Virtual Patching Best Practices

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This paper presents a virtual patching framework that organizations can follow to maximize the timely implementation of virtual patches. It also demonstrates, as an example, how a web application firewall, (WAF) such as ModSecurity, can be used to remediate a sampling of vulnerabilities in the OWASP WebGoat application. This document was initially developed as a collaborative outcome from the OWASP Global Summit 2011.

What is a Virtual Patch?

The term virtual patching was originally coined by Intrusion Prevention System (IPS) vendors a number of years ago. It is not a web application specific term, and may be applied to other protocols however currently it is more generally used as a term for Web Application Firewalls (WAF). It has been known by many different names including both External Patching and Just-in-time Patching. Whatever term you choose to use is irrelevant. What is important is that you understand exactly what a virtual patch is.


A security policy enforcement layer which prevents the exploitation of a known vulnerability.

The virtual patch works since the security enforcement layer analyzes transactions and intercepts attacks in transit, so malicious traffic never reaches the web application. The resulting impact of virtual patch is that, while the actual source code of the application itself has not been modified, the exploitation attempt does not succeed.

Value of Virtual Patching

When you consider the numerous situations when organizations can’t simply immediately edit the source code, the value of virtual patching becomes apparent. From an organizations perspective, the benefits are:

  • It is a scalable solution as it is implemented in few locations vs. installing patches on all hosts.
  • It reduces risk until a vendor-supplied patch is released or while a patch is being tested and applied.
  • There is less likelihood of introducing conflicts as libraries and support code files are not changed.
  • It provides protection for mission-critical systems that may not be taken offline.
  • It reduces or eliminates time and money spent performing emergency patching.
  • It allows organizations to maintain normal patching cycles.

From a web application security consultant’s perspective, virtual patching opens up another avenue for providing services to your clients. Traditionally, if source code could not be updated for any of the reasons previously specified, there wasn’t much else a consultant could do to help. Now, a consultant can offer to create virtual patches to externally address the issues outside of the application code.

Why Not Just Fix the Code?

From a purely technical perspective, the number one remediation strategy would be for an organization to correct the identified vulnerability within the source code of the web application. This concept is universally agreed upon by both web application security experts and system owners. Unfortunately, in real world business situations, there arise many scenarios where updating the source code of a web application is not easy. Common roadblocks to source code fixes include:

Patch Availability

If a vulnerability is identified within a commercial application, the customer most likely will not be able to modify the source code themselves. In this situation, the customer is held at the mercy of the Vendor as they have to wait for an official patch to be released. Vendors usually have extremely rigid patch release dates, which mean that an officially supported patch may not be available for an extended period of time.

Installation Time

Even in situations where an official patch is available, or a source code fix could be applied to a custom coded application, the normal patching processes of most organizations is time consuming. This is usually due to the extensive regression testing required after code changes. It is not uncommon for these testing gates to be measured in months. For example, the Symantec Internet Threat Report [1] stated that the average time it took for organizations to patch their systems was 55 days, while the Whitehat Security Web Security Statistics Report [2] documented that their customers time-to-fix average was 138 days to remediate SQL Injection vulnerabilities found in their web applications. Now contrast this patching data with the fact that Symantec also reported that it only took an average of 6 days for exploit code to be released to the public and it becomes clear that traditional source code patching processes are not adequate.

Fixing Custom Code May Be Cost Prohibitive

Web assessments that include source code reviews, vulnerability scanning and penetration tests will most assuredly identify vulnerabilities in your web application. Identification of the vulnerability is only the first half of the battle with the second half being the remediation actions. What many organizations are finding out is that the cost associated with the identification of the vulnerabilities often pales in comparison to that of actually fixing the issues. This is especially true when vulnerabilities are not found early in the design or testing phases but rather after an application is already in production. In these situations, it is usually deemed that it is just too expensive to recode the application.

Legacy Code

An organization may be using a commercial application and the vendor has gone out of business, or they are using a version that is no longer supported by the vendor. In these situations, legacy application code can’t be patched. An additional situation is when an organization is forced into using outdated vendor code due to in-house custom coded functionality being added on top of the original vendor code. This functionality is tied to a mission critical business application and prior upgrade attempts broke functionality.

Outsourced Code

As more and more businesses opt to outsource their application development, they are finding that executing vulnerability fixes would require an entirely new project. Many organizations are facing the harsh reality that poor contractual language oftentimes does on cover “secure coding” issues but only functional defects.

Virtual Patching Tools

There are a number of different tools that may be used for virtual patching efforts.

  • Intermediary device such as a WAF or IPS
  • Web server plugin such as ModSecurity
  • Application layer filter such as ESAPI WAF

Robust HTTP and HTML Parsing

The tool must use an HTTP and HTML parser to analyze the input stream. The parser must be able to understand specific protocol features including content encoding such as chunked encoding or multipart/form-data encoding, request and response compression and even XML payload.

In addition the parser must be flexible as the environment protected as many headers and protocol elements are not used according to RFC requirements. For example, while the RFC requires a single space between the method and the URI in the HTTP request line, Apache allows any sequence of whitespace between them. Another example is PHP unique use of parameters: in PHP leading and trailing spaces are removed from parameter names. In a proxy deployment a stricter parsing may be acceptable, however the tool has to be at least as flexible as the web server in order to prevent evasion. IDS/IPS systems that fail to do so can be easily evaded by attackers.

Protocol Analysis

Based on the parsed info, the tool must break up the HTTP stream into logical entities that can be inspected, such as headers, parameters and uploaded files. Each element is inspected separately not just for its content, but also for its length and count. In addition the tool must correctly divide the network stream when keep-alive HTTP connections are used to unique request and replies, and correctly match requests and replies.

Anti-Evasion Capabilities

Modern protocols such as HTTP and HTML allow the same information to be presented in multiple ways. As a result signature based detection of attacks must inspect the attack vector in any form it may be in. Attackers evade detection systems by using a less common presentation of the attack vector. Some common evasion techniques include using different character encodings for the attack vector or using none canonized path names. In order to prevent evasion the tool must transform the request to a normalized form before inspection.

The tools should be able to selectively employ normalization functions for different input fields for each inspection performed. For example, the tools should be able to normalize an HTML form field that accepts path names as input.

Rules instead of Signatures

Virtual patches must implement complex logic, as it cannot rely solely on signatures and requires a more robust rules language to define the tests. For example, the following features exist in the ModSecurity rules language: • Operators and logical expressions – can check an input field for attributed other than its content, such as its size or character distribution. Additionally ModSecurity can combine such atoms to create more complex conditions using logical operators. For example, it may inspect if a field length is too long only for a specific value of another field, or alternatively check if two different fields are empty. • Selectable anti-evasion transformation functions – as discussed above, each rule can employ specific transformation function. • Variables, sessions & state management – since the protocols inspected keep state, the rules language needs to include variables. Such variables can persist for a single transaction, for the life of a session, or globally. Using such variables enables ModSecurity to aggregate information and therefore detect an attack based on multiple indications during the life span of a transaction or a session. Attacks that require such mechanisms to detect are brute force attacks, application layer denial of service attacks and business logic flaws. • Control structures – the ModSecurity rules language includes control structures such as conditional execution. Such structures enable ModSecurity to perform different rules based on transaction content. For example, if the transaction payload is XML, an entirely different set of rules may be used.

A Virtual Patching Methodology

Virtual Patching, like most other security processes, is not something that should be approached haphazardly. Instead, a consistent, repeatable process should be followed that will provide the best chances of success. The following virtual patching workflow mimics the industry accepted practice for conducting IT Incident Response and consists of the following phases: Preparation, Identification, Analysis, Virtual Patch Creation, Implementation/Testing, and Recovery/Follow T Up.

Preparation Phase

The importance of properly utilizing the preparation phase with regards to virtual patching cannot be overstated. The idea is that you need to do a number of things to setup the virtual patching processes and framework prior to actually having to deal with an identified vulnerability, or worse yet, react to a live web application intrusion. The point is that during a live compromise is not the ideal time to be proposing installation of a web application firewall and the concept of a virtual patch. Tension is high during real incidents and time is of the essence, so lay the foundation of virtual patching when the waters are calm and get everything in place and ready to go when an incident does occur. Here are a few critical items that should be addressed during the preparation phase: • Ensure that you are signed up for on all vendor alert mail-lists for commercial software that you are using. This will ensure that you will be notified in the event that the vendor releases vulnerability information and patching data. • Virtual Patching Pre-Authorization – Virtual Patches need to be implemented quickly so the normal governance processes and authorizations steps for standard software patches need to be expedited. Since virtual patches are not actually modifying source code, they do not require the same amount of regression testing as normal software patches. I have found that categorizing virtual patches in the same group as Anti-Virus updates or Network IDS signatures helps to speed up the authorization process and minimize extended testing phases. • Deploy ModSecurity In Advance - As time is critical during incident response, it would be a poor time to have to get approvals to install new software. You can install ModSecurity in embedded mode on your Apache servers, or an Apache reverse proxy server. The advantage with this deployment is that you can create fixes for non-Apache back-end servers. Even if you do not use ModSecurity under normal circumstances, it is best to have it “on deck” ready to be enabled if need be. • Increase Audit Logged – The standard Common Log Format (CLF) utilized by most web servers does not provide adequate data for conducting proper incident response. Consider the following Apache access_log entry: - - [22/Apr/2007:18:55:53 –0400] \ “POST /xmlrpc.php HTTP/1.1” 200 293

We see that this request uses a POST Request Method. This means that critical data such as the Request Body (where the client is passing parameter data) is not logged. Without the full request payloads, it is next to impossible to accurately confirm either an attack attempt or a successful compromise. Fortunately, ModSecurity has a robust audit logging engine that is able to capture the entire request and response payloads. The following audit log entry is for the same xmlrpc.php request we showed from the Apache access_log file.

--ddb9bf17-A– [22/Apr/2007:18:55:53 –0400] dGgsYX8AAAEAABJkpY8AAACG 41376 80 –ddb9bf17-B– POST /xmlrpc.php HTTP/1.1 TE: deflate,gzip;q=0.3 Connection: TE, close Host: User-Agent: libwww-perl/5.805 Content-Length: 201 –ddb9bf17-C–

<?xml version=”1.0”?>

test.method',''));echo'_begin_';echo \`id;ls/;w\`;echo '_end_';exit;/\*</param>

As you can see, now that we can see the request body contents, we are able to identify that the client is attempting to exploit the php application and is attempting to execute OS command injection.

Identification Phase

The Identification Phase occurs when an organization becomes aware of a vulnerability within their web application. There are generally two different methods of identifying vulnerabilities: Proactive and Reactive. Proactive Identification This occurs when an organization takes it upon themselves to assess their web security posture and conducts the following tasks: • Vulnerability assessment (internal or external) and penetration tests • Source code reviews These tasks are extremely important for custom coded web applications as there would be external entity that has the same application code. Reactive Identification There are three main reactive methods for identifying vulnerabilities: • Vendor contact (e.g. pre-warning) - Occurs when a vendor discloses a vulnerability for commercial web application software that you are using. • Public disclosure - Public vulnerability disclosure for commercial/open source web application software that you are using. The threat level for public disclosure is increased as more people know about the vulnerability. • Security incident – This is the most urgent situation as the attack is active. In these situations, remediation must be immediate. Normal network security response measures include blocking the source IP of the attack at a firewall or edge security device. This technique does not work as well for web application attacks as you may prevent legitimate users from accessing the application. A virtual patch is more flexible as it is not necessarily where an attacker is coming from but what they are sending.

Analysis Phase

There are a number of tasks that must be completed during the analysis phase. What is the name of the vulnerability? This means that you need to have the proper CVE name/number identified by the vulnerability announcement, vulnerability scan, etc… What is the impact of the problem? It is always important to understand the level of criticality involved with a web vulnerability. Information leakages may not be treated in the same manner as an SQL Injection issue. What versions of software are affected? You need to identify what versions of software are listed so that you can determine if the version(s) you have installed are affected. What configuration is required to trigger the problem or how to tell if you are affected by the problem? Some vulnerabilities may only manifest themselves under certain configuration settings. Is proof of concept exploit code available? Many vulnerability announcements have accompanying exploit code that shows how to demonstrate the vulnerability. If this data is available, make sure to download it for analysis. This will be useful later on when both developing and testing the virtual patch. Is there a work around available without patching or upgrading? This is where virtual patching actually comes into play. It is a temporary work-around that will buy organizations time while they implement actual source code fixes. Is there a patch available? Unfortunately, vulnerabilities are often announced without an accompanying patch. This leaves organizations exposed and is why virtual patching has become an invaluable tool. If there is a patch available, then you initiate the proper patch management processes and simultaneously create a virtual patch.

Virtual Patch Creation Phase

The process of creating an accurate virtual patch is bound by two main tenants:

1. No false positives. Do not ever block legitimate traffic under any circumstances. This is always the top priority. 2. No false negatives. Do not miss attacks, even when the attacker intentionally tries to evade detection. This is a high priority.[4]

The virtual patch creator must keep these priorities, and their relative ordering, in mind at all times. A key distinction between virtual patch construction philosophies (log-only mode vs. a blocking mode) lies in the relative ranking of these two goals. The art of creating blocking virtual patches is generalizing the detection logic as much as possible to rigorously meet rule #2, without ever violating rule #1. Deriving a Zero False Negative Virtual Patch When performing technical vulnerability research, the virtual patch writer must first search for all of the necessary conditions for an attack to succeed. The researcher starts by obtaining technical data that triggers the vulnerability remotely (perhaps from proof of concept exploit code). The writer then varies or fuzzes all the “interesting-looking” parts of the attack. Changes are made one at a time, in steps, keeping careful notes. (Strings, length values, character encoding, white space… the list goes on. All are good things to vary.) If the attack succeeds even when a particular variable is set to a random value, that variable is not important for the virtual patch criteria. Eventually the researcher can identify the complete set of variables that are important to the attack’s success, and arrive at a set of criteria that must be collectively satisfied for any attack to succeed. If there are multiple distinct attack vectors, the researcher must perform this analysis on each one separately.

Given a set of criteria that must be satisfied for an attack to succeed, it is possible to describe virtual patching logic that has zero false negatives. That is, an attack simply cannot succeed unless the associated web application attack traffic has exactly the characteristics that the virtual patch is looking for. Deriving a Zero False Positive Virtual Patch Given a zero false negative virtual patch as previously described, the writer must also evaluate the accuracy of patch in terms of false positives. At this stage, the writer attempts to identify at least one characteristic that would never occur in normal web traffic. If a characteristic exists that is both anomalous compared to normal traffic and critical to the attack’s success, then the zero false negative virtual patch is also a zero false positive signature. Negative Security Virtual Patches A negative security model (or misuse based detection) is based on a set of rules that detect specific known attacks rather than allow only valid traffic. It is important to note that the differentiation between negative and positive security models is subjective and reflects how tight the security envelope around the application is. A good example would be limiting the characters allowed in an input field. Since the character set is a closed set, providing an allow list of permitted characters is actually similar to providing a deny list of forbidden characters including the characters complementing the 1st group. Positive Security Virtual Patches Positive security model is a comprehensive security mechanism that provides an independent input validation envelope to an application. The model specifies the characteristics of valid input (character set, length, etc…) and denies anything that does not conform. By defining rules for every parameter in every page in the application the application is protected by an additional security envelop independent from its code. Which Method is Better for Virtual Patching – Positive or Negative Security? A virtual patch may employ either a negative or positive security model. Which one you decide to use depends on the situation and a few different considerations. For example, negative security rules can usually be implemented more quickly, however the possible evasions are more likely.

Positive security rules, only the other hand, provides better protection however it is often a manual process and thus is not scalable and difficult to maintain for large/dynamic sites. While manual positive security rules for an entire site may not be feasible, a positive security model can be selectively employed when a vulnerability alert identifies a specific location with a problem. Beware of Exploit-Specific Virtual Patches You want to resist the urge to take the easy road and quickly create an exploit-specific rule. While it may provide some immediate protection, its long term value is significantly decreased. A case study of this concept in the IDS world is “bleeding edge” snort signature for Bugtraq vulnerability #21799. This vulnerability in the Cacti open source graphing software was picked quite at random. The exploit references on Bugtraq vulnerabilities archive is:

/cacti/cmd.php?1+1111)/**/UNION/**/SELECT/**/2,0,1,1,127.0.0 .1,null,1,null,null,161,500, proc,null,1,300,0, ls -la > ./rra/suntzu.log,null,null/**/FROM/**/host/*+11111

And the Snort signature is:

alert tcp $EXTERNAL_NET any -> $HTTP_SERVERS $HTTP_PORTS ( msg:”BLEEDING-EDGE WEB Cacti cmd.php Remote Arbitrary SQL Command Execution Attempt”; flow:to_server,established; uricontent:”/cmd.php?”; nocase; uricontent:”UNION”; nocase; uricontent:”SELECT”; nocase; reference:cve,CVE-2006-6799; reference:bugtraq,21799; classtype: web-application-attack; sid:2003334; rev:1; )

While snort has some anti-evasion techniques such as case insensitivity and URI decoding, this signature still falls short of detecting an exploit of the vulnerability. It is gears only towards detecting the specific attack vector shown above. Any other exploit such as blind SQL injection would not be detected. It also searches for the keywords only in the request line, while many development environments would allow for parameters to be provided both in the POST and GET payload.

Additionally this signature is prone to false positives as both select and union are common English words and since the signature do not require any word delimiters the signature will also be satisfied by the words “Selection” and “Reunion”. In many cases such a signature has to be turned off.

For examples of poorly written ModSecurity rules, let’s look at the following GotRoot rule:

SecDefaultAction “log,deny,phase:2,status:500,t:urlDecodeUni,

  1. WEB-CGI csSearch.cgi arbitrary command execution attempt

SecRule REQUEST_URI “/csSearch\.cgi\?” chain SecRule REQUEST_URI “\`”

In the first line, the SecDefaultAction is specifying the use of the “t:lowercase” transformation function. This is often used to normalize input data for anti-evasion. When this is used, care should be taken to specify only lowercase letters in the operator payload section. In this rule example, however, the rule writer mistakenly used mixed-case and thus this rule would not trigger (false negative).

Implementation/Testing Phase

In order to accurately test out the newly created virtual patches, it may be necessary to use an application other than a web browser. Some useful tools are: • Command line web clients such as Curl and Wget. • Local Proxy Servers such as WebScarab ( and Burp Proxy ( • ModSecurity AuditViewer ( – which allows you to load a ModSecurity audit log file, manipulate it and then re-inject the data back into any web server. These tools will allow you to manipulate the request data in any way desired. ModSecurity’s Debug Log File In order to verify exactly how your new rule is working, you should review the ModSecurity SecDebugLog file data. The Debug log provides extensive details on the rule processing order and in many cases is the only true way to verify that the rule is working exactly as you expected. You will most likely need to increase the SecDebugLogLevel directive setting to get enough detail to validate the patch processing. You can selectively increase the logging based on source IP address so that you don’t impact performance on the entire web server. Below is an excerpt of the debug log data during rule processing (some data deleted for readibility):

Recipe: Invoking rule 82211d8. Executing operator !rx with param “^(POST)$” against REQUEST_METHOD. Target value: POST Operator completed in 17 usec. Rule returned 0. No match, not chained -> mode NEXT_RULE. Recipe: Invoking rule 82214b0. Rule returned 0. No match, not chained -> mode NEXT_RULE. Recipe: Invoking rule 82360d0. Executing operator !rx with param “^(\w{0,32})$” against ARGS:username. Target value: 0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 Operator completed in 13 usec. Rule returned 1. Match, intercepted -> returning. Access denied with code 501 (phase 2). Match of “rx ^(\w{0,32})$” against “ARGS:username” required. [id “1”] [msg “Postparameter username failed validity check. Value domain: Username.”] [severity “ERROR”]

Recovery/Follow-Up Phase

Although you may need to expedite the implementation of virtual patches, you should still track them in your normal Patch Management processes. This means that you should create proper change request tickets, etc… so that their existence and functionality is documented.

You should also have periodic re-assessments to verify if/when you can remove previous virtual patches if the web application code has been updated with the real source code fix. I have found that many people opt to keep virtual patches in place due to better identification/logging vs. application or db capabilities.



Detailed definitions and more in-depth descriptions concerning WAS - Web Application Security - can be found at:

ModSecurity Core Rule Set Project ModSecurity Core Rule Set Project Category:OWASP Document Category:How To