Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.
CWE-180 Variant Draft
Incorrect Behavior Order: Validate Before Canonicalize
This vulnerability occurs when a system checks user input for malicious content before standardizing its format, allowing specially crafted data to bypass security checks.
Definition
What is CWE-180?
This vulnerability occurs when a system checks user input for malicious content before standardizing its format, allowing specially crafted data to bypass security checks.
When validation runs before canonicalization (the process of converting data into a standard, consistent form), attackers can exploit the gap between these two steps. They can submit input that appears safe during the initial check but transforms into a dangerous payload after it's standardized. For example, an attacker might use alternate character encodings, multiple slashes, or dot sequences that resolve to a forbidden path after canonicalization.
This flaw effectively neutralizes security defenses like allow-lists or injection filters, creating a false sense of security. To prevent this, always canonicalize input first—convert it to its simplest, canonical form—and then perform validation and sanitization on that standardized data. This ensures your security logic evaluates the actual data the application will use.
Real-world impact
Real-world CVEs caused by CWE-180
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Product allows remote attackers to view restricted files via an HTTP request containing a "*" (wildcard or asterisk) character.
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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 a .hts extension.
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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 attacks.
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Overlaps "fakechild/../realchild"
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Product checks URI for "<" and other literal characters, but does it before hex decoding the URI, so "%3E" and other sequences are allowed.
How attackers exploit it
Step-by-step attacker path
- 1
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/".
- 2
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 "/".
- 3
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.
Vulnerable code example
Vulnerable Java
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/".
String path = getInputPath();
if (path.startsWith("/safe_dir/"))
{
File f = new File(path);
return f.getCanonicalPath();
} Secure code example
Secure Java
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.
String path = getInputPath();
File f = new File(path);
if (f.getCanonicalPath().startsWith("/safe_dir/"))
{
return f.getCanonicalPath();
} What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist
How to prevent CWE-180
- 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.
Detection signals
How to detect CWE-180
Run dynamic application security testing against the live endpoint.
Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.
Code review: flag any new code that handles input from this surface without using the validated framework helpers.
CWE-180
Don't catalog this weakness. Prove it's reachable.
Plexicus turns CWE catalogs into evidence: every CWE-pattern is matched against your real code graph, reach is proven on a sandbox clone, and verified findings ship as reviewed PRs.
Frequently asked questions What is CWE-180?
How serious is CWE-180?
What languages or platforms are affected by CWE-180?
How can I prevent CWE-180?
How does Plexicus detect and fix CWE-180?
Where can I learn more about CWE-180?
Frequently asked questions
What is CWE-180?
This vulnerability occurs when a system checks user input for malicious content before standardizing its format, allowing specially crafted data to bypass security checks.
How serious is CWE-180?
MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.
What languages or platforms are affected by CWE-180?
MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.
How can I prevent CWE-180?
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.
How does Plexicus detect and fix CWE-180?
Plexicus's SAST engine matches the data-flow signature for CWE-180 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.
Where can I learn more about CWE-180?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/180.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-180
CWE-179 Parent
Incorrect Behavior Order: Early Validation
This vulnerability occurs when an application validates user input before applying security filters or data normalization. Attackers can…
CWE-181 Sibling
Incorrect Behavior Order: Validate Before Filter
This vulnerability occurs when a system checks user input for validity before cleaning or filtering it. This flawed sequence allows…
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