Hardening: refuse a JSON "dir" JWE without protected header and "aad", whose content can be an A*GCMKW encrypted key

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
48/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
php
Domain
backend-api-design, security

Research direction

Read the decrypter and builder paths for JSON JWE handling, then trace how protected headers and aad determine content authentication data for dir. Check the existing tests for JSON serialization and decryption. Done means the decrypter rejects the described empty-AAD shape, the builder no longer emits it, and documentation recommends setting alg on keys shared across algorithms.

Written by the indexing model from the issue text.

Description

enhancement
Version(s) affected

All released versions (3.4.x, 4.0.x, 4.1.x, 4.2.x), and the 4.3.x and 5.0.x branches.

Description

AES-GCM key wrapping (A128GCMKW, A192GCMKW, A256GCMKW) encrypts the CEK with AES-GCM under the key and an empty AAD. In the JSON serializations, a JWE may carry alg and enc outside the protected header. When it has neither a protected header nor an aad member, the AAD of its content is empty too, and with dir the content is then encrypted by the same computation as a GCM key wrap.

A decrypter that registers dir, an AGCMKW algorithm and the AGCM algorithm of the same size, and holds the key-wrapping key without an alg parameter, therefore accepts the encrypted key of an A*GCMKW token presented as the content of a dir JWE, and returns the CEK of that token as payload. Nothing is gained unless the application reveals decrypted payloads to the sender. If it does, the CEK opens the original token and allows new content under its encrypted key. Conversely, a JSON dir JWE issued without a protected header is a valid GCM key wrap of its plaintext.

This is a property of the JSON serializations rather than of this library alone: other implementations behave the same when they are given the raw key bytes (checked with panva/jose 6.2.2). Compact JWEs, and therefore JWTs, cannot take this shape.

Mitigations available today:

  • an alg parameter on every key (the key analyzer already reports keys without it);
  • an AlgorithmChecker that reads the protected header only;
  • not registering dir next to key wrapping algorithms in the same decrypter.
Possible Solution
  • The decrypter refuses a dir JWE whose content AAD is empty (no protected header and no aad), and the builder no longer produces one. This refuses a token shape the JSON serializations allow, so 4.3.0 with an upgrade note.
  • Documentation: recommend alg on the keys of a key set shared by several algorithms.

Related: #761 (key_ops does not tell content decryption from key unwrapping).

Additional Context

Found by the Anthropic OSS Scanner (ANT-2026-35BMRM17).

Dominant language
PHP
Stars
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Forks
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Avg merge
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Merged PRs (30d)
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