Secret key base exposed in stack trace arguments on wrong-typed Plug.Crypto arguments

Overview

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.

Affected

• plug >= 1.6.3 < 1.7.0
• elixir-plug/plug >= 56000c33d8a0a2fbac491f8be06b9c5112bc2276 < *
• plug_crypto >= 1.0.0 < 2.2.1
• elixir-plug/plug_crypto >= 2d078b7b8264c565f0ff71235c515f4f46882dcf < 981887493dada0fce7003c61861c00dfbb0fc8c5

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Workarounds

Check argument types in the calling code before entering Plug.Crypto, so no head guard ever fails. Pass a token to Phoenix.Token.verify/4, Phoenix.Token.decrypt/4, Plug.Crypto.verify/4 or Plug.Crypto.decrypt/4 only when is_binary(token) holds, and treat anything else as an invalid token. Call Plug.Crypto.secure_compare/2 only when both operands are binaries, treating a missing header or parameter as a failed comparison rather than passing nil.

Where the leaked value matters more than the crash, configuring the error tracker to drop stack-frame arguments also keeps the secret out of the forwarded report. For Sentry this means not sending vars with the crash report. This narrows exposure but leaves the value in the local error log.