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The regular expressions can be defined for request paths, query parameters, form parameters, headers, XML elements (in an XML payload defined using XPath), JSON object attributes (in a JSON payload defined using JSONPath).
The following example RegularExpressionProtection policy protects the backend from SQL injection attacks:
<!-- /antipatterns/examples/greedy-1.xml --> <RegularExpressionProtection async="false" continueOnError="false" enabled="true" name="RegexProtection"> <DisplayName>RegexProtection</DisplayName> <Properties/> <Source>request</Source> <IgnoreUnresolvedVariables>false</IgnoreUnresolvedVariables> <QueryParam name="query"> <Pattern>[\s]*(?i)((delete)|(exec)|(drop\s*table)| (insert)|(shutdown)|(update)|(\bor\b))</Pattern> </QueryParam> </RegularExpressionProtection>
The default quantifiers (
?) are greedy in
nature: they start to match with the longest possible sequence. When no match is found, they
backtrack gradually to try to match the pattern. If the resultant string matching the pattern is
very short, then using greedy quantifiers can take more time than necessary. This is especially
true if the payload is large (in the tens or hundreds of KBs).
The following example expression uses multiple instances of
.*, which are greedy
<Pattern>.*Exception in thread.*</Pattern>
In this example, the RegularExpressionProtection policy first tries to match the longest possible
sequence—the entire string. If no match is found, the policy then backtracks
gradually. If the matching string is close to the start or middle of the payload, then using a
greedy quantifier like
.* can take a lot more time and processing power than reluctant
.*? or (less commonly) possessive quantifiers like
Reluctant quantifiers (like
X??) start by trying
to match a single character from the beginning of the payload and gradually add characters.
Possessive quantifiers (like
X++) try to match the
entire payload only once.
Given the following sample text for the above pattern:
Hello this is a sample text with Exception in thread with lot of text after the Exception text.
Using the greedy
.* is non-performant in this case. The pattern
.*Exception in thread.* takes 141 steps to match. If you used the pattern
.*?Exception in thread.* (which uses a reluctant quantifier) instead, the result would
be only 55 steps.
Using greedy quantifiers like wildcards (
*) with the
RegularExpressionProtection policy can lead to:
- An increase in overall latency for API requests for a moderate payload size (up to 1MB)
- Longer time to complete the execution of the RegularExpressionProtection policy
- API requests with large payloads (>1MB) failing with 504 Gateway Timeout Errors if the predefined timeout period elapses on the Edge Router
- High CPU utilization on Message Processors due to large amount of processing which can further impact other API requests
- Avoid using greedy quantifiers like
.*in regular expressions with the RegularExpressionProtection policy. Instead, use reluctant quantifiers like
.*?or possessive quantifiers like
.*+(less commonly) wherever possible.