What’s Vulnerable › Vulnerabilities

CVE-2026-56813: Cookie attribute injection in Plug.Conn.Cookies.encode/2

• Severity: low (CVSS 2.1 v4.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-07-10 · updated 2026-10-24
• Weakness: CWE-141

Description

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 1.19.1 before 1.19.5, and from 1.20.0 before 1.20.3.

Affected products

• elixir-plug 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 1.19.1 before 1.19.5; from 1.20.0 before 1.20.3 — fixed in 1.16.6, 1.17.4, 1.18.5, 1.19.5, 1.20.3

How to fix

Upgrade to a fixed version: elixir-plug plug 1.16.6, 1.17.4, 1.18.5, 1.19.5. Check the vendor advisory for mitigations.

References

• https://cna.erlef.org/cves/CVE-2026-56813.html
• https://github.com/elixir-plug/plug/commit/149d9ed68fee0b4f77efd1e835ce5d785856697b
• https://github.com/elixir-plug/plug/commit/3f00dfad4e20ba88472e315c90a25742bf178f8e
• https://github.com/elixir-plug/plug/commit/4167981747fe9ce75f374b94a28861ae950ea992
• https://github.com/elixir-plug/plug/commit/a6d1248659022749869963fd302687165ecf8c8b
• https://github.com/elixir-plug/plug/commit/eceb8315ce9a31ef784943a95a8624ebd1bc7e06
• https://github.com/elixir-plug/plug/commit/f26876aa67aaeb38e616638aa3efbcc2fe2906a5
• https://github.com/elixir-plug/plug/security/advisories/GHSA-wpmj-jh88-rpgm
• https://osv.dev/vulnerability/EEF-CVE-2026-56813

Get alerted when CVE-2026-56813 or the next vulnerability affects your stack