CWE-179: Incorrect Behavior Order: Early Validation

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.

Extended Description:
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.

Affected languages: Not Language-Specific
Parent weaknesses: CWE-20, CWE-696

Consequences:
  - Access Control: Bypass Protection Mechanism

Mitigations:
  - [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 i

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

This script creates a subdirectory within a user directory and sets the user as the owner.
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.
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); }

Real-world examples:
  - 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.