The software stores security-critical state information about
its users, or the software itself, in a location that is accessible to
unauthorized actors.
Extended Description
If an attacker can modify the state information without detection, then it
could be used to perform unauthorized actions or access unexpected
resources, since the application programmer does not expect that the state
can be changed.
State information can be stored in various locations such as a cookie, in
a hidden web form field, input parameter or argument, an environment
variable, a database record, within a settings file, etc. All of these
locations have the potential to be modified by an attacker. When this state
information is used to control security or determine resource usage, then it
may create a vulnerability. For example, an application may perform
authentication, then save the state in an "authenticated=true" cookie. An
attacker may simply create this cookie in order to bypass the
authentication.
Time of Introduction
Architecture and Design
Implementation
Applicable Platforms
Languages
All
Technology Classes
Web-Server: (Often)
Common Consequences
Scope
Effect
Integrity
An attacker could potentially modify the state in malicious ways. If
the state is related to the privileges or level of authentication that
the user has, then state modification might allow the user to bypass
authentication or elevate privileges.
Confidentiality
The state variables may contain sensitive information that should not
be known by the client.
Availability
By modifying state variables, the attacker could violate the
application's expectations for the contents of the state, leading to a
denial of service due to an unexpected error condition.
Likelihood of Exploit
High
Enabling Factors for Exploitation
An application maintains its own state and/or user state (i.e. application
is stateful).
State information can be affected by the user of an application through
some means other than the legitimate state transitions (e.g. logging into
the system, purchasing an item, making a payment, etc.)
An application does not have means to detect state tampering and behave in
a fail safe manner.
Demonstrative Examples
Example 1
In the following example, an authentication flag is read from a
browser cookie, thus allowing for external control of user state
data.
(Bad Code)
Java
Cookie[] cookies = request.getCookies();
for (int i =0; i< cookies.length; i++) {
Cookie c = cookies[i];
if (c.getName().equals("authenticated") &&
Boolean.TRUE.equals(c.getValue())) {
authenticated = true;
}
}
Example 2
The following code segment implements a basic server that uses the
"ls" program to perform a directory listing of the directory that is listed
in the "HOMEDIR" environment variable. The code intends to allow the user to
specify an alternate "LANG" environment variable. This causes "ls" to
customize its output based on a given language, which is an important
capability when supporting internationalization.
(Bad Code)
Perl
$ENV{"HOMEDIR"} = "/home/mydir/public/";
my $stream = AcceptUntrustedInputStream();
while (<$stream>) {
chomp;
if (/^ENV ([\w\_]+) (.*)/) {
$ENV{$1} = $2;
}
elsif (/^QUIT/) { ... }
elsif (/^LIST/) {
open($fh, "/bin/ls -l $ENV{HOMEDIR}|");
while (<$fh>) {
SendOutput($stream, "FILEINFO: $_");
}
close($fh);
}
}
The programmer takes care to call a specific "ls" program and sets the
HOMEDIR to a fixed value. However, an attacker can use a command such as
"ENV HOMEDIR /secret/directory" to specify an alternate directory,
enabling a path traversal attack (CWE-22). At the same time, other
attacks are enabled as well, such as OS command injection (CWE-78) by
setting HOMEDIR to a value such as "/tmp; rm -rf /". In this case, the
programmer never intends for HOMEDIR to be modified, so input validation
for HOMEDIR is not the solution. A partial solution would be a whitelist
that only allows the LANG variable to be specified in the ENV command.
Alternately, assuming this is an authenticated user, the language could
be stored in a local file so that no ENV command at all would be
needed.
While this example may not appear realistic, this type of problem
shows up in code fairly frequently. See CVE-1999-0073 in the observed
examples for a real-world example with similar behaviors.
Server allows client to specify the search path,
which can be modified to point to a program that the client has
uploaded.
Potential Mitigations
Phase
Description
Architecture and Design
Understand all the potential locations that are accessible to
attackers. For example, some programmers assume that cookies and hidden
form fields cannot be modified by an attacker, or they may not consider
that environment variables can be modified before a privileged program
is invoked.
Architecture and Design
Do not keep state information on the client without using encryption
and integrity checking, or otherwise having a mechanism on the server
side to catch state tampering. Use a message authentication code (MAC)
algorithm, such as Hash Message Authentication Code (HMAC). Apply this
against the state data that you have to expose, which can guarantee the
integrity of the data - i.e., that the data has not been modified.
Ensure that you use an algorithm with a strong hash function
(CWE-328).
Architecture and Design
Store state information on the server side only. Ensure that the
system definitively and unambiguously keeps track of its own state and
user state and has rules defined for legitimate state transitions. Do
not allow any application user to affect state directly in any way other
than through legitimate actions leading to state transitions.
Architecture and Design
With a stateless protocol such as HTTP, use a framework that maintains
the state for you.
Examples include ASP.NET View State and the OWASP ESAPI Session
Management feature.
Be careful of language features that provide state support, since
these might be provided as a convenience to the programmer and may not
be considering security.
Architecture and Design
For any security checks that are performed on the client side, ensure
that these checks are duplicated on the server side, in order to avoid
CWE-602. Attackers can bypass the client-side checks by modifying values
after the checks have been performed, or by changing the client to
remove the client-side checks entirely. Then, these modified values
would be submitted to the server.
Operation
Implementation
If you are using PHP, configure your application so that it does not
use register_globals. During implementation, develop your application so
that it does not rely on this feature, but be wary of implementing a
register_globals emulation that is subject to weaknesses such as CWE-95,
CWE-621, and similar issues.
Testing
Use automated static analysis tools that target this type of weakness.
Many modern techniques use data flow analysis to minimize the number of
false positives. This is not a perfect solution, since 100% accuracy and
coverage are not feasible.
Testing
Use dynamic tools and techniques that interact with the software using
large test suites with many diverse inputs, such as fuzz testing
(fuzzing), robustness testing, and fault injection. The software's
operation may slow down, but it should not become unstable, crash, or
generate incorrect results.
Testing
Use tools and techniques that require manual (human) analysis, such as
penetration testing, threat modeling, and interactive tools that allow
the tester to record and modify an active session. These may be more
effective than strictly automated techniques. This is especially the
case with weaknesses that are related to design and business
rules.
Description
