Incomplete List of Disallowed Inputs

Opis

The product implements a protection mechanism that relies on a list of inputs (or properties of inputs) that are not allowed by policy or otherwise require other action to neutralize before additional processing takes place, but the list is incomplete.

Inne nazwy

Denylist / Deny List

This is used by CWE and CAPEC instead of other commonly-used terms. Its counterpart is allowlist.

Blocklist / Block List

This is often used by security tools such as firewalls, email or web gateways, proxies, etc.

Blacklist / Black List

This term is frequently used, but usage has been declining as organizations have started to adopt other terms.

Skutki

• Access Control

  Bypass Protection Mechanism

  Attackers may be able to find other malicious inputs that were not expected by the developer, allowing them to bypass the intended protection mechanism.

Zapobieganie

• Implementation · Input Validation

  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.

Przykłady podatności

• CVE-2024-6091 Chain: AI agent platform does not restrict pathnames containing internal "/./" sequences (CWE-55), leading to an incomplete denylist (CWE-184) that does not prevent OS command injection (CWE-78)
• CVE-2024-4315 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.
• CVE-2024-44335 Chain: filter only checks for some shell-injection characters (CWE-184), enabling OS command injection (CWE-78)
• CVE-2008-2309 product uses a denylist to identify potentially dangerous content, allowing attacker to bypass a warning
• CVE-2005-2782 PHP remote file inclusion in web application that filters "http" and "https" URLs, but not "ftp".
• CVE-2004-0542 Programming language does not filter certain shell metacharacters in Windows environment.
• CVE-2004-0595 XSS filter doesn't filter null characters before looking for dangerous tags, which are ignored by web browsers. MIE and validate-before-cleanse.
• CVE-2005-3287 Web-based mail product doesn't restrict dangerous extensions such as ASPX on a web server, even though others are prohibited.
• CVE-2004-2351 Resultant XSS when only <script> and <style> are checked.
• CVE-2005-2959 Privileged program does not clear sensitive environment variables that are used by bash. Overlaps multiple interpretation error.
• CVE-2005-1824 SQL injection protection scheme does not quote the "\" special character.
• CVE-2005-2184 Detection of risky filename extensions prevents users from automatically executing .EXE files, but .LNK is accepted, allowing resultant Windows symbolic link.
• CVE-2007-1343 Product uses list of protected variables, but accidentally omits one dangerous variable, allowing external modification
• CVE-2007-5727 Chain: product only removes SCRIPT tags (CWE-184), enabling XSS (CWE-79)
• CVE-2006-4308 Chain: product only checks for use of "javascript:" tag (CWE-184), allowing XSS (CWE-79) using other tags
• CVE-2007-3572 Chain: OS command injection (CWE-78) enabled by using an unexpected character that is not explicitly disallowed (CWE-184)
• CVE-2002-0661 "\" not in list of disallowed values for web server, allowing path traversal attacks when the server is run on Windows and other OSes.

Słabość nadrzędna

• CWE-693 Protection Mechanism Failure
• CWE-1023 Incomplete Comparison with Missing Factors

Może prowadzić do

• CWE-79 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
• CWE-78 Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')
• CWE-434 Unrestricted Upload of File with Dangerous Type
• CWE-98 Improper Control of Filename for Include/Require Statement in PHP Program ('PHP Remote File Inclusion')

Słabości podrzędne

• CWE-692 Incomplete Denylist to Cross-Site Scripting

Kiedy powstaje

• Implementation — Developers often try to protect their products against malicious input by checking against lists of known bad inputs, such as special characters that can invoke new commands. However, such lists often only address the most well-known bad inputs. As a quick fix, developers might rely on these lists instead of addressing the root cause of the issue. See [REF-141].
• Architecture and Design — The design might rely solely on detection of malicious inputs as a protection mechanism.

Dotyczy

Not Language-Specific, Not Technology-Specific

Uwagi

Multiple interpretation errors can indirectly introduce inputs that should be disallowed. For example, a list of dangerous shell metacharacters might not include a metacharacter that only has meaning in one particular shell, not all of them; or a check for XSS manipulations might ignore an unusual construct that is supported by one web browser, but not others.

Źródła

1. Exploiting Software: How to Break Code — Greg Hoglund, Gary McGraw, Addison-Wesley, 2004
2. Blacklist defenses as a breeding ground for vulnerability variants — Steve Christey, 2006
3. The Art of Software Security Assessment — Mark Dowd, John McDonald, Justin Schuh, Chapter 8, "Eliminating Metacharacters", Page 435, Addison Wesley, 2006