Why is this an issue?
Validation of X.509 certificates is essential to create secure SSL/TLS sessions not vulnerable to man-in-the-middle attacks.
The certificate chain validation includes these steps:
- The certificate is issued by its parent Certificate Authority or the root CA trusted by the system.
- Each CA is allowed to issue certificates.
- Each certificate in the chain is not expired.
It’s not recommended to reinvent the wheel by implementing custom certificate chain validation.
TLS libraries provide built-in certificate validation functions that should be used.
Noncompliant code example
libcurl
#include <curl/curl.h>
CURL *curl;
curl_global_init(CURL_GLOBAL_DEFAULT);
curl = curl_easy_init();
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/");
curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0L); // Noncompliant; CURLOPT_SSL_VERIFYPEER is set to 0, no peer's SSL certificate will be verified
//Perform the request
curl_easy_perform(curl);
OpenSSL
#include <openssl/ssl.h>
const SSL_METHOD *method = TLS_method();
SSL_CTX *ctx = SSL_CTX_new(method);
SSL_CTX_set_verify(ctx, SSL_VERIFY_NONE, NULL); // Noncompliant; SSL_VERIFY_NONE means no automatic certificate verification
SSL *ssl = SSL_new(ctx);
// ...
SSL_connect(ssl);
#include <openssl/ssl.h>
static int verify_callback(int preverify_ok, X509_STORE_CTX *ctx) { return 1; } // This callback always validate the certificate
const SSL_METHOD *method = TLS_method();
SSL_CTX *ctx = SSL_CTX_new(method);
SSL_CTX_set_verify(ctx, CURLOPT_SSL_VERIFYPEER, verify_callback); // Noncompliant; the verify callback result overrides OpenSSL built-in verification enabled by CURLOPT_SSL_VERIFYPEER option.
SSL *ssl = SSL_new(ctx);
// ...
SSL_connect(ssl);
botan
#include <botan/tls_client.h>
#include <botan/tls_callbacks.h>
#include <botan/tls_session_manager.h>
#include <botan/tls_policy.h>
#include <botan/auto_rng.h>
#include <botan/certstor.h>
#include <botan/certstor_system.h>
class Callbacks : public Botan::TLS::Callbacks
{
// ...
virtual void tls_verify_cert_chain(
const std::vector<Botan::X509_Certificate> &cert_chain,
const std::vector<std::shared_ptr<const Botan::OCSP::Response>> &ocsp_responses,
const std::vector<Botan::Certificate_Store *> &trusted_roots,
Botan::Usage_Type usage,
const std::string &hostname,
const Botan::TLS::Policy &policy) override {} // Noncompliant (secondary location), tls_verify_cert_chain never throws. Always accept server certificate and doesn't verify hostname
};
class Client_Credentials : public Botan::Credentials_Manager
{
// ...
};
Callbacks callbacks;
Botan::AutoSeeded_RNG rng;
Botan::TLS::Session_Manager_In_Memory session_mgr(rng);
Client_Credentials creds;
Botan::TLS::Strict_Policy policy;
// open the tls connection
Botan::TLS::Client client(callbacks, session_mgr, creds, policy, rng,
Botan::TLS::Server_Information("example.com", 443),
Botan::TLS::Protocol_Version::TLS_V12); // Noncompliant; uses an implementation of Botan::TLS::Callbacks that doesn't validate server certificate
Compliant solution
libcurl
#include <curl/curl.h>
CURL *curl;
curl_global_init(CURL_GLOBAL_DEFAULT);
curl = curl_easy_init();
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/");
curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 1L); // Compliant; CURLOPT_SSL_VERIFYPEER is set to 1
//Perform the request
curl_easy_perform(curl);
OpenSSL
#include <openssl/ssl.h>
const SSL_METHOD *method = TLS_method();
SSL_CTX *ctx = SSL_CTX_new(method);
SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER, NULL); // Compliant; CURLOPT_SSL_VERIFYPEER enable OpenSSL's built-in verification of the peer certificate.
SSL *ssl = SSL_new(ctx);
// ...
SSL_connect(ssl);
botan
#include <botan/tls_client.h>
#include <botan/tls_callbacks.h>
#include <botan/tls_session_manager.h>
#include <botan/tls_policy.h>
#include <botan/auto_rng.h>
#include <botan/certstor.h>
#include <botan/certstor_system.h>
// Compliant use the default implementation of tls_verify_cert_chain method which verify server certificate and hostname
class Callbacks : public Botan::TLS::Callbacks
{
// ...
};
class Client_Credentials : public Botan::Credentials_Manager
{
// ...
};
Callbacks callbacks;
Botan::AutoSeeded_RNG rng;
Botan::TLS::Session_Manager_In_Memory session_mgr(rng);
Client_Credentials creds;
Botan::TLS::Strict_Policy policy;
// open the tls connection
Botan::TLS::Client client(callbacks, session_mgr, creds, policy, rng,
Botan::TLS::Server_Information("example.com", 443),
Botan::TLS::Protocol_Version::TLS_V12); // Compliant; uses an implementation of Botan::TLS::Callbacks that validate server certificate
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