Fuzzing Overview
Learn about fuzzing HTTP requests with Nuclei
Nuclei supports fuzzing of HTTP requests based on rules defined in the fuzzing
section of the HTTP request. This allows creating templates for generic Web Application vulnerabilities like SQLi, SSRF, CMDi, etc without any information of the target like a classic web fuzzer. We call this concept as Fuzzing for Unknown Vulnerabilities.
pre-condition
More often than not, we want to only attempt fuzzing on those requests where it makes sense. For example,
- Fuzz Body When Body is Present
- Ignore PreFlight and CONNECT requests
and so on. With Nuclei v3.2.4 we have introduced a new pre-condition
section which contains conditions when the fuzzing template should be executed.
pre-condition can be considered a twin of matchers in nuclei. They support all matcher types, including DSL, and the only difference is that this serves a different purpose.
For example, to only execute template on POST request with some body, you can use the following filter.
When writing/executing a template, you can use the -v -svd flags to see all variables available in filters before applying the filter.
Part
Part specifies what part of the request should be fuzzed based on the specified rules. Available options for this parameter are -
query (default
) - fuzz query parameters for URL
path - fuzz path parameters for requests
header - fuzz header parameters for requests
cookie - fuzz cookie parameters for requests
body - fuzz body parameters for requests
Special Part
request - fuzz the entire request (all parts mentioned above)
Multiple selective parts
Multiple parts can be selected for fuzzing by defining a parts
field which is the plural of above allowing selected multiple parts to be fuzzed.
Type
Type specifies the type of replacement to perform for the fuzzing rule value. Available options for this parameter are -
- replace (
default
) - replace the value with payload - prefix - prefix the value with payload
- postfix - postfix the value with payload
- infix - infix the value with payload (place in between)
- replace-regex - replace the value with payload using regex
Key-Value Abstraction
In a HTTP request, there are various parts like query, path, headers, cookies, and body and each part has different in various formats. For example, the query part is a key-value pair, the path part is a list of values, the body part can be a JSON, XML, or form-data.
To effectively abstract these parts and allow them to be fuzzed, Nuclei exposes these values as key
and value
pairs. This allows users to fuzz based on the key or value of the request part.
For example, Below sample HTTP request can be abstracted as key-value pairs as shown below.
part: Query
key | value |
---|---|
token | x0x0x0 |
source | app |
part: Path
key | value |
---|---|
value | /reset-password |
part: Header
key | value |
---|---|
Host | 127.0.0.1:8082 |
User-Agent | Go-http-client/1.1 |
Content-Length | 23 |
Content-Type | application/json |
Accept-Encoding | gzip |
Connection | close |
part: Cookie
key | value |
---|---|
PHPSESSID | 1234567890 |
part: Body
key | value |
---|---|
password | 12345678 |
Note: XML, JSON, Form, Multipart-FormData will be in kv format, but if the Body is binary or in any other format, the entire Body will be represented as a single key-value pair with key as value
and value as the entire Body.
key | value |
---|---|
value | ”\x08\x96\x01\x12\x07\x74” |
This abstraction really levels up the game since you only need to write a single rule for the Body, and it will be applied to all formats. For example, if you check for SQLi in body values, a single rule will work on all formats, i.e., JSON, XML, Form, Multipart-FormData, etc.
Mode
Mode specifies the mode in which to perform the replacements. Available modes are -
- multiple (
default
) - replace all values at once - single - replace one value at a time
Note: default values are set/used when other options are not defined.
Component Data Filtering
Multiple filters are supported to restrict the scope of fuzzing to only interesting parameter keys and values. Nuclei HTTP Fuzzing engine converts request parts into Keys and Values which then can be filtered by their related options.
The following filter fields are supported -
- keys - list of parameter names to fuzz (exact match)
- keys-regex - list of parameter regex to fuzz
- values - list of value regex to fuzz
These filters can be used in combination to run highly targeted fuzzing based on the parameter input. A few examples of such filtering are provided below.
Fuzz
Fuzz specifies the values to replace with a type
for a parameter. It supports payloads, DSL functions, etc and allows users to fully utilize the existing nuclei feature-set for fuzzing purposes.
Analyzer
Analyzers is a new concept introduced in nuclei fuzzing which allow the engine to make additional verification requests based on a certain logic to verify the vulnerability.
time_delay
The time_delay
analyzer verifies that the response time of the request is controllable by the fuzzed payload. It uses a Linear Regression algorithm ported from ZAP with alternating requests to determine the server time is actually controllable rather than just noise. You can configure it like so
The following dynamic placeholders are available in payloads with time_delay
analyzer.
[SLEEPTIME]
- The sleep time in seconds for the time delay analyzer.[INFERENCE]
- The inference condition (%d=%d) for the time delay analyzer.
These values are substituted at runtime with the actual values for the analyzer. The following is how a usual verification process looks.
- Send the request with the payload to the target with 5 second delay.
- If the response time is less than 5, do nothing.
- Send the request to the analyzer which queues it with 5 seconds delay.
- Next a 1 second delay
- Next a 5 second delay
- Finally, the last 1 second delay.
If the response time is controllable, the analyzer will report the vulnerability.
Matching for the analyzer matches is pretty straightforward as well. Simiar to interactsh, you can use part: analyzer
to match the analyzer response.
Optionally, you can also extract the analyzer_details
from the analyzer for matches.
Example Fuzzing template
An example sample template for fuzzing XSS vulnerabilities is provided below.
More complete examples are provided here
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