projectdiscovery/nuclei · error

host concurrency must be at least 1

Error message

host concurrency must be at least 1

What it means

After net/url.Parse succeeds, an smb:// URL with an empty host is rejected: u.Host must be non-empty because the SMB bridge must dial a concrete server on port 445. This catches URLs where the host was omitted or the whole authority slid into the path.

Source

Thrown at lib/config.go:136

type Concurrency struct {
	TemplateConcurrency           int // number of templates to run concurrently (per host in host-spray mode)
	HostConcurrency               int // number of hosts to scan concurrently  (per template in template-spray mode)
	HeadlessHostConcurrency       int // number of hosts to scan concurrently for headless templates  (per template in template-spray mode)
	HeadlessTemplateConcurrency   int // number of templates to run concurrently for headless templates (per host in host-spray mode)
	JavascriptTemplateConcurrency int // number of templates to run concurrently for javascript templates (per host in host-spray mode)
	TemplatePayloadConcurrency    int // max concurrent payloads to run for a template (a good default is 25)
	ProbeConcurrency              int // max concurrent http probes to run (a good default is 50)
}

// WithConcurrency sets concurrency options
func WithConcurrency(opts Concurrency) NucleiSDKOptions {
	return func(e *NucleiEngine) error {
		// minimum required is 1
		if opts.TemplateConcurrency <= 0 {
			return errors.New("template threads must be at least 1")
		}
		if opts.HostConcurrency <= 0 {
			return errors.New("host concurrency must be at least 1")
		}
		if opts.HeadlessHostConcurrency <= 0 {
			return errors.New("headless host concurrency must be at least 1")
		}
		if opts.HeadlessTemplateConcurrency <= 0 {
			return errors.New("headless template threads must be at least 1")
		}
		if opts.JavascriptTemplateConcurrency <= 0 {
			return errors.New("js must be at least 1")
		}
		if opts.TemplatePayloadConcurrency <= 0 {
			return errors.New("payload concurrency must be at least 1")
		}
		if opts.ProbeConcurrency <= 0 {
			return errors.New("probe concurrency must be at least 1")
		}
		e.opts.TemplateThreads = opts.TemplateConcurrency
		e.opts.BulkSize = opts.HostConcurrency

View on GitHub (pinned to 265b3a3dec)

Solutions

1. Include the host: smb://server/share/file.txt
2. Check the string between // and the first / is not empty
3. Pre-validate generated URLs before scanning

Example fix

# before
smb:///share/file.txt
# after
smb://fileserver/share/file.txt

import "net/url"

func smbURLHasHost(raw string) bool {
    u, err := url.Parse(raw)
    if err != nil {
        return false
    }
    return u.Host != ""
}

When it happens

Trigger: smb:///share/file.txt (three slashes) or smb:// - no host component to dial.

Common situations: Typos deleting the hostname; template variables for the host expanding to an empty string.

Related errors

• headless template threads must be at least 1
• no input provider found
• template threads must be at least 1
• http: invalid url: %w
• http: url must include scheme and host

AI-assisted analysis of projectdiscovery/nuclei@265b3a3dec (2026-08-15). Data as JSON: /api/errors/be87630c4e57a203. Report an issue: GitHub.