Prometheus basic auth password starting with YAML directive characters fails

[michael-burt]

Component(s)

receiver/prometheus

What happened?

Description

Starting in 0.130.0, basic auth passwords that begin with a YAML directive began crashing the collector on startup. I believe this line is the problem:

opentelemetry-collector-contrib/receiver/prometheusreceiver/internal/targetallocator/config.go

Line 116 in 18eb74f

return []byte(s), nil

Steps to Reproduce

Minimal config

cat << 'EOF' > config.yaml
receivers:
  prometheus:
    config:
      scrape_configs:
        - job_name: "foo"
          static_configs:
            - targets: ["target:8000"]
          basic_auth:
            username: user
            password: "%password"
exporters:
  otlphttp:
    endpoint: https://example.com:4318
service:
  pipelines:
    metrics:
      receivers: [prometheus]
      processors: []
      exporters: [otlphttp]
EOF

This command succeeds:

docker run -d \
  -v "$(pwd)/config.yaml:/etc/otelcol-contrib/config.yaml:ro" \
  otel/opentelemetry-collector-contrib:0.129.1

But this command fails:

docker run -d \
  -v "$(pwd)/config.yaml:/etc/otelcol-contrib/config.yaml:ro" \
  otel/opentelemetry-collector-contrib:0.130.0

Expected Result

❯ docker run -i -v "$(pwd)/config.yaml:/etc/otelcol-contrib/config.yaml:ro" otel/opentelemetry-collector-contrib:0.129.1

2025-11-21T04:34:15.007Z	info	service@v0.129.0/service.go:197	Setting up own telemetry...	{"resource": {"service.instance.id": "8e683d4d-d515-415a-8ef5-9b673f22b5a0", "service.name": "otelcol-contrib", "service.version": "0.129.0"}}
2025-11-21T04:34:15.008Z	info	service@v0.129.0/service.go:257	Starting otelcol-contrib...	{"resource": {"service.instance.id": "8e683d4d-d515-415a-8ef5-9b673f22b5a0", "service.name": "otelcol-contrib", "service.version": "0.129.0"}, "Version": "0.129.0", "NumCPU": 10}
...

Actual Result

❯ docker run -i -v "$(pwd)/config.yaml:/etc/otelcol-contrib/config.yaml:ro" otel/opentelemetry-collector-contrib:0.130.0

2025-11-21T04:35:18.937Z	info	service@v0.130.0/service.go:197	Setting up own telemetry...	{"resource": {"service.instance.id": "8a67957b-56cb-4e17-8092-76357ad179a2", "service.name": "otelcol-contrib", "service.version": "0.130.0"}}
2025-11-21T04:35:18.938Z	error	service@v0.130.0/service.go:187	error found during service initialization	{"resource": {"service.instance.id": "8a67957b-56cb-4e17-8092-76357ad179a2", "service.name": "otelcol-contrib", "service.version": "0.130.0"}, "error": "failed to build pipelines: failed to create \"prometheus\" receiver for data type \"metrics\": failed to reload Prometheus config: prometheus receiver: failed to marshal config to yaml: [1:1] unexpected directive value. document not started\n>  1 | %password\n       ^\n"}
go.opentelemetry.io/collector/service.New.func1
	go.opentelemetry.io/collector/service@v0.130.0/service.go:187
go.opentelemetry.io/collector/service.New
	go.opentelemetry.io/collector/service@v0.130.0/service.go:218
go.opentelemetry.io/collector/otelcol.(*Collector).setupConfigurationComponents
	go.opentelemetry.io/collector/otelcol@v0.130.0/collector.go:197
go.opentelemetry.io/collector/otelcol.(*Collector).Run
	go.opentelemetry.io/collector/otelcol@v0.130.0/collector.go:312
go.opentelemetry.io/collector/otelcol.NewCommand.func1
	go.opentelemetry.io/collector/otelcol@v0.130.0/command.go:39
github.com/spf13/cobra.(*Command).execute
	github.com/spf13/cobra@v1.9.1/command.go:1015
github.com/spf13/cobra.(*Command).ExecuteC
	github.com/spf13/cobra@v1.9.1/command.go:1148
github.com/spf13/cobra.(*Command).Execute
	github.com/spf13/cobra@v1.9.1/command.go:1071
main.runInteractive
	github.com/open-telemetry/opentelemetry-collector-releases/contrib/main.go:70
main.run
	github.com/open-telemetry/opentelemetry-collector-releases/contrib/main_others.go:10
main.main
	github.com/open-telemetry/opentelemetry-collector-releases/contrib/main.go:63
runtime.main
	runtime/proc.go:283
Error: failed to build pipelines: failed to create "prometheus" receiver for data type "metrics": failed to reload Prometheus config: prometheus receiver: failed to marshal config to yaml: [1:1] unexpected directive value. document not started
>  1 | %password
       ^

2025/11/21 04:35:18 collector server run finished with error: failed to build pipelines: failed to create "prometheus" receiver for data type "metrics": failed to reload Prometheus config: prometheus receiver: failed to marshal config to yaml: [1:1] unexpected directive value. document not started
>  1 | %password
       ^

Collector version

0.130.0

Environment information

Environment

OS: (e.g., "Ubuntu 20.04")
Compiler(if manually compiled): (e.g., "go 14.2")

OpenTelemetry Collector configuration

Log output

Additional context

No response

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[github-actions]

Pinging code owners:

• receiver/prometheus: @Aneurysm9 @dashpole @ArthurSens @krajorama

See Adding Labels via Comments if you do not have permissions to add labels yourself.

[dashpole]

cc @erikburt as well

[erikburt]

I believe I have the fix for this in #44602