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Remediate server-side request forgery (SSRF)

Recipe IDorg.openrewrite.java.security.FixCwe918
Artifactorg.openrewrite.recipe:rewrite-java-security

Inserts a guard that validates URLs constructed from user-controlled input do not resolve to internal, reserved, or otherwise unsafe network addresses, blocking server-side request forgery (SSRF) attacks. The block list covers IPv4 and IPv6, including IPv4-mapped IPv6 (::ffff:0:0/96), IPv6 ULA (fc00::/7), NAT64 (64:ff9b::/96), 6to4 (2002::/16), and Teredo (2001::/32) — all of which embed or translate to addresses that would otherwise bypass a naïve isSiteLocalAddress / isLoopbackAddress check. The guard does not by itself prevent DNS rebinding. URL.openConnection() re-resolves the host at connect time, so a rapidly shifting authoritative DNS response can answer with a public IP during validation and an internal IP at connect time. Closing this time-of-check / time-of-use window requires binding the TCP connection to the validated IP literal — typically via a hardened HTTP client with a custom DNS resolver (HttpClient.Builder, Apache HttpClient's DnsResolver, OkHttp's Dns, etc.) — and is HTTP-client-specific, so it is out of scope for this recipe. The block list reflects IANA special-use registries at the time of this recipe's release and is not, and cannot be, permanently complete. New special-use ranges are assigned periodically — 3fff::/20 was added as IPv6 documentation space in 2024 (RFC 9637), for example — so the list will need ongoing maintenance to keep pace with the IANA IPv4 and IPv6 Special-Purpose Address Registries.

Single recipeJavaCWE-918Moderne Proprietary License
Try in PlatformTry this recipe in the Moderne platform. Not a user yet? You’ll get a no-setup demo environment, with nothing to install or configure.

Examples​

java
Before
import javax.servlet.http.HttpServletRequest;

import java.net.URL;

class Test {
    void handleRequest(HttpServletRequest request) throws Exception {
        String target = request.getParameter("url");
        URL url = new URL(target);
        url.openConnection();
    }
}
After
import javax.servlet.http.HttpServletRequest;

import java.net.URL;

class Test {
    void handleRequest(HttpServletRequest request) throws Exception {
        String target = request.getParameter("url");
        URL url = new URL(target);
        ssrfGuardValidateUrl(url);
        url.openConnection();
    }

    private static void ssrfGuardValidateUrl(java.net.URL url) {
        String[] blockedCidrs = {
                "0.0.0.0/8",
                "10.0.0.0/8",
                "100.64.0.0/10",
                "127.0.0.0/8",
                "169.254.0.0/16",
                "172.16.0.0/12",
                "192.0.0.0/24",
                "192.0.2.0/24",
                "192.168.0.0/16",
                "198.18.0.0/15",
                "198.51.100.0/24",
                "203.0.113.0/24",
                "224.0.0.0/3",
                "::/128",
                "::1/128",
                "::ffff:0:0/96",
                "64:ff9b::/96",
                "100::/64",
                "2001::/32",
                "2001:10::/28",
                "2001:db8::/32",
                "2002::/16",
                "fc00::/7",
                "fe80::/10",
                "fec0::/10",
                "ff00::/8"
        };
        try {
            for (java.net.InetAddress addr : java.net.InetAddress.getAllByName(url.getHost())) {
                byte[] addrBytes = addr.getAddress();
                for (String cidr : blockedCidrs) {
                    int slash = cidr.indexOf('/');
                    byte[] prefixBytes = java.net.InetAddress.getByName(cidr.substring(0, slash)).getAddress();
                    if (prefixBytes.length != addrBytes.length) {
                        continue;
                    }
                    int prefixLen = Integer.parseInt(cidr.substring(slash + 1));
                    int fullBytes = prefixLen >>> 3;
                    int remainingBits = prefixLen & 7;
                    boolean match = true;
                    for (int i = 0; i < fullBytes && match; i++) {
                        if (addrBytes[i] != prefixBytes[i]) {
                            match = false;
                        }
                    }
                    if (match && remainingBits > 0) {
                        int mask = 0xFF << (8 - remainingBits);
                        if ((addrBytes[fullBytes] & mask) != (prefixBytes[fullBytes] & mask)) {
                            match = false;
                        }
                    }
                    if (match) {
                        throw new SecurityException("Blocked request to internal address");
                    }
                }
            }
        } catch (java.net.UnknownHostException e) {
            throw new SecurityException("Unable to resolve host", e);
        }
    }
}

Usage​

Run this recipe

This recipe has no required configuration options. Users of Moderne can run it via the Moderne CLI.

You will need to have configured the Moderne CLI on your machine before you can run the following command.

shell
mod run . --recipe FixCwe918

If the recipe is not available locally, then you can install it using:

mod config recipes jar install org.openrewrite.recipe:rewrite-java-security:3.33.1

Data tables​

Source files that had results
org.openrewrite.table.SourcesFileResults

Source files that were modified by the recipe run.

ColumnDescription
Source path before the runThe source path of the file before the run. null when a source file was created during the run.
Source path after the runA recipe may modify the source path. This is the path after the run. null when a source file was deleted during the run.
Parent of the recipe that made changesIn a hierarchical recipe, the parent of the recipe that made a change. Empty if this is the root of a hierarchy or if the recipe is not hierarchical at all.
Recipe that made changesThe specific recipe that made a change.
Estimated time savingAn estimated effort that a developer to fix manually instead of using this recipe, in unit of seconds.
CycleThe recipe cycle in which the change was made.
Source files that had search results
org.openrewrite.table.SearchResults

Search results that were found during the recipe run.

ColumnDescription
Source path of search result before the runThe source path of the file with the search result markers present.
Source path of search result after run the runA recipe may modify the source path. This is the path after the run. null when a source file was deleted during the run.
ResultThe trimmed printed tree of the LST element that the marker is attached to.
DescriptionThe content of the description of the marker.
Recipe that added the search markerThe specific recipe that added the Search marker.
Source files that errored on a recipe
org.openrewrite.table.SourcesFileErrors

The details of all errors produced by a recipe run.

ColumnDescription
Source pathThe file that failed to parse.
Recipe that made changesThe specific recipe that made a change.
Stack traceThe stack trace of the failure.
Recipe performance
org.openrewrite.table.RecipeRunStats

Statistics used in analyzing the performance of recipes.

ColumnDescription
The recipeThe recipe whose stats are being measured both individually and cumulatively.
Source file countThe number of source files the recipe ran over.
Source file changed countThe number of source files which were changed in the recipe run. Includes files created, deleted, and edited.
Cumulative scanning time (ns)The total time spent across the scanning phase of this recipe.
Max scanning time (ns)The max time scanning any one source file.
Cumulative edit time (ns)The total time spent across the editing phase of this recipe.
Max edit time (ns)The max time editing any one source file.