CWE-696: Incorrect Behavior Order

Description

The product performs multiple related behaviors, but the behaviors are performed in the wrong order in ways that may produce resultant weaknesses.

Potential Impact

Integrity

Alter Execution Logic

Demonstrative Examples

The following code attempts to validate a given input path by checking it against an allowlist and then return the canonical path. In this specific case, the path is considered valid if it starts with the string "/safe_dir/".

Bad

String path = getInputPath();if (path.startsWith("/safe_dir/")){File f = new File(path);return f.getCanonicalPath();}

The problem with the above code is that the validation step occurs before canonicalization occurs. An attacker could provide an input path of "/safe_dir/../" that would pass the validation step. However, the canonicalization process sees the double dot as a traversal to the parent directory and hence when canonicized the path would become just "/".

To avoid this problem, validation should occur after canonicalization takes place. In this case canonicalization occurs during the initialization of the File object. The code below fixes the issue.

Good

String path = getInputPath();File f = new File(path);if (f.getCanonicalPath().startsWith("/safe_dir/")){return f.getCanonicalPath();}

This function prints the contents of a specified file requested by a user.

Bad

function printFile($username,$filename){
                        
                           
                           //read file into string
                           $file = file_get_contents($filename);if ($file && isOwnerOf($username,$filename)){echo $file;return true;}else{echo 'You are not authorized to view this file';}return false;
                     }

This code first reads a specified file into memory, then prints the file if the user is authorized to see its contents. The read of the file into memory may be resource intensive and is unnecessary if the user is not allowed to see the file anyway.

Assume that the module foo_bar implements a protected register. The register content is the asset. Only transactions made by user id (indicated by signal usr_id) 0x4 are allowed to modify the register contents. The signal grant_access is used to provide access.

Bad

module foo_bar(data_out, usr_id, data_in, clk, rst_n);
       			output reg [7:0] data_out;
       			input wire [2:0] usr_id;
       			input wire [7:0] data_in; 
       			input wire clk, rst_n;
       			wire grant_access;
       			always @ (posedge clk or negedge rst_n)
       			begin
       			
				  if (!rst_n)
				  
					data_out = 0;
				  
				  else
				  
					data_out = (grant_access) ? data_in : data_out;
					assign grant_access = (usr_id == 3'h4) ? 1'b1 : 1'b0;
				  
			    
			    end
			    endmodule

This code uses Verilog blocking assignments for data_out and grant_access. Therefore, these assignments happen sequentially (i.e., data_out is updated to new value first, and grant_access is updated the next cycle) and not in parallel. Therefore, the asset data_out is allowed to be modified even before the access control check is complete and grant_access signal is set. Since grant_access does not have a reset value, it will be meta-stable and will randomly go to either 0 or 1.

Flipping the order of the assignment of data_out and grant_access should solve the problem. The correct snippet of code is shown below.

Good

always @ (posedge clk or negedge rst_n)
       			begin
			    
				  if (!rst_n)
				  
					data_out = 0;
				  
				  else
				  
					assign grant_access = (usr_id == 3'h4) ? 1'b1 : 1'b0;
					data_out = (grant_access) ? data_in : data_out;
				  
			    
			    end
       			endmodule

Real-World CVE Examples

CVE ID	Description
CVE-2019-9805	Chain: Creation of the packet client occurs before initialization is complete (CWE-696) resulting in a read from uninitialized memory (CWE-908), causing memory corruption.
CVE-2007-5191	file-system management programs call the setuid and setgid functions in the wrong order and do not check the return values, allowing attackers to gain unintended privileges
CVE-2007-1588	C++ web server program calls Process::setuid before calling Process::setgid, preventing it from dropping privileges, potentially allowing CGI programs to be called with higher privileges than intended
CVE-2022-37734	Chain: lexer in Java-based GraphQL server does not enforce maximum of tokens early enough (CWE-696), allowing excessive CPU consumption (CWE-1176)

Taxonomy Mappings

CERT C Secure Coding: POS36-C — Observe correct revocation order while relinquishing privileges

Frequently Asked Questions

What is CWE-696?

CWE-696 (Incorrect Behavior Order) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Class-level weakness. The product performs multiple related behaviors, but the behaviors are performed in the wrong order in ways that may produce resultant weaknesses.

How can CWE-696 be exploited?

Attackers can exploit CWE-696 (Incorrect Behavior Order) to alter execution logic. This weakness is typically introduced during the Architecture and Design, Implementation phase of software development.

How do I prevent CWE-696?

Follow secure coding practices, conduct code reviews, and use automated security testing tools (SAST/DAST) to detect this weakness early in the development lifecycle.

What is the severity of CWE-696?

CWE-696 is classified as a Class-level weakness (High abstraction). It has been observed in 4 real-world CVEs.