OpenTelemetry dotnet: Unbounded `grpc-status-details-bin` parsing in OTLP/gRPC retry handling

Summary

When exporting telemetry over gRPC using the OpenTelemetry Protocol (OTLP), the exporter may parse a server-provided grpc-status-details-bin trailer during retry handling. Prior to the fix, a malformed trailer could encode an extremely large length-delimited protobuf field which was used directly for allocation, allowing excessive memory allocation and potential denial of service (DoS).

Details

#5980 introduced a retry path that parses grpc-status-details-bin to extract gRPC retry delay information for retryable responses.

On that path:

• OtlpGrpcExportClient captures grpc-status-details-bin from retryable status responses (ResourceExhausted / Unavailable).
• OtlpRetry invokes GrpcStatusDeserializer.TryGetGrpcRetryDelay using this untrusted trailer value.
• GrpcStatusDeserializer.DecodeBytes decoded a protobuf varint length and allocated new byte[length] without validating the bounds against the remaining payload size.

A malicious or compromised collector (or a MitM in weakly-protected deployments) could return a crafted grpc-status-details-bin payload that forces oversized allocation and memory exhaustion in the instrumented process.

Impact

If an OTLP/gRPC endpoint is attacker-controlled (or traffic is intercepted), a crafted retryable response can trigger large allocations during trailer parsing, which may exhaust memory and cause process instability/crash (availability impact / DoS).

Mitigation

The application's configured back-end/collector endpoint needs to behave maliciously. If the collector/back-end is a well-behaved implementation response bodies should not be excessively large if a request error occurs.

Workarounds

None known.

Remediation

#7064 updates GrpcStatusDeserializer to validate decoded length-delimited field sizes before allocation by ensuring the requested length is sane and does not exceed the remaining payload.

This causes malformed or truncated grpc-status-details-bin payloads to fail safely instead of attempting unbounded allocation.