An attacker could exploit the vulnerability – tracked as CVE-2021-40346 (CVSS score of 8.6) – to bypass duplicate HTTP Content-Length header checks. Thus, the attacker could smuggle HTTP requests to the backend server without the proxy server noticing it, or launch a response-splitting attack.

“Our analysis confirmed that the duplication is achieved by making use of the memory layout of HAProxy’s internal representation of an HTTP message to slip a select character from the header’s name to its value. Due to the difficulty in executing such an attack, the risk is low,” according to an HAProxy advisory.