CVE-2026-56813
LowCVSS 2.1Summary
A vulnerability in the Elixir Plug library, specifically in the Plug.Conn.Cookies.encode/2 function, fails to neutralize the ';' delimiter in cookie attributes. This allows an attacker to inject additional attributes into the Set-Cookie header, potentially overriding existing cookie attributes.
Risk Assessment
An attacker can exploit this to override cookie attributes such as Domain and Path scope, or drop Secure and HttpOnly flags, enabling cookie tossing and session fixation attacks. This poses a risk to user session security.
Recommendation
Update the Plug library to version 1.16.6, 1.17.4, 1.18.5, 1.19.5, or 1.20.3, depending on your branch. If updating is not possible, avoid placing attacker-controlled data into cookie values or attributes.
Original NVD description (English source)
Improper Neutralization of Parameter/Argument Delimiters vulnerability in elixir-plug plug allows an attacker to inject or override HTTP cookie attributes. The Plug.Conn.Cookies.encode/2 function in lib/plug/conn/cookies.ex builds the Set-Cookie response header by interpolating the cookie value and its path, domain, same_site, and extra attributes directly into the header without neutralizing the ';' delimiter that separates cookie attributes. An application that places attacker-controlled data into a cookie value or attribute (for example via Plug.Conn.put_resp_cookie/4 when reflecting a username or preference) lets an attacker inject a ';' to append or override cookie attributes (such as Domain and Path scope, or dropping the Secure and HttpOnly flags), enabling cookie tossing and session fixation. Carriage return, line feed, and null bytes are rejected by Plug.Conn header validation, so HTTP response splitting is not possible, but attribute injection through ';' is not prevented. This issue affects plug: from 0.1.0 before 1.16.6, from 1.17.0 before 1.17.4, from 1.18.0 before 1.18.5, from 1.19.0 before 1.19.5, from 1.20.0 before 1.20.3.