Exploitation of a vulnerability with commonly-used manipulations might fail, but minor variations might succeed.
CWE-184 Base Draft
Incomplete List of Disallowed Inputs
This vulnerability occurs when a security filter or validation mechanism relies on a 'denylist'—a predefined list of forbidden inputs—but that list is missing critical entries. Attackers can bypass…
Definition
What is CWE-184?
This vulnerability occurs when a security filter or validation mechanism relies on a 'denylist'—a predefined list of forbidden inputs—but that list is missing critical entries. Attackers can bypass the protection by using variations or inputs the developers didn't anticipate.
Denylist-based security is fundamentally fragile because it requires developers to predict and block every possible bad input. This approach often fails against sophisticated attacks, as attackers constantly evolve their techniques to use new encodings, character sequences, or command variations that aren't on the list. Relying solely on a denylist creates a false sense of security and is difficult to maintain over time.
For stronger protection, prioritize 'allowlisting' (only permitting known-good patterns) over denylisting. If a denylist is unavoidable, complement it with other defenses like strict input validation, output encoding, and security libraries designed to handle the specific threat. Always assume your denylist is incomplete and design your system to fail safely when an unexpected input is encountered.
Real-world impact
Real-world CVEs caused by CWE-184
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Chain: API for text generation using Large Language Models (LLMs) does not include the "\" Windows folder separator in its denylist (CWE-184) when attempting to prevent Local File Inclusion via path traversal (CWE-22), allowing deletion of arbitrary files on Windows systems.
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product uses a denylist to identify potentially dangerous content, allowing attacker to bypass a warning
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PHP remote file inclusion in web application that filters "http" and "https" URLs, but not "ftp".
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Programming language does not filter certain shell metacharacters in Windows environment.
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XSS filter doesn't filter null characters before looking for dangerous tags, which are ignored by web browsers. MIE and validate-before-cleanse.
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Web-based mail product doesn't restrict dangerous extensions such as ASPX on a web server, even though others are prohibited.
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Resultant XSS when only and are checked.
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Privileged program does not clear sensitive environment variables that are used by bash. Overlaps multiple interpretation error.
How attackers exploit it
Step-by-step attacker path
- 1
Identify a code path that handles untrusted input without validation.
- 2
Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.
- 3
Deliver the payload through a normal request and observe the application's reaction.
- 4
Iterate until the response leaks data, executes attacker code, or escalates privileges.
Vulnerable code example
Vulnerable Java
The following code attempts to stop XSS attacks by removing all occurences of "script" in an input string.
public String removeScriptTags(String input, String mask) {
return input.replaceAll("script", mask);
} Secure code example
Secure pseudo
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
const safe = validateAndEscape(input);
return executeWithGuards(safe);
} 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-184
- Implementation Do not rely exclusively on detecting disallowed inputs. There are too many variants to encode a character, especially when different environments are used, so there is a high likelihood of missing some variants. Only use detection of disallowed inputs as a mechanism for detecting suspicious activity. Ensure that you are using other protection mechanisms that only identify "good" input - such as lists of allowed inputs - and ensure that you are properly encoding your outputs.
Detection signals
How to detect CWE-184
Plexicus auto-detects CWE-184 and opens a fix PR in under 60 seconds.
Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.
Frequently asked questions What is CWE-184?
How serious is CWE-184?
What languages or platforms are affected by CWE-184?
How can I prevent CWE-184?
How does Plexicus detect and fix CWE-184?
Where can I learn more about CWE-184?
Frequently asked questions
What is CWE-184?
This vulnerability occurs when a security filter or validation mechanism relies on a 'denylist'—a predefined list of forbidden inputs—but that list is missing critical entries. Attackers can bypass the protection by using variations or inputs the developers didn't anticipate.
How serious is CWE-184?
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-184?
MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.
How can I prevent CWE-184?
Do not rely exclusively on detecting disallowed inputs. There are too many variants to encode a character, especially when different environments are used, so there is a high likelihood of missing some variants. Only use detection of disallowed inputs as a mechanism for detecting suspicious activity. Ensure that you are using other protection mechanisms that only identify "good" input - such as lists of allowed inputs - and ensure that you are properly encoding your outputs.
How does Plexicus detect and fix CWE-184?
Plexicus's SAST engine matches the data-flow signature for CWE-184 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-184?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/184.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-184
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