system/HTTP/ContentSecurityPolicy.php
public function setDefaultSrc($uri, ?bool $explicitReporting = null)
	{
		$this->defaultSrc = [(string) $uri => $explicitReporting ?? $this->reportOnly];

		return $this;
	}
public function setDefaultSrc($uri, ?bool $explicitReporting = null)
	{
		$this->defaultSrc = [(string) $uri => $explicitReporting ?? $this->reportOnly];

		return $this;
	}
Adds a new valid endpoint for a form's action. Can be either
a URI class or a simple string.

default_src is the URI that is used for many of the settings when
no other source has been set.

@see http://www.w3.org/TR/CSP/#directive-default-src

@param string|array $uri
@param boolean|null $explicitReporting

@return $this