---
id: "CVE-2026-105963"
kind: "vulnerability"
aliases: ["CVE-2026-105963"]
title: "plug_crypto: wrong-typed arguments leak secret key base into logs"
severity: "medium"
cvss: 5.1
epss: 0.00715
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["elixir", "error-tracking", "package", "phoenix", "plug", "plug_crypto", "self-hosted"]
not_affected_if: ["no-plug-cowboy-or-bandit", "patched-version"]
published_at: "2026-10-23T10:16:40+00:00"
first_seen_at: "2026-10-23T21:57:32+00:00"
updated_at: "2026-10-25T04:28:52+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-105963", "first_seen_at": "2026-10-23T21:57:32+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-105963", "first_seen_at": "2026-10-24T07:15:51+00:00"}]
---

# plug_crypto: wrong-typed arguments leak secret key base into logs

A failed type guard in plug_crypto makes the Erlang VM log a FunctionClauseError that carries the raw argument values, including the secret key base or the value an API key is compared against. An unauthenticated remote request can trigger it through Phoenix.Token.verify/4 or a missing header passed to secure_compare/2, and the secret then sits in application logs and error trackers.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the plug_crypto version in `mix.lock` or with `mix deps | grep plug_crypto`; affected if below `2.2.1`
- Check the plug version in `mix.lock`; affected if from `1.6.3` up to but not including `1.7.0`
- Search application and error-tracker logs for FunctionClauseError entries from Plug.Crypto that show crash_reason metadata

## Fix

- Upgrade plug_crypto to `2.2.1` or later
- Upgrade plug to `1.7.0` or later
- Rotate the secret key base and any API keys if affected log entries were forwarded to third-party error trackers

## Timeline

- 2026-10-23T21:57:32+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-105963
- 2026-10-24T07:15:51+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-105963

## From the sources

Externally-Generated Error Message Containing Sensitive Information vulnerability in plug_crypto in elixir-plug plug_crypto allows an unauthenticated remote attacker to expose the secret key base through application logs via a wrong-typed argument. Every public function in Plug.Crypto validates argument types in a function-head guard. A failed guard makes the Erlang VM raise FunctionClauseError with the actual argument values in the top stack frame, so the secret key base, a derived key, or the value an API key is compared against lands in that frame. The library's rescue blocks and Plug.Crypto.prune_args_from_stacktrace/1 only reach exceptions raised inside a function body, so they never see a guard failure. Plug.Cowboy and Bandit write the frame to the error log and attach it as crash_reason Logger metadata, which error trackers forward.

Two entry points take request data. Plug.Crypto.verify/4 exposes the secret key base when the token is neither a binary nor nil, which Phoenix.Token.verify/4 reaches because it forwards the token unguarded. Plug.Crypto.secure_compare/2 exposes the other operand when one side is not a binary, as happens when a missing request header yields nil. Reading the leaked value needs separate access to the logs or the error tracker, so the request alone does not disclose the secret to the attacker.

This issue affects plug: from 1.6.3 before 1.7.0; plug_crypto: from 1.0.0 before 2.2.1.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_