Zapros, a Python HTTP client, prior to version 0.14.0 is vulnerable to denial of service when an application requests content from an untrusted server, or follows a redirect to one, because a malicious response containing an excessive number of chained `Content-Encoding` values causes Zapros to construct a deeply nested decompression chain that consumes excessive resources. Version 0.14.0 patches the vulnerability by limiting responses to five content-encoding layers and raising `DecodingError` when that limit is exceeded. As a workaround, applications can add response middleware that inspects the `Content-Encoding` header and rejects responses containing more than a safe number of encoding layers.

Affected: PyPI: zapros

### Impact

**Who is impacted**: - Any application using Zapros to make HTTP requests to untrusted servers - Applications that follow redirects to attacker-controlled hosts

**Attack vector**: - A malicious HTTP server returns a response with many chained content encodings. When the client attempts to decode, it creates a deeply nested decompression chain consuming excessive resources.

### Patches

Fixed in version 0.14.0.

The fix adds a hardcoded limit of **5** `Content-Encoding` layers. Responses exceeding this limit raise `DecodingError`.

### Workarounds

Add middleware that checks for a malicious Content-Encoding header.

```python from typing import cast

from zapros import ( AsyncBaseHandler, AsyncBaseMiddleware, BaseHandler, BaseMiddleware, Client, DecodingError, Request, Response, )

MAX_DECODE_LAYERS = 5

class ContentEncodingCheckMiddleware(BaseMiddleware, AsyncBaseMiddleware): def __init__( self, next_handler: BaseHandler | AsyncBaseHandler, *, max_layers: int = MAX_DECODE_LAYERS, ) -> None: self.next = cast(BaseHandler, next_handler) self.async_next = cast(AsyncBaseHandler, next_handler) self._max_layers = max_layers

def _check(self, response: Response) -> None: encoding_header = response.headers.get("Content-Encoding") if not encoding_header: return

layers = [enc.strip().lower() for enc in encoding_header.split(","…

Affected: PyPI: zapros

### Impact

**Who is impacted**: - Any application using Zapros to make HTTP requests to untrusted servers - Applications that follow redirects to attacker-controlled hosts

**Attack vector**: - A malicious HTTP server returns a response with many chained content encodings. When the client attempts to decode, it creates a deeply nested decompression chain consuming excessive resources.

### Patches

Fixed in version 0.14.0.

The fix adds a hardcoded limit of **5** `Content-Encoding` layers. Responses exceeding this limit raise `DecodingError`.

### Workarounds

Add middleware that checks for a malicious Content-Encoding header.

```python from typing import cast

from zapros import ( AsyncBaseHandler, AsyncBaseMiddleware, BaseHandler, BaseMiddleware, Client, DecodingError, Request, Response, )

MAX_DECODE_LAYERS = 5

class ContentEncodingCheckMiddleware(BaseMiddleware, AsyncBaseMiddleware): def __init__( self, next_handler: BaseHandler | AsyncBaseHandler, *, max_layers: int = MAX_DECODE_LAYERS, ) -> None: self.next = cast(BaseHandler, next_handler) self.async_next = cast(AsyncBaseHandler, next_handler) self._max_layers = max_layers

def _check(self, response: Response) -> None: encoding_header = response.headers.get("Content-Encoding") if not encoding_header: return

layers = [enc.strip().lower() for enc in encoding_header.split(","…