Reliance on Untrusted Inputs in a Security Decision

Description

The product uses a protection mechanism that relies on the existence or values of an input, but the input can be modified by an untrusted actor in a way that bypasses the protection mechanism.

Extended Description

Developers may assume that inputs such as cookies, environment variables, and hidden form fields cannot be modified. However, an attacker could change these inputs using customized clients or other attacks. This change might not be detected. When security decisions such as authentication and authorization are made based on the values of these inputs, attackers can bypass the security of the software. Without sufficient encryption, integrity checking, or other mechanism, any input that originates from an outsider cannot be trusted.

Common Consequences 1

Scope: ConfidentialityAccess ControlAvailabilityOther

Impact: Bypass Protection MechanismGain Privileges or Assume IdentityVaries by Context

Attackers can bypass the security decision to access whatever is being protected. The consequences will depend on the associated functionality, but they can range from granting additional privileges to untrusted users to bypassing important security checks. Ultimately, this weakness may lead to exposure or modification of sensitive data, system crash, or execution of arbitrary code.

Detection Methods 8

Manual Static AnalysisHigh

Since this weakness does not typically appear frequently within a single software package, manual white box techniques may be able to provide sufficient code coverage and reduction of false positives if all potentially-vulnerable operations can be assessed within limited time constraints.

Automated Static Analysis - Binary or BytecodeSOAR Partial

According to SOAR [REF-1479], the following detection techniques may be useful: ``` Cost effective for partial coverage: ``` Bytecode Weakness Analysis - including disassembler + source code weakness analysis Binary Weakness Analysis - including disassembler + source code weakness analysis

Manual Static Analysis - Binary or BytecodeSOAR Partial

According to SOAR [REF-1479], the following detection techniques may be useful: ``` Cost effective for partial coverage: ``` Binary / Bytecode disassembler - then use manual analysis for vulnerabilities & anomalies

Dynamic Analysis with Automated Results InterpretationSOAR Partial

According to SOAR [REF-1479], the following detection techniques may be useful: ``` Cost effective for partial coverage: ``` Web Application Scanner Web Services Scanner Database Scanners

Dynamic Analysis with Manual Results InterpretationSOAR Partial

According to SOAR [REF-1479], the following detection techniques may be useful: ``` Cost effective for partial coverage: ``` Fuzz Tester Framework-based Fuzzer Monitored Virtual Environment - run potentially malicious code in sandbox / wrapper / virtual machine, see if it does anything suspicious

Manual Static Analysis - Source CodeHigh

According to SOAR [REF-1479], the following detection techniques may be useful: ``` Highly cost effective: ``` Manual Source Code Review (not inspections)

Automated Static Analysis - Source CodeSOAR Partial

According to SOAR [REF-1479], the following detection techniques may be useful: ``` Cost effective for partial coverage: ``` Source code Weakness Analyzer Context-configured Source Code Weakness Analyzer

Architecture or Design ReviewHigh

According to SOAR [REF-1479], the following detection techniques may be useful: ``` Highly cost effective: ``` Inspection (IEEE 1028 standard) (can apply to requirements, design, source code, etc.) Formal Methods / Correct-By-Construction ``` Cost effective for partial coverage: ``` Attack Modeling

Potential Mitigations 5

Phase: Architecture and Design

Strategy: Attack Surface Reduction

Store state information and sensitive data 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. If information must be stored on the client, do not do so without encryption and integrity checking, or otherwise having a mechanism on the server side to catch tampering. Use a message authentication code (MAC) algorithm, such as Hash Message Authentication Code (HMAC) [REF-529]. Apply this against the state or sensitive data that has to be exposed, which can guarantee the integrity of the data - i.e., that the data has not been modified. Ensure that a strong hash function is used (Use of Weak Hash).

Phase: Architecture and Design

Strategy: Libraries or Frameworks

Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. With a stateless protocol such as HTTP, use a framework that maintains the state for you. Examples include ASP.NET View State [REF-756] and the OWASP ESAPI Session Management feature [REF-45]. 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.

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, in order to avoid Client-Side Enforcement of Server-Side Security. 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.

Phase: OperationImplementation

Strategy: Environment Hardening

When using PHP, configure the application so that it does not use register_globals. During implementation, develop the 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 Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection'), Variable Extraction Error, and similar issues.

Phase: Architecture and DesignImplementation

Strategy: Attack Surface Reduction

Understand all the potential areas where untrusted inputs can enter your software: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide data to the application. Remember that such inputs may be obtained indirectly through API calls. Identify all inputs that are used for security decisions and determine if you can modify the design so that you do not have to rely on submitted inputs at all. For example, you may be able to keep critical information about the user's session on the server side instead of recording it within external data.

Demonstrative Examples 4

ID : DX-15

The following code excerpt reads a value from a browser cookie to determine the role of the user.

Code Example:Bad
Java
java

ID : DX-16

The following code could be for a medical records application. It performs authentication by checking if a cookie has been set.

Code Example:Bad
PHP
php

The programmer expects that the AuthenticateUser() check will always be applied, and the "authenticated" cookie will only be set when authentication succeeds. The programmer even diligently specifies a 2-hour expiration for the cookie.

However, the attacker can set the "authenticated" cookie to a non-zero value such as 1. As a result, the $auth variable is 1, and the AuthenticateUser() check is not even performed. The attacker has bypassed the authentication.

ID : DX-17

In the following example, an authentication flag is read from a browser cookie, thus allowing for external control of user state data.

Code Example:Bad
Java
java

ID : DX-93

The following code samples use a DNS lookup in order to decide whether or not an inbound request is from a trusted host. If an attacker can poison the DNS cache, they can gain trusted status.

Code Example:Bad
C
c

Code Example:Bad
Java
java

Code Example:Bad
C#
c#

IP addresses are more reliable than DNS names, but they can also be spoofed. Attackers can easily forge the source IP address of the packets they send, but response packets will return to the forged IP address. To see the response packets, the attacker has to sniff the traffic between the victim machine and the forged IP address. In order to accomplish the required sniffing, attackers typically attempt to locate themselves on the same subnet as the victim machine. Attackers may be able to circumvent this requirement by using source routing, but source routing is disabled across much of the Internet today. In summary, IP address verification can be a useful part of an authentication scheme, but it should not be the single factor required for authentication.

Observed Examples 5

CVE-2009-1549Attacker can bypass authentication by setting a cookie to a specific value.

CVE-2009-1619Attacker can bypass authentication and gain admin privileges by setting an "admin" cookie to 1.

CVE-2009-0864Content management system allows admin privileges by setting a "login" cookie to "OK."

CVE-2008-5784e-dating application allows admin privileges by setting the admin cookie to 1.

CVE-2008-6291Web-based email list manager allows attackers to gain admin privileges by setting a login cookie to "admin."

References 5

Top 25 Series - Rank 6 - Reliance on Untrusted Inputs in a Security Decision

Frank Kim

SANS Software Security Institute

05-03-2010

https://www.sans.org/blog/top-25-series-rank-6-reliance-on-untrusted-inputs-in-a-security-decision/(2023-04-07)

ID: REF-754

HMAC

Wikipedia

18-08-2011

https://en.wikipedia.org/wiki/HMAC(2023-04-07)

ID: REF-529

Understanding ASP.NET View State

Scott Mitchell

Microsoft

15-05-2004

https://learn.microsoft.com/en-us/previous-versions/dotnet/articles/ms972976(v=msdn.10)?redirectedfrom=MSDN(2023-04-07)

ID: REF-756

OWASP Enterprise Security API (ESAPI) Project

OWASP

https://owasp.org/www-project-enterprise-security-api/(2025-07-24)

ID: REF-45

State-of-the-Art Resources (SOAR) for Software Vulnerability Detection, Test, and Evaluation

Gregory Larsen, E. Kenneth Hong Fong, David A. Wheeler, and Rama S. Moorthy

07-2014

https://www.ida.org/-/media/feature/publications/s/st/stateoftheart-resources-soar-for-software-vulnerability-detection-test-and-evaluation/p-5061.ashx(2025-09-05)

ID: REF-1479

Likelihood of Exploit

High

Applicable Platforms

Languages:

Not Language-Specific : Undetermined

Modes of Introduction

Architecture and Design

Implementation

Related Weaknesses

ChildOf:

Protection Mechanism Failure (CWE-693)

Taxonomy Mapping

The CERT Oracle Secure Coding Standard for Java (2011)