XSS攻击通常指的是通过利用网页开发时留下的漏洞，通过巧妙的方法注入恶意指令代码到网页，使用户加载并执行攻击者恶意制造的网页程序。这些恶意网页程序通常是JavaScript，但实际上也可以包括Java、 VBScript、ActiveX、 Flash 或者甚至是普通的HTML。攻击成功后，攻击者可能得到包括但不限于更高的权限（如执行一些操作）、私密网页内容、会话和cookie等各种内容。

简单来说就是攻击者在某个表单提交了可执行的JavaScript或者html 盗取信息或者攻击网站

如何防御xss攻击？

import java.net.URLEncoder;

/**
 * 过滤非法字符工具类
 * 
 */
public class EncodeFilter {

    //过滤大部分html字符
    public static String encode(String input) {
        if (input == null) {
            return input;
        }
        StringBuilder sb = new StringBuilder(input.length());
        for (int i = 0, c = input.length(); i < c; i++) {
            char ch = input.charAt(i);
            switch (ch) {
                case '&': sb.append("&");
                    break;
                case '<': sb.append("<");
                    break;
                case '>': sb.append(">");
                    break;
                case '"': sb.append("\"");
                    break;
                case '\'': sb.append("\'");
                    break;
                case '/': sb.append("/");
                    break;
                default: sb.append(ch);
            }
        }
        return sb.toString();
    }

    //js端过滤
    public static String encodeForJS(String input) {
        if (input == null) {
            return input;
        }

        StringBuilder sb = new StringBuilder(input.length());

        for (int i = 0, c = input.length(); i < c; i++) {
            char ch = input.charAt(i);

            // do not encode alphanumeric characters and ',' '.' '_'
            if (ch >= 'a' && ch <= 'z' || ch >= 'A' && ch <= 'Z' ||
                    ch >= '0' && ch <= '9' ||
                    ch == ',' || ch == '.' || ch == '_') {
                sb.append(ch);
            } else {
                String temp = Integer.toHexString(ch);

                // encode up to 256 with \\xHH
                if (ch < 256) {
                    sb.append('\\').append('x');
                    if (temp.length() == 1) {
                        sb.append('0');
                    }
                    sb.append(temp.toLowerCase());

                // otherwise encode with \\uHHHH
                } else {
                    sb.append('\\').append('u');
                    for (int j = 0, d = 4 - temp.length(); j < d; j ++) {
                        sb.append('0');
                    }
                    sb.append(temp.toUpperCase());
                }
            }
        }

        return sb.toString();
    }

    /**
     * css非法字符过滤
     * http://www.w3.org/TR/CSS21/syndata.html#escaped-characters
    */
    public static String encodeForCSS(String input) {
        if (input == null) {
            return input;
        }

        StringBuilder sb = new StringBuilder(input.length());

        for (int i = 0, c = input.length(); i < c; i++) {
            char ch = input.charAt(i);

            // check for alphanumeric characters
            if (ch >= 'a' && ch <= 'z' || ch >= 'A' && ch <= 'Z' ||
                    ch >= '0' && ch <= '9') {
                sb.append(ch);
            } else {
                // return the hex and end in whitespace to terminate
                sb.append('\\').append(Integer.toHexString(ch)).append(' ');
            }
        }
        return sb.toString();
    }

    /**
     * URL参数编码 
     * http://en.wikipedia.org/wiki/Percent-encoding     */ 
    public static String encodeURIComponent(String input) {
        return encodeURIComponent(input, "utf-8");
    }

    public static String encodeURIComponent(String input, String encoding) {
        if (input == null) {
            return input;
        }
        String result;
        try {
            result = URLEncoder.encode(input, encoding);
        } catch (Exception e) {
            result = "";
        }
        return result;
    }

    public static boolean isValidURL(String input) {
        if (input == null || input.length() < 8) {
            return false;
        }
        char ch0 = input.charAt(0);
        if (ch0 == 'h') {
            if (input.charAt(1) == 't' &&
                input.charAt(2) == 't' &&
                input.charAt(3) == 'p') {
                char ch4 = input.charAt(4);
                if (ch4 == ':') {
                    if (input.charAt(5) == '/' &&
                        input.charAt(6) == '/') {
  
                        return isValidURLChar(input, 7);
                    } else {
                        return false;
                    }
                } else if (ch4 == 's') {
                    if (input.charAt(5) == ':' &&
                        input.charAt(6) == '/' &&
                        input.charAt(7) == '/') {

                        return isValidURLChar(input, 8);
                    } else {
                        return false;
                    }
                } else {
                    return false;
                }
            } else {
                return false;
            }
  
        } else if (ch0 == 'f') {
            if( input.charAt(1) == 't' &&
                input.charAt(2) == 'p' &&
                input.charAt(3) == ':' &&
                input.charAt(4) == '/' &&
                input.charAt(5) == '/') {
  
                return isValidURLChar(input, 6);
            } else {
                return false;
            }
        }
        return false;
    }

    static boolean isValidURLChar(String url, int start) {
        for (int i = start, c = url.length(); i < c; i ++) {
            char ch = url.charAt(i);
            if (ch == '"' || ch == '\'') {
                return false;
            }
        }
        return true;
    }

}

SQL注入漏洞的防御

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class SqlBuilder {
    protected StringBuilder sqlBuf = new StringBuilder();
    protected List<Object> values = new ArrayList<Object>();
    protected Map<String, Object> paramMap = new HashMap<String, Object>();

    public SqlBuilder appendSql(String sql) {
        sqlBuf.append(sql);
        return this;
    }

    public SqlBuilder appendValue(Object value) {
        sqlBuf.append('?');
        values.add(value);
        return this;
    }

    public SqlBuilder appendValues(Object[] values) {
        sqlBuf.append('(');
        for (int i = 0, c = values.length; i < c; ++i) {
            sqlBuf.append('?').append(',');
            this.values.add(values[i]);
        }
        int last = sqlBuf.length() - 1;
        if (last > 0 && sqlBuf.charAt(last) == ',') {
            sqlBuf.setCharAt(last, ')');
        }
        return this;
    }
  
    public SqlBuilder appendEqParam(String param, Object value) {
        sqlBuf.append(param).append(" = :").append(param);
        paramMap.put(param, value);
        return this;
    }
  
    public SqlBuilder appendLtParam(String param, Object value) {
        sqlBuf.append(param).append(" < :").append(param);
        paramMap.put(param, value);
        return this;
    }
  
    public SqlBuilder appendGtParam(String param, Object value) {
        sqlBuf.append(param).append(" > :").append(param);
        paramMap.put(param, value);
        return this;
    }
  
    public SqlBuilder appendInParam(String param, Object ... values) {
        if(values == null) {
            return this;
        }
        sqlBuf.append(param).append(" in (");
        int len = values.length;
        for(int i = 0; i < len; i++) {
            if(i != 0) {
                sqlBuf.append(", ");
            }
            sqlBuf.append(":").append(param).append(i);
            paramMap.put(param+i, values[i]);
        }
        sqlBuf.append(")");
        return this;
    }
  
    public SqlBuilder appendLikeParam(String param, Object value) {
        sqlBuf.append(param).append(" like :").append(param);
        paramMap.put(param, "%"+value+"%");
        return this;
    }

    public String getSql() {
        return sqlBuf.toString();
    }

    public Object[] getValues() {
        return values.toArray();
    }
  
    public Map<String, Object> getParamMap() {
        return paramMap;
    }
}

以上两段代码来自cnblog

link[/link]