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+---
+gem: jwt
+cve: 2026-45363
+notes: 'CVE has been reserved, but not filled in.'
+ghsa: c32j-vqhx-rx3x
+url: https://github.com/jwt/ruby-jwt/security/advisories/GHSA-c32j-vqhx-rx3x
+title: 'ruby-jwt: Empty-key HMAC bypass; cross-language sibling of CVE-2026-44351'
+date: 2026-05-18
+description: |
+  `JWT.decode(token, '', true, algorithm: 'HS256')` accepts an
+  attacker-forged token. `OpenSSL::HMAC.digest('SHA256', '', payload)`
+  returns a valid digest under an empty key, and no
+  `raise InvalidKeyError if key.empty?` precondition exists in the HMAC
+  algorithm.
+
+  ```
+  JWT.decode(token, "", true, algorithm: 'HS256')
+    -> JWA::Hmac.verify(verification_key: "", ...)
+    -> OpenSSL::HMAC.digest('SHA256', "", signing_input) == signature
+  ```
+
+  The same path is reached when a keyfinder block or key_finder: argument
+  returns "", nil, or an array containing nil for an unknown key.
+  JWT::Decode#find_key only rejects literal nil and empty arrays, and
+  JWT::JWA::Hmac silently coerces nil to "" (signing_key ||= '') before
+  signing.
+
+  ```
+  JWT.decode(token, nil, true, algorithms: ['HS256']) { |_h| "" }
+    -> find_key returns ""               # "" && !Array("").empty? == true
+    -> JWA::Hmac.verify(verification_key: "", ...)
+    -> verifies
+  ```
+
+  Common application patterns that produce the unsafe value:
+  `redis.get("kid:#{kid}").to_s`, ORM string columns with `default: ''`,
+  `ENV['SECRET'] || ''`, `Hash.new('')` lookups, `[primary, fallback]`
+  where fallback may be nil. Applications passing a non-empty static
+  `key:`, or whose keyfinder returns nil / raises on miss, are not
+  affected.
+
+  The existing `enforce_hmac_key_length` option would block this but
+  defaults to false. On OpenSSL ≥ 3.5 the empty-key HMAC.digest call no
+  longer raises, so the OpenSSL-3.0 rescue in JWA::Hmac#sign does not
+  fire.
+
+  Affects HS256/HS384/HS512 via both JWT.decode (positional key and block
+  keyfinder) and `JWT::EncodedToken#verify_signature!(key_finder:)`.
+cvss_v3: 7.4
+patched_versions:
+  - ">= 3.2.0"
+related:
+  url:
+    - https://github.com/jwt/ruby-jwt/security/advisories/GHSA-c32j-vqhx-rx3x
+    - https://github.com/jwt/ruby-jwt/commit/db560b769a07bd9724e77ff505011ac01872106f
+    - https://github.com/jwt/ruby-jwt/releases/tag/v3.2.0
+    - https://github.com/advisories/GHSA-c32j-vqhx-rx3x
+    - https://www.cve.org/CVERecord?id=CVE-2026-45363