Incorrect Regular Expression

The software 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.


The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The following code takes phone numbers as input, and uses a regular expression to reject invalid phone numbers.

$phone = GetPhoneNumber();
if ($phone =~ /\d+-\d+/) {
  # looks like it only has hyphens and digits
  system("lookup-phone $phone");
else {
  error("malformed number!");

An attacker could provide an argument such as: "; ls -l ; echo 123-456" This would pass the check, since "123-456" is sufficient to match the "\d+-\d+" portion of the regular expression.

See Also

SFP Secondary Cluster: Tainted Input to Command

This category identifies Software Fault Patterns (SFPs) within the Tainted Input to Command cluster (SFP24).

Comprehensive CWE Dictionary

This view (slice) covers all the elements in CWE.

Weaknesses without Software Fault Patterns

CWE identifiers in this view are weaknesses that do not have associated Software Fault Patterns (SFPs), as covered by the CWE-888 view. As such, they represent gaps in...

CWE Cross-section

This view contains a selection of weaknesses that represent the variety of weaknesses that are captured in CWE, at a level of abstraction that is likely to be useful t...

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