GHSA-6xvm-j4wr-6v98

### Summary A remote, unauthenticated attacker can trigger a denial of service in applications using vulnerable `quinn` versions by sending a crafted QUIC Initial packet containing malformed `quic_transport_parameters`. `In quinn-proto` parsing logic, attacker-controlled varints are decoded with `unwrap()`, so truncated encodings cause `Err(UnexpectedEnd)` and `panic`. This is reachable over the network with a single packet and no prior trust or authentication.

### Details The issue is panic-on-untrusted-input in QUIC transport parameter parsing. In `quinn-proto` (observed in `quinn-proto 0.11.13`), parsing of some transport parameters uses a fallible varint decode followed by `unwrap()`. For malformed/truncated parameter values, decode returns `UnexpectedEnd`, and `unwrap()` panics.

#### Observed output: ``` thread 'tokio-rt-worker' (2366474) panicked at quinn-proto/src/transport_parameters.rs:473:67: called `Result::unwrap()` on an `Err` value: UnexpectedEnd ```

### PoC #### Reproduces against the upstream Quinn server example. 1. Start server: ``` cargo run --example server -- ./ ``` 2. Prepare PoC client environment: ``` python3 -m venv .venv source .venv/bin/activate pip install aioquic ``` 3. Run PoC script [attack.py](https://github.com/user-attachments/files/25741713/attack.py) against server QUIC listener (default example target shown): ``` python attack.py ``` #### Observed output ``` thread 'tokio-rt-worker' (2366903) panicked at quinn-proto/src/transport_parameters.rs:473:67: called `Result::unwrap()` on an `Err` value: UnexpectedEnd ```

### Impact Vulnerability type: Remote Denial of Service (panic/crash) Attack requirements: Network reachability to UDP QUIC listener Authentication/privileges: None Who is impacted: Any server/application using affected `quinn/quinn-proto` versions where this parse path is reachable; process-level impact depends on integration panic handling policy

This vulnerability was originally submitted by @revofusion to the Ethereum Foundation bug bounty program