AsyncHTTPClient: CRLF Injection in HTTP request headers (CVE-2023-0040)

TL;DR: Users of AHC should update to 1.13.2, 1.12.1, 1.9.1, or 1.4.1 as soon as practically possible.

The Async HTTP Client team have just shipped a series of security patches to address CVE-2023-0040. Versions of Async HTTP Client prior to the patch versions listed above are vulnerable to a form of targeted request manipulation called CRLF injection. This vulnerability was the result of insufficient validation of HTTP header field values before sending them to the network. Users are vulnerable if they pass untrusted data into HTTP header field values without prior sanitisation. Common use-cases here might be to place usernames from a database into HTTP header fields, or to place form field data into HTTP header fields.

This vulnerability allows attackers to inject new HTTP header fields, or entirely new requests, into the data stream. This can cause requests to be understood very differently by the remote server than was intended. In general, this is unlikely to result in data disclosure, but it can result in a number of logical errors and other misbehaviours.

We have classified this as a medium severity security issue. Many users are not at any risk at all, either because they are not running web servers or because they do not place untrusted user data into HTTP header field values. Such users are not at risk. However, any user passing potentially untrusted data into HTTP header field values should update immediately.

This vulnerability was reported by Alessio Della Libera of Snyk Research Team (@dellalibera).