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



Architectural Paradigms

Client-Server: (Sometimes)

+ Common Consequences

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


+ 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.


(Good Code)
Example Language: Perl 
$server = "";
$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, "CHANGE-ADDRESS $username $address\n";
else {
print "ERROR: Invalid Authentication!\n";


(Bad Code)
$sock = acceptSocket(1234);
($cmd, $args) = ParseClientRequest($sock);
if ($cmd eq "AUTH") {
($username, $pass) = split(/\s+/, $args, 2);
$result = AuthenticateUser($username, $pass);
writeSocket($sock, "$result\n");
# does not close the socket on failure; assumes the
# user will try again
elsif ($cmd eq "CHANGE-ADDRESS") {
if (validateAddress($args)) {
$res = UpdateDatabaseRecord($username, "address", $args);
writeSocket($sock, "SUCCESS\n");
else {
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.

+ Observed Examples
CVE-2006-6994ASP program allows upload of .asp files by bypassing client-side checks.
CVE-2007-0163steganography products embed password information in the carrier file, which can be extracted from a modified client.
CVE-2007-0164steganography products embed password information in the carrier file, which can be extracted from a modified client.
CVE-2007-0100client allows server to modify client's configuration and overwrite arbitrary files.
+ Potential Mitigations

Phase: 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.

Phase: 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.

Phase: 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.

Phase: 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
(where the weakness exists independent of other weaknesses)
+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory254Security Features
Development Concepts (primary)699
ChildOfWeakness ClassWeakness Class669Incorrect Resource Transfer Between Spheres
Research Concepts (primary)1000
ChildOfWeakness ClassWeakness Class693Protection Mechanism Failure
Research Concepts1000
ChildOfCategoryCategory722OWASP Top Ten 2004 Category A1 - Unvalidated Input
Weaknesses in OWASP Top Ten (2004) (primary)711
ChildOfCategoryCategory7532009 Top 25 - Porous Defenses
Weaknesses in the 2009 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)750
CanPrecedeWeakness BaseWeakness Base471Modification of Assumed-Immutable Data (MAID)
Research Concepts1000
PeerOfWeakness BaseWeakness Base290Authentication Bypass by Spoofing
Research Concepts1000
PeerOfWeakness ClassWeakness Class300Channel Accessible by Non-Endpoint ('Man-in-the-Middle')
Research Concepts1000
ParentOfWeakness BaseWeakness Base565Reliance on Cookies without Validation and Integrity Checking
Research Concepts1000
ParentOfWeakness BaseWeakness Base603Use of Client-Side Authentication
Research Concepts (primary)1000
+ Research Gaps

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.

+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
OWASP Top Ten 2004A1CWE More SpecificUnvalidated Input
+ Related Attack Patterns
CAPEC-IDAttack Pattern Name
(CAPEC Version: 1.4)
21Exploitation of Session Variables, Resource IDs and other Trusted Credentials
31Accessing/Intercepting/Modifying HTTP Cookies
122Exploitation of Authorization
162Manipulating hidden fields to change the normal flow of transactions (eShoplifting)
202Create Malicious Client
207Removing Important Functionality from the Client
208Removing/short-circuiting 'Purse' logic: removing/mutating 'cash' decrements
+ References
[REF-11] M. Howard and D. LeBlanc. "Writing Secure Code". Chapter 23, "Client-Side Security Is an Oxymoron" Page 687. 2nd Edition. Microsoft. 2002.
+ Content History
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Time of Introduction
2008-09-08CWE Content TeamMITREInternal
updated Relationships, Other Notes, Taxonomy Mappings, Weakness Ordinalities
2009-01-12CWE Content TeamMITREInternal
updated Demonstrative Examples, Description, Likelihood of Exploit, Name, Observed Examples, Other Notes, Potential Mitigations, Relationships, Research Gaps, Time of Introduction
2009-03-10CWE Content TeamMITREInternal
updated Potential Mitigations
2009-05-27CWE Content TeamMITREInternal
updated Demonstrative Examples
2009-07-27CWE Content TeamMITREInternal
updated Related Attack Patterns, Relationships
2009-10-29CWE Content TeamMITREInternal
updated Applicable Platforms, Common Consequences, Description
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Client-Side Enforcement of Server-Side Security
2009-01-12Design Principle Violation: Client-Side Enforcement of Server-Side Security