Incorrect Regular Expression

Opis

The product specifies a regular expression in a way that causes data to be improperly matched or compared.

When the regular expression is used in protection mechanisms such as filtering or validation, this may allow an attacker to bypass the intended restrictions on the incoming data.

Skutki

• Other

  Unexpected State; Varies by Context

  When the regular expression is not correctly specified, data might have a different format or type than the rest of the program expects, producing resultant weaknesses or errors.

• Access Control

  Bypass Protection Mechanism

  In PHP, regular expression checks can sometimes be bypassed with a null byte, leading to any number of weaknesses.

Zapobieganie

• Implementation · Refactoring

  Regular expressions can become error prone when defining a complex language even for those experienced in writing grammars. Determine if several smaller regular expressions simplify one large regular expression. Also, subject the regular expression to thorough testing techniques such as equivalence partitioning, boundary value analysis, and robustness. After testing and a reasonable confidence level is achieved, a regular expression may not be foolproof. If an exploit is allowed to slip through, then record the exploit and refactor the regular expression.

Przykłady podatności

• CVE-2002-2109 Regexp isn't "anchored" to the beginning or end, which allows spoofed values that have trusted values as substrings.
• CVE-2005-1949 Regexp for IP address isn't anchored at the end, allowing appending of shell metacharacters.
• CVE-2001-1072 Bypass access restrictions via multiple leading slash, which causes a regular expression to fail.
• CVE-2000-0115 Local user DoS via invalid regular expressions.
• CVE-2002-1527 chain: Malformed input generates a regular expression error that leads to information exposure.
• CVE-2005-1061 Certain strings are later used in a regexp, leading to a resultant crash.
• CVE-2005-2169 MFV. Regular expression intended to protect against directory traversal reduces ".../...//" to "../".
• CVE-2005-0603 Malformed regexp syntax leads to information exposure in error message.
• CVE-2005-1820 Code injection due to improper quoting of regular expression.
• CVE-2005-3153 Null byte bypasses PHP regexp check.
• CVE-2005-4155 Null byte bypasses PHP regexp check.

Słabość nadrzędna

• CWE-697 Incorrect Comparison

Może prowadzić do

• CWE-187 Partial String Comparison
• CWE-182 Collapse of Data into Unsafe Value

Słabości podrzędne

• CWE-186 Overly Restrictive Regular Expression
• CWE-625 Permissive Regular Expression

Kiedy powstaje

• Implementation

Dotyczy

Not Language-Specific

Uwagi

While there is some overlap with allowlist/denylist problems, this entry is intended to deal with incorrectly written regular expressions, regardless of their intended use. Not every regular expression is intended for use as an allowlist or denylist. In addition, allowlists and denylists can be implemented using other mechanisms besides regular expressions.

Regexp errors are likely a primary factor in many MFVs, especially those that require multiple manipulations to exploit. However, they are rarely diagnosed at this level of detail.

Źródła

1. Writing Secure Code — Michael Howard, David LeBlanc, Chapter 10, "Using Regular Expressions for Checking Input" Page 350, Microsoft Press, 2002