What causes "Unable to create any system cron task" in shadow1ng/fscan?

Run with root privileges so /etc/cron.* files are writable.

Unable to create any system cron task

Error message

Unable to create any system cron task

What it means

addSystemCronJobs writes script/cron entries into system cron directories (e.g. /etc/cron.d, /etc/cron.daily) and collects the paths it successfully modified. If every write fails (len(modified) == 0) it returns this error, meaning no system cron task could be installed at all.

Source

Thrown at plugins/local/crontask.go:261

		modified = append(modified, cronFile)
	}

	// 在每个cron目录中创建脚本
	for _, cronDir := range cronDirs[1:] { // 跳过cron.d
		if _, err := os.Stat(cronDir); os.IsNotExist(err) {
			continue
		}

		scriptFile := filepath.Join(cronDir, ".system-check")
		scriptContent := fmt.Sprintf("#!/bin/bash\n%s >/dev/null 2>&1 &\n", execPath)

		if err := os.WriteFile(scriptFile, []byte(scriptContent), 0755); err == nil {
			modified = append(modified, scriptFile)
		}
	}

	if len(modified) == 0 {
		return nil, fmt.Errorf("%s", i18n.GetText("crontask_system_create_none"))
	}

	return modified, nil
}

// addAtJob 添加at延时任务
func (p *CronTaskPlugin) addAtJob(execPath string) error {
	// 检查at命令是否可用
	if _, err := exec.LookPath("at"); err != nil {
		return err
	}

	// 创建5分钟后执行的任务
	atCommand := fmt.Sprintf("echo '%s >/dev/null 2>&1' | at now + 5 minutes", execPath)
	cmd := exec.Command("sh", "-c", atCommand)
	return cmd.Run()
}

View on GitHub (pinned to 95cc12e753)

Solutions

Run with root privileges so /etc/cron.* files are writable.
Verify the system actually has cron installed and its directories exist (which crontab; ls /etc/cron.d).
Fall back to the current user's crontab (crontab -e / spool) instead of system cron dirs.
Check SELinux/AppArmor audit logs if running as root yet writes still fail.

Example fix

// before
$ systemctl run-agent ... # User=agent (non-root)
// after
# run as root or grant capability:
$ sudo ./agent --plugin crontask

Defensive patterns

Strategy: fallback

Validate before calling

if os.Geteuid() != 0 {
    return errors.New("system cron persistence requires root")
}
if _, err := os.Stat("/etc/cron.d"); err != nil {
    return errors.New("system cron directories unavailable")
}

Try / catch

res := plugin.Scan(ctx, cfg)
if !res.Success && strings.Contains(res.Error.Error(), "system cron") {
    // fall back to user crontab or skip persistence
}

Prevention

Check euid/privileges before enabling system-level persistence.
Verify cron is installed and /etc/cron.* exists on the target distro.
Keep a user-crontab fallback path for non-root deployments.

When it happens

Trigger: All os.WriteFile calls into system cron locations failed — typically EACCES because the process is not root, or the target directories do not exist on the distribution.

Common situations: Non-root service account attempting persistence on a standard server; hardened images with cron dirs removed or cron service disabled; SELinux/AppArmor blocking writes to /etc/cron.*.

Understand the failure class

Background: "failed to write file", "Could not save figure", "Error saving remote file" — file write failed: causes and fixes across languages and libraries — this error's family across 38 libraries.

Related errors

AI-assisted analysis of shadow1ng/fscan@95cc12e753 (2026-09-06). Data as JSON: /api/errors/aa38ad83227d9b28. Report an issue: GitHub.