Description

The opentelemetry_cowboy telemetry handler crashes when processing an HTTP/2 request with a malformed :scheme pseudo-header. Once the handler crashes, the Erlang telemetry library permanently detaches it, causing all subsequent requests on that node to lose OpenTelemetry tracing.

Root Cause

In opentelemetry_cowboy.erl:239, the extract_scheme/2 function only handles <<"http">> and <<"https">>:

case ReqScheme of
    <<"http">> ->
        http;
    <<"https">> ->
        https
end;

Any other value causes a case_clause crash. Cowboy does not validate the :scheme pseudo-header and passes through arbitrary values, so malformed requests (from scanners, bots, or misconfigured proxies) can trigger this crash.

Reproduction

Minimal reproduction repository: https://github.com/velimir/scheme_crash

git clone https://github.com/velimir/scheme_crash
cd scheme_crash
mix deps.get
iex -S mix

In another terminal:

nghttp -v -n -H':scheme: http://example.com/?' http://localhost:8080/

Error Output

[error] Handler {:opentelemetry_cowboy, :otel_cowboy} has failed and has been detached. Class=:error
Reason={:case_clause, "http://example.com/?"}
Stacktrace=[
  {:opentelemetry_cowboy, :extract_scheme, 2,
   [file: '.../opentelemetry_cowboy.erl', line: 239]},
  {:opentelemetry_cowboy, :handle_event, 4,
   [file: '.../opentelemetry_cowboy.erl', line: 295]},
  ...
]

Versions

• opentelemetry_cowboy: 1.0.0
• cowboy: 2.14.2
• opentelemetry: 1.7.0
• Erlang/OTP: 27.x