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docs: add warnings and a use case on false positive management #247

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Update 2-3-false-positives-and-tuning.md
touchweb-vincent Nov 4, 2025
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airween Nov 4, 2025
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Update content/2-how-crs-works/2-3-false-positives-and-tuning.md
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Update 2-3-false-positives-and-tuning.md
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theseion Nov 11, 2025
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EsadCetiner May 7, 2026
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12 changes: 11 additions & 1 deletion content/2-how-crs-works/2-3-false-positives-and-tuning.md
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Expand Up @@ -229,6 +229,12 @@ SecRuleRemoveById 933151

**Scenario:** Several different parts of a web application are causing false positives with various SQL injection rules. None of the web services behind the WAF make use of SQL, so it is deemed safe to tune away these false positives by removing all the SQLi detection rules.

{{% notice warning %}}
This type of broad exclusion is dangerous and should only be considered if the rules are causing problems _and_ the application is not vulnerable to the specific attack, e.g., the PHP rules cause issues and no application behind the WAF uses PHP.

In general, the less exclusions the better, since every rule reduces the amount of bad traffic arriving at the protected application.
{{% /notice %}}

**Rule Exclusion:**

```apache
Expand Down Expand Up @@ -310,7 +316,7 @@ SecRule REQUEST_URI "@beginsWith /web_app_1/content" \

```apache
# CRS Rule Exclusion: 941150 - XSS Filter - Category 5: Disallowed HTML Attributes
SecRule REQUEST_URI "@beginsWith /dynamic/new_post" \
SecRule REQUEST_URI "@rx ^/dynamic/new_post" \
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@theseion theseion Nov 6, 2025

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Why? @beginsWith is much easier to understand.

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@touchweb-vincent touchweb-vincent Nov 6, 2025

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I’m not sure I understand your point - using ^ in a regex is just as straightforward as @beginswith, and it also provides a slightly different example. Since @beginswith is already used several times, it makes sense to vary the syntax a bit

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@theseion theseion Nov 10, 2025

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What is your reasoning behind changing the operator here? @rx is harder to understand.

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@touchweb-vincent touchweb-vincent Nov 10, 2025

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I still don’t understand why you think @rx ^ is more complicated than beginsWith - and honestly, if they’re at the point of saying that, then mod_security2 is probably not the right tool for them, imo, since they’ll be writing regexes all day long just to deal with their false positives.

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@theseion theseion Nov 12, 2025

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Please explain why you've made this change.

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@touchweb-vincent touchweb-vincent Nov 13, 2025

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I replaced @beginswith with @rx because it adds a concrete, real-world example of a pattern they’ll use almost every day.

Using @rx (regular expression) shows a more flexible and practical case than @beginswith, since it can handle a wider variety of inputs - something they'll frequently need in production scenarios.

In short, this change makes the rule both more instructive and more representative of what developers will actually encounter.

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@EsadCetiner EsadCetiner May 7, 2026

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@touchweb-vincent I see where your coming from, but I think kind of information is better suited in the section about creating rules. I think we should keep the false positive tuning doc simple to avoid overloading the reader with too much information.

"id:1020,\
phase:1,\
pass,\
Expand All @@ -325,6 +331,10 @@ SecRule REQUEST_URI "@beginsWith /dynamic/new_post" \

**Scenario:** The values of request cookie 'uid' are causing false positives with various SQL injection rules when trying to log in to a web service at location '/webapp/login.html'. It is decided that it is not a risk to allow SQL-like content in this specific cookie's values for the login page, however it is deemed unacceptable to disable the SQLi detection rules for anything apart from the specific request cookie in question at the login page only. It is decided to tune away these false positives by excluding only the problematic request cookie from the SQLi detection rules, and only when accessing '/webapp/login.html'.

{{% notice warning %}}
This type of broad exclusion is dangerous. Even though the decision in this example may seem trivial, rule exclusions always carry a risk. It is important to understand the risk of disabling a rule, i.e., which attack vectors become available by disabling the rule, even if only a single target is affected.
{{% /notice %}}

**Rule Exclusion:**

```apache
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