Code: int mbedtls_x509_crt_verify_with_profile( mbedtls_x509_crt *crt,
                     mbedtls_x509_crt *trust_ca,
                     mbedtls_x509_crl *ca_crl,
                     const mbedtls_x509_crt_profile *profile,
                     const char *cn, uint32_t *flags,
                     int (*f_vrfy)(void *, mbedtls_x509_crt *, int, uint32_t *),
                     void *p_vrfy )
{
    size_t cn_len;
    int ret;
    int pathlen = 0, selfsigned = 0;
    mbedtls_x509_crt *parent;
    mbedtls_x509_name *name;
     mbedtls_x509_sequence *cur = NULL;
     mbedtls_pk_type_t pk_type;
 
    if( profile == NULL )
        return( MBEDTLS_ERR_X509_BAD_INPUT_DATA );
     *flags = 0;
 
     if( cn != NULL )
     {
         name = &crt->subject;
        cn_len = strlen( cn );

        if( crt->ext_types & MBEDTLS_X509_EXT_SUBJECT_ALT_NAME )
        {
            cur = &crt->subject_alt_names;

            while( cur != NULL )
            {
                if( cur->buf.len == cn_len &&
                    x509_memcasecmp( cn, cur->buf.p, cn_len ) == 0 )
                    break;

                if( cur->buf.len > 2 &&
                    memcmp( cur->buf.p, "*.", 2 ) == 0 &&
                    x509_check_wildcard( cn, &cur->buf ) == 0 )
                {
                    break;
                }

                cur = cur->next;
            }

            if( cur == NULL )
                *flags |= MBEDTLS_X509_BADCERT_CN_MISMATCH;
        }
        else
        {
            while( name != NULL )
            {
                if( MBEDTLS_OID_CMP( MBEDTLS_OID_AT_CN, &name->oid ) == 0 )
                {
                    if( name->val.len == cn_len &&
                        x509_memcasecmp( name->val.p, cn, cn_len ) == 0 )
                        break;

                    if( name->val.len > 2 &&
                        memcmp( name->val.p, "*.", 2 ) == 0 &&
                        x509_check_wildcard( cn, &name->val ) == 0 )
                        break;
                }

                name = name->next;
            }

            if( name == NULL )
                *flags |= MBEDTLS_X509_BADCERT_CN_MISMATCH;
        }
    }

    /* Check the type and size of the key */
    pk_type = mbedtls_pk_get_type( &crt->pk );

    if( x509_profile_check_pk_alg( profile, pk_type ) != 0 )
        *flags |= MBEDTLS_X509_BADCERT_BAD_PK;

    if( x509_profile_check_key( profile, pk_type, &crt->pk ) != 0 )
        *flags |= MBEDTLS_X509_BADCERT_BAD_KEY;

    /* Look for a parent in trusted CAs */
    for( parent = trust_ca; parent != NULL; parent = parent->next )
    {
        if( x509_crt_check_parent( crt, parent, 0, pathlen == 0 ) == 0 )
            break;
    }

    if( parent != NULL )
    {
         ret = x509_crt_verify_top( crt, parent, ca_crl, profile,
                                    pathlen, selfsigned, flags, f_vrfy, p_vrfy );
         if( ret != 0 )
            return( ret );
     }
     else
     {
        /* Look for a parent upwards the chain */
        for( parent = crt->next; parent != NULL; parent = parent->next )
            if( x509_crt_check_parent( crt, parent, 0, pathlen == 0 ) == 0 )
                break;

        /* Are we part of the chain or at the top? */
        if( parent != NULL )
        {
             ret = x509_crt_verify_child( crt, parent, trust_ca, ca_crl, profile,
                                          pathlen, selfsigned, flags, f_vrfy, p_vrfy );
             if( ret != 0 )
                return( ret );
         }
         else
         {
             ret = x509_crt_verify_top( crt, trust_ca, ca_crl, profile,
                                        pathlen, selfsigned, flags, f_vrfy, p_vrfy );
             if( ret != 0 )
                return( ret );
         }
     }
 
     if( *flags != 0 )
         return( MBEDTLS_ERR_X509_CERT_VERIFY_FAILED );
 
    return( 0 );
}

Vulnerability Type: Bypass 
CWE ID: CWE-287
Summary: ARM mbed TLS before 1.3.21 and 2.x before 2.1.9, if optional authentication is configured, allows remote attackers to bypass peer authentication via an X.509 certificate chain with many intermediates. NOTE: although mbed TLS was formerly known as PolarSSL, the releases shipped with the PolarSSL name are not affected.
Commit Message: Improve behaviour on fatal errors

If we didn't walk the whole chain, then there may be any kind of errors in the
part of the chain we didn't check, so setting all flags looks like the safe
thing to do.