elixir-plug up to 1.20.2 Cookies lib/plug/conn/cookies.ex encode injection

Summaryinfo

A vulnerability described as critical has been identified in elixir-plug plug up to 1.16.5/1.17.3/1.18.4/1.19.4/1.20.2. This issue affects the function encode of the file lib/plug/conn/cookies.ex of the component Cookies. Such manipulation leads to injection. This vulnerability is referenced as CVE-2026-56813. It is possible to launch the attack remotely. No exploit is available.

Detailsinfo

A vulnerability, which was classified as critical, has been found in elixir-plug plug up to 1.16.5/1.17.3/1.18.4/1.19.4/1.20.2. Affected by this issue is the function encode of the file lib/plug/conn/cookies.ex of the component Cookies. The manipulation with an unknown input leads to a injection vulnerability. Using CWE to declare the problem leads to CWE-74. The product constructs all or part of a command, data structure, or record using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify how it is parsed or interpreted when it is sent to a downstream component. Impacted is confidentiality, integrity, and availability. CVE summarizes:

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.

The advisory is shared for download at github.com. This vulnerability is handled as CVE-2026-56813 since 06/23/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. Successful exploitation requires user interaction by the victim. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1055.

Upgrading to version 1.16.6, 1.17.4, 1.18.5, 1.19.5 or 1.20.3 eliminates this vulnerability.

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Productinfo

Vendor

• elixir-plug

Name

• plug

Version

• 1.16.0
• 1.16.1
• 1.16.2
• 1.16.3
• 1.16.4
• 1.16.5
• 1.17.0
• 1.17.1
• 1.17.2
• 1.17.3
• 1.18.0
• 1.18.1
• 1.18.2
• 1.18.3
• 1.18.4
• 1.19.0
• 1.19.1
• 1.19.2
• 1.19.3
• 1.19.4
• 1.20.0
• 1.20.1
• 1.20.2

Website

• Product: https://github.com/elixir-plug/plug/

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