CWE-602: Client-Side Enforcement of Server-Side Security
Client-Side Enforcement of Server-Side Security
Weakness ID: 602 (Weakness Base)
Status: Draft
Description
Description Summary
The software is composed of a server that relies on the client
to implement a mechanism that is intended to protect the
server.
Extended Description
When the server relies on protection mechanisms placed on the client side,
an attacker can modify the client-side behavior to bypass the protection
mechanisms resulting in potentially unexpected interactions between the
client and server. The consequences will vary, depending on what the
mechanisms are trying to protect.
Time of Introduction
Architecture and Design
Applicable Platforms
Languages
All
Architectural Paradigms
Client-Server: (Sometimes)
Common Consequences
Scope
Effect
Integrity
Client-side validation checks can be easily bypassed, allowing
malformed or unexpected input to pass into the application, potentially
as trusted data. This may lead to unexpected states, behaviors and
possibly a resulting crash.
Access Control
Client-side checks for authentication can be easily bypassed, allowing
clients to escalate their access levels and perform unintended
actions.
Likelihood of Exploit
Medium
Enabling Factors for Exploitation
Consider a product that consists of two or more processes or nodes that
must interact closely, such as a client/server model. If the product uses
protection schemes in the client in order to defend from attacks against the
server, and the server does not use the same schemes, then an attacker could
modify the client in a way that bypasses those schemes. This is a
fundamental design flaw that is primary to many weaknesses.
Demonstrative Examples
Example 1
This example contains client-side code that checks if the user
authenticated successfully before sending a command. The server-side code
performs the authentication in one step, and executes the command in a
separate step.
CLIENT-SIDE (client.pl)
(Good Code)
Perl
$server = "server.example.com";
$username = AskForUserName();
$password = AskForPassword();
$address = AskForAddress();
$sock = OpenSocket($server, 1234);
writeSocket($sock, "AUTH $username $password\n");
$resp = readSocket($sock);
if ($resp eq "success") {
# username/pass is valid, go ahead and update the info!
writeSocket($sock, "FAILURE -- address is
malformed\n");
}
}
The server accepts 2 commands, "AUTH" which authenticates the user,
and "CHANGE-ADDRESS" which updates the address field for the username.
The client performs the authentication and only sends a CHANGE-ADDRESS
for that user if the authentication succeeds. Because the client has
already performed the authentication, the server assumes that the
username in the CHANGE-ADDRESS is the same as the authenticated user. An
attacker could modify the client by removing the code that sends the
"AUTH" command and simply executing the CHANGE-ADDRESS.
client allows server to modify client's
configuration and overwrite arbitrary files.
Potential Mitigations
Phase
Description
Architecture and Design
For any security checks that are performed on the client side, ensure
that these checks are duplicated on the server side. 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.
Even though client-side checks provide minimal benefits with respect
to server-side security, they are still useful. First, they can support
intrusion detection. If the server receives input that should have been
rejected by the client, then it may be an indication of an attack.
Second, client-side error-checking can provide helpful feedback to the
user about the expectations for valid input. Third, there may be a
reduction in server-side processing time for accidental input errors,
although this is typically a small savings.
Architecture and Design
If some degree of trust is required between the two entities, then use
integrity checking and strong authentication to ensure that the inputs
are coming from a trusted source. Design the product so that this trust
is managed in a centralized fashion, especially if there are complex or
numerous communication channels, in order to reduce the risks that the
implementer will mistakenly omit a check in a single code path.
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.
Weakness Ordinalities
Ordinality
Description
Primary
(where the
weakness exists independent of other weaknesses)
Server-side enforcement of client-side security is conceptually likely to
occur, but some architectures might have these strong dependencies as part
of legitimate behavior, such as thin clients.
updated Relationships, Other Notes, Taxonomy Mappings,
Weakness Ordinalities
2009-01-12
CWE Content Team
MITRE
Internal
updated Demonstrative Examples, Description,
Likelihood of Exploit, Name, Observed Examples, Other Notes,
Potential Mitigations, Relationships, Research Gaps,
Time of Introduction
2009-03-10
CWE Content Team
MITRE
Internal
updated Potential Mitigations
2009-05-27
CWE Content Team
MITRE
Internal
updated Demonstrative Examples
2009-07-27
CWE Content Team
MITRE
Internal
updated Related Attack Patterns,
Relationships
2009-10-29
CWE Content Team
MITRE
Internal
updated Applicable Platforms, Common Consequences,
Description
Description
