Input Validation

James Walden

Northern Kentucky University

Topics

1. The Nature of Trust

2. Validating Input

3. Entry Points

4. Web Application Input

Trust Relationships

Relationship between multiple entities. Assumptions that certain properties are true.

- example: input has a certain format

Assumptions that other properties are false.- example: input never longer than X bytes

Trustworthy entities satisfy assumptions.

Who do you trust?

Client users example: encryption key embedded in client

Operating system example: dynamicly loaded libraries

Calling program example: environment variables

Vendor example: Borland Interbase backdoor 1994-

2001, only discovered when program made open source

Trust is Transitive

If you call another program, you are trusting the entities that it trusts. Processes you spawn run with your privileges. Did you run the program you think you did?

- PATH and IFS environment variables

What input format does it use?- Shell escapes in editors and mailers

What output does it send you?

Validate All Input

Never trust input. Assume dangerous until proven safe.

Prefer rejecting data to filtering data. Difficult to filter out all dangerous input

Every component should validate data. Trust is transitive. Don’t trust calling component. Don’t trust called component: shell, SQL

Validation Techniques

Indirect Selection Allow user to supply index into a list of

legitimate values. Application never directly uses user input.

Whitelist List of valid patterns or strings. Input rejected unless it matches list.

Blacklist List of invalid patterns or strings. Input reject if it matches list.

Trust Boundaries

Safe SyntaxApp

Logic

Syntax Validation Semantic Validation

RawInput

RawInput

RawInput

Trust Boundaries

Wrap Dangerous Functions

Input is context sensitive. Need more context than is

available at front end.

Soln: create secure API Apply context-sensitive

input validation to all input. Maintain input validation

login in one place. Ensure validation always

applied. Use static analysis to check

for use of dangerous functions replaced by API.

OWASP ESAPI

Usability Validation ≠ Security Validation

Usability Validation helps legimitate users Catch common errors. Provide easy to understand feedback. Client-side feedback is helpful for speed.

Security Validation mitigates vulnerabilities Catches potential attacks, including unusual,

unfriendly types of input. Provide little to no feedback on reasons for

blocking input. Cannot trust client. Always server side.

Check Input Length

Long input can result in buffer overflows. Can also cause DoS due to low memory.

Truncation vulnerabilities 8-character long username column in DB. User tries to enter ‘admin x’ as username. DB returns no match since name is 9 chars. App inserts data into DB, which truncates. Later SQL queries will return both names,

since MySQL ignores trailing spaces on string comparisons.

Entry Points

1. Command line arguments

2. Environment variables

3. File descriptors

4. Signal handlers

5. Format strings

6. Paths

7. Shell input

8. Web application input

9. Database input

10. Other input types

Command Line Arguments

Available to program as **argv.

execve() allows user to specify arguments.

May be of any length even program name, argv[0] argv[0] may even be NULL

Environment Variables

Default: inherit parent’s environment.

execve() allows you to specify environment variables for exec’d process. environment variables can be of any length.

Telnet environment propagation to server Server receives client shell’s environment. Server runs setuid program login. ssh may use user’s ~/.ssh/environment file.

Dangerous Environment Variables

LD_PRELOAD Programs loads functions from library specified in

LD_PRELOAD before searching for system libraries. Can replace any library function. setuid root programs don’t honor this variable.

LD_LIBRARY_PATH Specify list of paths to search for shared libs. Store hacked version of library in first directory. Modern libc implementation disallow for setuid/setgid.

Dangerous Environment Variables

PATH Search path for binaries Attacker puts directory with hacked binary first in

PATH so his ls used instead of system ls Avoid “.” as attacker may place hacked binaries in

directory program sets CWD to

IFS Internal field separator for shell Used to separate command line into arguments Attacker sets to “/”: /bin/ls becomes “bin” and “ls”

Environment Storage Format

Access Functions setenv(), getenv()

Internal Storage Format array of character pointers, NULL terminated string format: “NAME=value”, NULL term Multiple env variables can have same name. Did you check the same variable that you

fetched? First or last variable that matches?

Securing Your Environment/* BSS, pp. 318-319 */extern char **environ;static char *def_env[] = {“PATH=/bin:/usr/bin”,“IFS= \t\n”,0

};static void clean_environment() {int i = -1;while( environ[++i] != 0 );while(i--)

environ[i] = 0;while(def_env[i])

putenv(def_env[i++]);}

Securing Your Environment

Secure Environment in Shell/usr/bin/env – PATH=/bin:/usr/bin IFS=“ \t\n” cmd

Secure Environment in Perl%ENV = (

PATH => “/bin:/usr/bin”,IFS => “ \t\n”

);

File Descriptors Default: inherited from parent process

stdin, stdout, stderr usually fd’s 0, 1, and 2 Parent process may have closed or redirected

standard file descriptors Parent may have left some fd’s open

Cannot assume first file opened will have fd 3 Parent process may not have left enough file

descriptors for your program Check using code from BSS, p. 315

Signal Handlers

Default: inherited from parent process.

/* BSS, p. 316 */#include <signal.h>

int main( int argc, char **argv ) {int i;for(i=0; i<NSIG; i++)

signal(I, SIG_DFL);}

Format Strings

Formatted output functions use format lang. Percent(%) symbols in string indicate substitutions. %[flags][width][.precision][length]specifier

Example format specifiers “%010d”, 2009: 0000002009 “%4.2f”, 3.1415926: 3.14

Example functions printf() scanf() syslog()

printf() family dangers

User-specified format strings userstring = “foo %x”; printf( userstring ); Where can it find arguments to replace %x?

- The Stack: %x reads 4-bytes higher in stack

Solution: Useprintf( “%s”, userstring ) orfputs( userstring )

printf() family dangers

Buffer overflowschar buf[256];sprintf( buf, “The data is %s\n”, userstring );

Specify “precision” of string substitutionsprintf(buf,“The data is .32%s\n”,userstring );

Use snprintf (C99 standard function)snprintf(buf, 255, “The data is %s\n”, userstring );

%n format command

Number of characters written so far is stored into the integer indicated by the int * pointer argument.

char buf[] = "0123456789";int *n;printf(“buf=%s%n\n", buf, n);printf("n=%d\n", *n);

Output: buf=0123456789 n=14

%n format attack

Plan of Attack Find address of variable to overwrite Place address of variable on stack (as part of

format string) so %n will write to that address Write # of characters equal to value to insert

into variable (use precision, e.g., %.64x)

Use %n to write anywhere in memory Address on stack can point to any location

Paths

If attacker controls paths used by program Can read files accessible by program. Can write files accessible by program.

Vuln if access is different than attackers Privileged (SETUID) local programs. Remote server applications, including web.

Directory traversal Use “../../..” to climb out of application’s

directory and access files.

Canonicalization

How to make correct access control decisions when there are many names? config ./config /etc/program/config ../program/config /tmp/../etc/program/config

Canonical Name: standard form of a name Generally simplest form. Canonicalize name then apply access control. Use realpath() in C to canonicalize.

Common Naming Issues

. represents current directory .. represents previous directory Case sensitivity Windows allows both / and \ in URLs. Windows 8.3 representation of long names

Two names for each file for backwards compat.

Trailing dot in DNS names www.nku.edu. == www.nku.edu

URL encoding

Win/Apache Directory Traversal

Found in Apache 2.0.39 and earlier.

To view the file winnt\win.ini, use: http://127.0.0.1/error/%5c%2e%2e%5c%2e%2e%5c%2e%2e%5c%2e%2e%5cwinnt%5cwin.ini

which is the escaped form of

http://127.0.0.1/error/\..\..\..\..\winnt\win.ini

Command Injection

Find program that invokes a subshell command with user input

UNIX C: system(), popen(), …

Windows C: CreateProcess(), ShellExecute()

Java: java.lang.Runtime.exec()

Perl: system(), ``, open()

Use shell meta-characters to insert user-defined code into the command.

UNIX Shell Metacharacters

`command` will execute command‘;’ separates commands‘|’ creates a pipe between two commands‘&&’ and ‘||’ logical operators which may execute

following command‘!’ logical negation—reverses truth value of test‘-’ could convert filename into an argument‘*’ and ‘?’ glob, matching files, which may be

interpreted as args: what if “-rf” is file?‘#’ comments to end of line

Command Injection in C

/* Mail to root with user-defined subject */

int main( int argc, char **argv ) {

char buf[1024];

sprintf( buf, “/bin/mail –s %s root </tmp/message”, argv[1] );

system( buf );

}

Command Injection in C

How to exploit?./mailprog \`/path/to/hacked_bin\`

/path/to/hacked_bin will be run by mailprog

How to fix?Verify input matches list of safe strings.

Run /bin/mail using fork/exec w/o a subshell.

Command Injection in Java

String btype = request.getParameter("backuptype");

String cmd = new String("cmd.exe /K \"c:\\util\\rmanDB.bat "+btype+"&&c:\\utl\\cleanup.bat\"");

System.Runtime.getRuntime().exec(cmd);

Command Injection in Java

How to exploit?Edit HTTP parameter via web browser.

Set bype to be “&& del c:\\dbms\\*.*”

How to defend?Verify input matches list of safe strings.

Run commands separately w/o cmd.exe.

Web-based Input

Sources of Input: URLs, including paths + parameters POST form parameters HTTP headers Cookies

Common Types of Input: HTML Javascript URL-encoded parameters XML/JSON

Different Perspectives

Client Dangers

Dangerous code- ActiveX- ActionScript- Javascript- Java

Client-side storage- Cookies- Flash LSOs- DOM storage

Server Dangers

No data sent to client is secret: Hidden fields Cookies

User controls client. Can bypass validation. Can access URLs in

any order. Can alter client-side

storage.

URL Parameters

<proto>://<user>@<host>:<port>/<path>?<qstr>

Whitespace marks end of URL

“@” separates userinfo from host

“?” marks beginning of query string

“&” separates query parameters

%HH represents character with hex values ex: %20 represents a space

HTML Special Characters

“<“ begins a tag

“>” ends a tagsome browsers will auto-insert matching “<“

“&” begins a character entityex: &lt; represents literal “<“ character

Quotes(‘ and “) used to enclose attribute values, but don’t have to be used.

Character Set Encoding

Default: ISO-8859-1 (Latin-1) Char sets dictate which chars are special UTF-8 allows multiple representations Force Latin-1 encoding of web page with:

<META http-equiv=“Content-Type” content=“text/html; charset=ISO-8859-1”>

Cookies

Parameters Name Value Expiration Date Domain Path Secure Connections Only

Cookies

Server to ClientContent-type: text/html

Set-Cookie: foo=bar; path=/; expires Fri, 20-Feb-2004 23:59:00 GMT

Client to ServerContent-type: text/html

Cookie: foo=bar

Secure Cookie Authentication

Encrypt cookie so user cannot read Include expiration time inside cookie Include client IP address to avoid hijacking

Use another cookie with MAC of first cookie to detect tampering Use secret key as part of MAC so client does

not have necessary information to forge

Database Input

SQL Injection Most common flaw in database input parsing. Don’t pass unvalidated data to database. Whitelist for known safe character set.

- Alphanumerics- How many symbols do you need to accept?

Don’t trust input from database. Check that you receive expected # of rows. Check for safe data to avoid stored XSS and

second order SQL injection attacks.

Other Inputs

Default file permissions umask(066);

Resource Limits May suffer DoS if parent imposes strict limits

on CPU time, # processes, file size, stack size.

Use setrlimit() to limit core dump size to zero if program ever contains confidential data in memory, e.g., unencrypted passwords.

Key Points

1. Validate input from all sources.CLI args, env vars, config files, database, etc.

2. Use the strongest possible technique.1. Indirect Selection

2. Whitelist

3. Blacklist

3. Reject bad input, don’t attempt to fix it.

4. Trust is transitive.

5. Architect for validation: establish trust boundaries, wrap dangerous functions.

References

1. Brian Chess and Jacob West, Secure Programming with Static Analysis, Addison-Wesley, 2007.

2. Steve McConnell, Code Complete, 2/e, Microsoft Press, 2004.3. Gary McGraw, Software Security, Addison-Wesley, 2006.4. PCI Security Standards Council, PCI DSS Requirements and

Security Assessment Procedures, v1.2, 2008.5. Mark Graff and Kenneth van Wyk, Secure Coding: Principles &

Practices, O’Reilly, 2003.6. Michael Howard and David LeBlanc, Writing Secure Code, 2nd

edition, Microsoft Press, 2003.7. Michael Howard, David LeBlanc, and John Viega, 19 Deadly Sins

of Software Security, McGraw-Hill Osborne, 2005.8. John Viega, and Gary McGraw, Building Secure Software,

Addison-Wesley, 2002.9. David Wheeler, Secure Programming for UNIX and Linux

HOWTO, http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/index.html, 2003.