The software uses a regular expression to perform neutralization, but the regular expression is not anchored and may allow malicious or malformed data to slip through.
Extended Description
When performing tasks such as whitelist validation, data is examined and possibly modified to ensure that it is well-formed and adheres to a list of safe values. If the regular expression is not anchored, malicious or malformed data may be included before or after any string matching the regular expression. The type of malicious data that is allowed will depend on the context of the application and which anchors are omitted from the regular expression.
Time of Introduction
Implementation
Common Consequences
Scope
Effect
Availability
Confidentiality
Access Control
Technical Impact: Bypass protection
mechanism
An unanchored regular expression in the context of a whitelist will
possibly result in a protection mechanism failure, allowing malicious or
malformed data to enter trusted regions of the program. The specific
consequences will depend on what functionality the whitelist was
protecting.
Likelihood of Exploit
Low to Medium
Demonstrative Examples
Example 1
Consider a web application that supports multiple languages. It
selects messages for an appropriate language by using the lang
parameter.
(Bad Code)
Example
Language: PHP
$dir = "/home/cwe/languages";
$lang = $_GET['lang'];
if (preg_match("/[A-Za-z0-9]+/", $lang)) {
include("$dir/$lang");
}
else {
echo "You shall not pass!\n";
}
The previous code attempts to match only alphanumeric values so that language values such as "english" and "french" are valid while also protecting against path traversal, CWE-22. However, the regular expression anchors are omitted, so any text containing at least one alphanumeric character will now pass the validation step. For example, the attack string below will match the regular expression.
(Attack)
../../etc/passwd
If the attacker can inject code sequences into a file, such as the web
server's HTTP request log, then the attacker may be able to redirect the
lang parameter to the log file and execute arbitrary code.
Potential Mitigations
Phase: Implementation
Be sure to understand both what will be matched and what will not be
matched by a regular expression. Anchoring the ends of the expression
will allow the programmer to define a whitelist strictly limited to what
is matched by the text in the regular expression. If you are using a
package that only matches one line by default, ensure that you can match
multi-line inputs if necessary.
Background Details
Regular expressions are typically used to match a pattern of text. Anchors
are used in regular expressions to specify where the pattern should match:
at the beginning, the end, or both (the whole input).
Description
