CVE-2026-40891

### 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](https://github.com/open-telemetry/opentelemetry-dotnet/pull/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.