REcollapse

REcollapse is a helper tool for black-box regex fuzzing to bypass validations and discover normalizations in web applications.

It can also be helpful to bypass WAFs and weak vulnerability mitigations. For more information, take a look at the REcollapse blog post.

The goal of this tool is to generate payloads for testing. Actual fuzzing shall be done with other tools like Burp (intruder), Caido (automate), ffuf, or similar.

---

Installation

Requirements: Python 3

python3 setup.py install or pip3 install .

Docker

docker build -t recollapse . or docker pull 0xacb/recollapse

---

Usage

$ recollapse -h
usage: recollapse [-h] [-m MODES] [-e {1,2,3,4}] [-r RANGE] [-s SIZE] [-f FILE] [-an] [-mn MAXNORM] [-mt MAXTRUNC] [-nt] [-tt] [-ct]
                  [--html] [--version]
                  [input]

REcollapse is a helper tool for black-box regex fuzzing to bypass validations and discover normalizations in web applications

positional arguments:
  input                 original input

options:
  -h, --help            show this help message and exit
  -m, --modes, -p, --positions MODES
                        variation modes. Example: 1,2,3,4,5,6,7 (default). 1: starting, 2: separator, 3: normalization, 4:
                        termination, 5: regex metacharacters, 6: case folding/upper/lower, 7: byte truncation
  -e, --encoding {1,2,3,4}
                        1: URL-encoded format (default), 2: Unicode format, 3: Raw format, 4: Double URL-encoded format
  -r, --range RANGE     range of bytes for fuzzing. Example: 0,0xff (default)
  -s, --size SIZE       number of fuzzing bytes (default: 1)
  -f, --file FILE       read input from file
  -an, --alphanum       include alphanumeric bytes in fuzzing range
  -mn, --maxnorm MAXNORM
                        maximum number of normalizations (default: 3)
  -mt, --maxtrunc MAXTRUNC
                        maximum number of truncations (default: 3)
  -nt, --normtable      print normalization table
  -tt, --trunctable     print truncation table
  -ct, --casetable      print case table
  --html                output tables in HTML format
  --version             show recollapse version

---

Detailed options explanation

Let's consider this_is.an_example as the input.

Modes

1. Fuzz the beginning of the input: $this_is.an_example
2. Fuzz the before and after special characters: this$_$is$.$an$_$example
3. Fuzz normalization positions: replace all possible bytes according to the normalization table
4. Fuzz the end of the input: this_is.an_example$
5. Fuzz regex metacharacters: replace all possible regex metacharacters: .^$*+-?()[]{}\|
6. Fuzz case folding/upper/lower: replace all possible bytes according to the case table
7. Fuzz byte truncation: replace all possible bytes according to the truncation table

Encoding

1. URL-encoded format to be used with application/x-www-form-urlencoded or query/body parameters: %22this_is.an_example
2. Unicode format to be used with application/json: \u0022this_is.an_example
3. Raw format to be used with multipart/form-data: "this_is.an_example
4. Double URL-encoded format

Range

Specify a range of bytes for fuzzing: -r 1-127. This will exclude alphanumeric characters unless the -an option is provided.

Size

Specify the size of fuzzing for modes 1, 2 and 4. The default approach is to fuzz all possible values for one byte. Increasing the size will consume more resources and generate many more inputs, but it can lead to finding new bypasses.

File

Input can be provided as a positional argument, stdin, or a file through the -f option.

Alphanumeric

By default, alphanumeric characters will be excluded from output generation, which is usually not interesting in terms of responses. You can allow this with the -an option.

Maximum number or normalizations

Not all normalization libraries have the same behavior. By default, three possibilities for normalizations are generated for each input index, which is usually enough. Use the -mn option to go further.

Tables

Use the -nt option to show the normalization table, the -ct option to show the case table, and the -tt option to show the truncation table. You can also use the --html option to output tables in HTML format.

$ recollapse -nt --html > normalization_table.html
$ recollapse -tt --html > truncation_table.html
$ recollapse -ct --html > case_table.html

---

Examples

Using Recollapse as a command-line tool:

$ recollapse -e 1 -m 1,2,4 -r 10-11 https://legit.example.com
%0ahttps://legit.example.com
%0bhttps://legit.example.com
https%0a://legit.example.com
https%0b://legit.example.com
...

$ echo "a@b.com" | recollapse 
%00a@b.com
%01a@b.com
...

$ echo "<svg/onload=alert(1)>" | recollapse | ffuf -w - -u "https://example.com/?param=FUZZ" -mc 200,403,500

Using Recollapse as a library:

from recollapse import Recollapse

recollapse = Recollapse(modes=Recollapse.DEFAULT_MODES,
                        encoding=Recollapse.ENCODING_RAW)
variants = recollapse.generate("<script")
for variant in variants:
    print(variant)

---

Resources

This technique was originally presented on BSidesLisbon 2022

Blog post: https://0xacb.com/2022/11/21/recollapse/

Slides:

• nahamcon_2022_eu_till_recollapse.pdf
• bsideslisbon_2022_till_recollapse.pdf

Videos:

• NahamCon 2022 EU
• BSidesLisbon 2022

Tables:

• Normalization table: https://0xacb.com/normalization_table
• Case table: https://0xacb.com/case_table
• Truncation table: https://0xacb.com/truncation_table

---

Thanks

• @regala_
• @0xz3z4d45
• @jllis
• @samwcyo
• @yassineaboukir
• @0xteknogeek
• @vgpinho
• @ryancbarnett
• BBAC

and

• @ethiack team
• @0xdisturbance team
• @hacker0x01 team

---

⚠ Legal Disclaimer ⚠

This project is made for educational and ethical testing purposes only. Usage of this tool for attacking targets without prior mutual consent is illegal. Developers assume no liability and are not responsible for any misuse or damage caused by this tool.