get(“/proxy”).handler(ctx -> { String userUrl = ctx.request().getParam(“url”);
try { java.net.URI uri = new java.net.URI(userUrl); String host = uri.getHost();
// 1. Enforce HTTPS only if (!"https".equalsIgnoreCase(uri.getScheme())) { ctx.response().setStatusCode(400).end("Invalid protocol"); return; } // 2. Strict Host Allowlisting if (host == null || !ALLOWED_DOMAINS.contains(host)) { ctx.response().setStatusCode(403).end("Forbidden Domain"); return; } // 3. Prevent internal IP access via DNS resolution (Simplified check) // In production, use a custom NameResolver to block RFC1918 addresses client.getAbs(uri.toString()) .timeout(2000) .send() .onSuccess(res -> ctx.response().end(res.bodyAsString())) .onFailure(err -> ctx.fail(502));
} catch (Exception e) { ctx.response().setStatusCode(400).end(“Malformed URL”); } });
Your Vert.x API
might be exposed to SSRF (Server Side Request Forgery)
74% of Vert.x apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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