Many problems security, to systems that are accessible via Web applications. One example Cross-Site  Scripting  attacks  (XSS),  wherein  a  script is injected into an application and executed by the browser or  server,  granting  a  malicious  user  unauthorized  access to  system  resources  or  sensitive  information.  On  the  Top10  2010  list  published  by  Open  Web  Application  Security Project  (OWASP),  XSS  attack  is  listed  as  number  two. “Cross-Site Scripting (XSS) attacks occur when: 1. Data enters a Web application through an untrusted source, most frequently a web request. 2. The data is included in dynamic content that is sent to a web user without being validated for malicious code” [1].XSS attacks of type 1 are commonly referred to as First Order, or reflected, XSS attacks. This type of attack reflects information immediately from the server. The user is either the  attacker  himself  or  the  victim  of  a  social  engineering ruse  (e.g.,  an  emailed  link  containing  the  attack).  Second Order,  or  stored,  XSS  attacks  are  first  stored  permanently on the server, then later launched by an unsuspecting user. In both cases, the XSS attacks are successful only when certain  vulnerabilities  exist  in  the  application;  more  precisely,  when  such  vulnerabilities  remain  unresolved.  These vulnerabilities primarily involve allowing executable inputs from users (First Order XSS attack) or other applications or existing database elements (Second Order XSS attack) to be directly or indirectly assigned the outputs of Web application without proper sanitization. As outputs of Web applications are executed by the browser, inputs that influence the output scan inject unwanted/malicious codes that are then executed. This  classification  of  XSS  injection  attacks  aligns  with the  Code  Injection  Attack  definition  provided  by  Ray  and Ligatti [2].Research   on   XSS   aims   to   find   vulnerabilities   and/or prevent  attacks.  One  challenge  in  attack  prevention  is  discerning  which  inputs  will  result  in  attacks.  Some  basic approaches  have  been  deployed  to  detect  and  stop  attacks by checking for script tags and same-origin verification via HTML  “referer”  tags1[3].  However,  these  techniques may  not  be  effective,  as  sophisticated  attacks  can  easily circumvent tag-based detection and HTML “referer” tags are  optional  and  possibly  unavailable.  In  the  recent  past,  a number of more complex detection techniques [4]–[11] have been  proposed  that  are  either  based  on  static  analysis  or runtime monitoring. Static  analysis  uses  traditional  program  analysis  techniques to identify vulnerable code segments via taint analysis and instruments these segments to avoid their exploitation at runtime. Typically, they are not suited to find exploitation of vulnerabilities  resulting  from  conditional  copy.  Conditional copy  is  a  technique  used  in  code-interference  based  injection  attacks,  wherein  one  variable  value  is  transferred  into another  (e.g.,  character-by-character  in  a  conditional  code segment) with direct dependency of the copies or other data operations. Runtime monitoring, on the other hand, relies on a library. of attack patterns or specification of non-attack (allowable)patterns  to  detect  potential  attacks.  As  a  result,  runtime monitoring  can  incur  prohibitively  large  overhead  if  non-vulnerable  variables  are  unnecessarily  monitored  against attack patterns. Our  two-phase  method  employs  both  static  analysis  and runtime monitoring. The static analysis phase includes application translation and concolic unit testing techniques. The runtime monitoring phase uses the facts learned from static analysis to monitor the relevant variables in the application. Any existing runtime monitoring method can be used in ourruntime monitoring phase as long as it is coupled with the information generated in our static analysis phase. Therefore, in this paper, we emphasize the static analysis phase.

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Our method consists of the following steps:

1)Preprocessing:  Determine  the  inputs  and  outputs  ac-cording  to  the  grammar  of  the  Web  application  language (JSP in our case). We  statically  process  the  JSP  Web  application  one  file at  a  time  to  identify  the  inputs  and  outputs  of  each  page generated from the files. We define a grammar based on the application  language  to  capture  all  variables,  request  and response  parameters,  session  variables,  tags,  etc.  They  are easily enumerated from constructs of JSP (or any Web application language) which includes HTML tags and embedded Java code. Any information that the application expects from other  pages  within  the  application  (e.g.,  session  variables)is  also  identified  as  input,  since  we  treat  the  JSP  file  as an  atomic  unit.  This  aligns  with  Wu  and  Offutt  [18];  the authors consider “HTML file or section of a server program that  prints  HTML”  as  an  atomic  section  in  modeling  and testing Web-based applications.

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2)Translation: Convert relevant parts of the Web application to a test-program (Java in our case) that can be tested easily using existing concolic testing engines. Once preprocessing  is  complete,  the  file  translation  is performed.  Note  that  the  result  of  translation,  the  Java program, maintains the following aspects of the original JSP application:  the  relationships  between  program  variables, inputs  and  outputs,  and  control  flow  of  the  application. Finite state automaton representing vulnerable output All  other  elements  of  the  JSP  application,  that  relates  to generating  the  HTML  page  are  discarded  (commented  or put in print statements in the result of translation).The  JSP  application  of  Figure  1(a)  is  translated  to  the resulting  Java  program  as  shown  in  Figure  1(b).  The  in-puts,  outputs,  and  the  JSP  application  variables  are  also present  in  the  Java  program.  The  string  variables  are  con-verted to character arrays, as concolic testing cannot handle strings;  however,  this  does  not  result  in  any  over/under-approximation   in   the   context   of   vulnerability   detection. Lines 1–8 in Figure 1(b) encode a constructor which mimics external  input  (in  this  case, username parameter  in  JSP application).  Line  13  in  Figure  1(b)  is  the  translation  of Line  3  of  JSP  application  in  Figure  1(a).  Lines  15–18  are the  translation  of  conditional  copy  if  the  JSP  application, and Line 19 corresponds to assignment of unameto session variable uName in the JSP application.

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3)Testing:  Execute the  test-program  in  a  testing  engine which  automatically  generates  test  cases  determining vulnerabilities  in  original  Web  applications  in  term sof input/output dependencies. We  use  the  concolic  testing  tool  jCute  [12]  to  automatically  generate  input  test  cases  that  determine  vulnerable outputs:  outputs  that  are  directly  or  indirectly  assigned from inputs and can contain scripting tags. Concolic testing utilizes  a  combination  of  concrete  and  symbolic  execution to generate test cases and to effectively guide exploration of the  new  program  paths,  respectively.  The  method  relies  on careful placement of assertion statements and identifies testcases that can violate the assertions. As  our  focus  is  injection  attacks,  we  check  whether  the output  of  the  Java  program  can  evaluate  to  a  sequence  of characters representing a script. In other words, we need to check  whether  the  sequence  contains  “<”  proceeded  by  a“>”. The finite state automaton (FSA) in Figure 2 represents such a sequence; zero or more characters followed by “<”,followed by a finite sequence of characters, followed by “>”,and finally, followed by zero or more characters

4)Instrumentation:  Insert  calls  to  a  monitor  (Java  class in  our  case)  in  the  original  Web  application  at  pro-gram  points  before  vulnerable  outputs  are  used.  This monitor detects potential attacks. Applying  jCute,  we  automatically  obtain  test  cases  for which  some  output  variable  can  be  assigned  to  a  three-character sequence starting with “<” and ending with “>”.These  are  the  outputs  that  can  be  exploited  by  injection attacks,  and  therefore  their  valuation  must  be  monitored  at runtime.  We  refer  to  these  outputs  as  vulnerable  outputs. Note that there can be many outputs in the application and a  small  number  of  them  may  be  vulnerable.  By  utilizing jCute,  we  minimize  the  number  of  outputs  that  need  to  be monitored.

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