CWE-179: Incorrect Behavior Order: Early Validation

Q: How can CWE-179 be exploited?

Attackers can exploit CWE-179 (Incorrect Behavior Order: Early Validation) to bypass protection mechanism, execute unauthorized code or commands. This weakness is typically introduced during the Implementation phase of software development.

Q: How do I prevent CWE-179?

Key mitigations include: Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (C

Q: What is the severity of CWE-179?

CWE-179 is classified as a Base-level weakness (Medium abstraction). It has been observed in 7 real-world CVEs.

Description

The product validates input before applying protection mechanisms that modify the input, which could allow an attacker to bypass the validation via dangerous inputs that only arise after the modification.

Product needs to validate data at the proper time, after data has been canonicalized and cleansed. Early validation is susceptible to various manipulations that result in dangerous inputs that are produced by canonicalization and cleansing.

Potential Impact

Access Control, Integrity

Bypass Protection Mechanism, Execute Unauthorized Code or Commands

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 script creates a subdirectory within a user directory and sets the user as the owner.

Bad

function createDir($userName,$dirName){$userDir = '/users/'. $userName;if(strpos($dirName,'..') !== false){echo 'Directory name contains invalid sequence';return;}
                        //filter out '~' because other scripts identify user directories by this prefix
                        $dirName = str_replace('~','',$dirName);$newDir = $userDir . $dirName;mkdir($newDir, 0700);chown($newDir,$userName);}

While the script attempts to screen for '..' sequences, an attacker can submit a directory path including ".~.", which will then become ".." after the filtering step. This allows a Path Traversal (CWE-21) attack to occur.

Mitigations & Prevention

Implementation

Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.

Real-World CVE Examples

CVE ID	Description
CVE-2002-0433	List files in web server using "*.ext"
CVE-2003-0332	Product modifies the first two letters of a filename extension after performing a security check, which allows remote attackers to bypass authentication via a filename with a .ats extension instead of
CVE-2002-0802	Database consumes an extra character when processing a character that cannot be converted, which could remove an escape character from the query and make the application subject to SQL injection attac
CVE-2000-0191	Overlaps "fakechild/../realchild"
CVE-2004-2363	Product checks URI for "<" and other literal characters, but does it before hex decoding the URI, so "%3E" and other sequences are allowed.
CVE-2002-0934	Directory traversal vulnerability allows remote attackers to read or modify arbitrary files via invalid characters between two . (dot) characters, which are filtered and result in a ".." sequence.
CVE-2003-0282	Directory traversal vulnerability allows attackers to overwrite arbitrary files via invalid characters between two . (dot) characters, which are filtered and result in a ".." sequence.

Taxonomy Mappings

PLOVER: — Early Validation Errors

Frequently Asked Questions

What is CWE-179?

CWE-179 (Incorrect Behavior Order: Early Validation) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Base-level weakness. The product validates input before applying protection mechanisms that modify the input, which could allow an attacker to bypass the validation via dangerous inputs that only arise after the modificat...

How can CWE-179 be exploited?

Attackers can exploit CWE-179 (Incorrect Behavior Order: Early Validation) to bypass protection mechanism, execute unauthorized code or commands. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-179?