What version of Go are you using (go version)?

$ go version
1.17.9

Does this issue reproduce with the latest release?

Cannot confirm yet - we've only seen this in production systems with private user configuration.

What operating system and processor architecture are you using (go env)?

$ go env

What did you do?

The Prometheus Alertmanager uses x/text/case's title casing when examining headers in email notifier configurations. When some strings are fed to x/case it panics.

Panics have happened with private customer configuration that we are unable to access. We're still trying to find reproducable strings using the fuzzer, but nothing yet.

What did you expect to see?

An error returned for strings that cannot be title-cased.

What did you see instead?

The program panics with this stack trace:

/vendor/gopkg.in/yaml.v2/decode.go:270 +0xa7
gopkg.in/yaml%2ev2.(*decoder).callUnmarshaler(0xc20be3f980, 0xc20c116e70, {0x7f170dda42b8, 0xc0430126c0})
/vendor/github.com/prometheus/alertmanager/config/notifiers.go:196 +0x1b9
github.com/prometheus/alertmanager/config.(*EmailConfig).UnmarshalYAML(0xc0430126c0, 0x2340b20?)
	/vendor/golang.org/x/text/cases/cases.go:51
golang.org/x/text/cases.Caser.String(...)
	/vendor/golang.org/x/text/transform/transform.go:650 +0xbe5
golang.org/x/text/transform.String({0x7f170e30ea18, 0xc0002d2a00}, {0xc20fd3aeb8, 0x2})
...
runtime error: slice bounds out of range [11:7]

the panic itself happens inside x/text/transform:

n := copy(src, s[pSrc:])

There is additional context on this issue for the Prometheus Alertmanager