CVE-2019-10071

Root

Cause

CVE-2019-10071 affects Apache Tapestry, a Java web framework. The vulnerability lies in the code that validates HMAC signatures on form submissions. Instead of using a constant-time comparison algorithm, the framework employs String.equals(), which performs byte-by-byte comparison and short-circuits on the first mismatch [1]. This creates a timing side channel that leaks information about the correct signature.

Exploitation

An attacker can exploit this by sending a series of crafted payloads to a Tapestry application that includes form submissions with HMAC validation. By measuring the server's response times—or more precisely, the time taken to compare HMAC values—the attacker can incrementally deduce the correct signature for their payload [1]. No special privileges are required; the attacker only needs network access to the vulnerable application. Because String.equals() stops after the first differing byte, the attacker can observe slight timing differences that reveal which prefix bytes are correct.

Impact

Successfully determining the correct HMAC signature allows the attacker to bypass integrity checks on form data. This can lead to arbitrary data manipulation and, critically, remote code execution (RCE) if the attacker can craft a payload that is subsequently processed by the application [1]. The lack of constant-time comparison effectively undermines the security of the HMAC mechanism.

Mitigation

Apache Tapestry has released patches that replace the insecure comparison with a constant-time algorithm [1]. Users should upgrade to a fixed version as specified in the advisory. No workaround is available; a complete fix requires updating the framework libraries. The vulnerability is rated high severity (CVSS 7.5) and, although not yet observed in exploitation campaigns, its potential for RCE makes timely patching critical.