Affected: authlib

Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.5, Authlib’s JWE zip=DEF path performs unbounded DEFLATE decompression. A very small ciphertext can expand into tens or hundreds of megabytes on decrypt, allowing an attacker who can supply decryptable tokens to exhaust memory and CPU and cause denial of service. This issue has been patched in version 1.6.5. Workarounds for this issue involve rejecting or stripping zip=DEF for inbound JWEs at the application boundary, forking and add a bounded decompression guard via decompressobj().decompress(data, MAX_SIZE)) and returning an error when output exceeds a safe limit, or enforcing strict maximum token sizes and fail fast on oversized inputs; combine with rate limiting.

Affected: PyPI: authlib

### Summary _Authlib’s JWE `zip=DEF` path performs unbounded DEFLATE decompression. A very small ciphertext can expand into tens or hundreds of megabytes on decrypt, allowing an attacker who can supply decryptable tokens to exhaust memory and CPU and cause denial of service._

### Details - Affected component: Authlib JOSE, JWE `zip=DEF` (DEFLATE) support. - In `authlib/authlib/jose/rfc7518/jwe_zips.py`, `DeflateZipAlgorithm.decompress` calls `zlib.decompress(s, -zlib.MAX_WBITS)` without a maximum output limit. This permits unbounded expansion of compressed payloads. - In the JWE decode flow (`authlib/authlib/jose/rfc7516/jwe.py`), when the protected header contains `"zip": "DEF"`, the library routes the decrypted ciphertext into the `decompress` method and assigns the fully decompressed bytes to the plaintext field before returning it. No streaming limit or quota is applied. - Because DEFLATE achieves extremely high ratios on highly repetitive input, an attacker can craft a tiny `zip=DEF` ciphertext that inflates to a very large plaintext during decrypt, spiking RSS and CPU. Repeated requests can starve the process or host.

Code references (from this repository version): - `authlib/authlib/jose/rfc7518/jwe_zips.py` – `DeflateZipAlgorithm.decompress` uses unbounded `zlib.decompress`. - `authlib/authlib/jose/rfc7516/jwe.py` – JWE decode path applies `zip_.decompress(msg)` when `zip=DEF` is present in the header.

Contrast: The `joserfc` project guards `zip=DEF` decompression…

Affected: PyPI: authlib

### Summary _Authlib’s JWE `zip=DEF` path performs unbounded DEFLATE decompression. A very small ciphertext can expand into tens or hundreds of megabytes on decrypt, allowing an attacker who can supply decryptable tokens to exhaust memory and CPU and cause denial of service._

### Details - Affected component: Authlib JOSE, JWE `zip=DEF` (DEFLATE) support. - In `authlib/authlib/jose/rfc7518/jwe_zips.py`, `DeflateZipAlgorithm.decompress` calls `zlib.decompress(s, -zlib.MAX_WBITS)` without a maximum output limit. This permits unbounded expansion of compressed payloads. - In the JWE decode flow (`authlib/authlib/jose/rfc7516/jwe.py`), when the protected header contains `"zip": "DEF"`, the library routes the decrypted ciphertext into the `decompress` method and assigns the fully decompressed bytes to the plaintext field before returning it. No streaming limit or quota is applied. - Because DEFLATE achieves extremely high ratios on highly repetitive input, an attacker can craft a tiny `zip=DEF` ciphertext that inflates to a very large plaintext during decrypt, spiking RSS and CPU. Repeated requests can starve the process or host.

Code references (from this repository version): - `authlib/authlib/jose/rfc7518/jwe_zips.py` – `DeflateZipAlgorithm.decompress` uses unbounded `zlib.decompress`. - `authlib/authlib/jose/rfc7516/jwe.py` – JWE decode path applies `zip_.decompress(msg)` when `zip=DEF` is present in the header.

Contrast: The `joserfc` project guards `zip=DEF` decompression…