homepage
Open menu
Go one level top
  • Train and Certify
    Train and Certify

    Immediately apply the skills and techniques learned in SANS courses, ranges, and summits

    • Overview
    • Courses
      • Overview
      • Full Course List
      • By Focus Areas
        • Cloud Security
        • Cyber Defense
        • Cybersecurity and IT Essentials
        • DFIR
        • Industrial Control Systems
        • Offensive Operations
        • Management, Legal, and Audit
      • By Skill Levels
        • New to Cyber
        • Essentials
        • Advanced
        • Expert
      • Training Formats
        • OnDemand
        • In-Person
        • Live Online
      • Course Demos
    • Training Roadmaps
      • Skills Roadmap
      • Focus Area Job Roles
        • Cyber Defence Job Roles
        • Offensive Operations Job Roles
        • DFIR Job Roles
        • Cloud Job Roles
        • ICS Job Roles
        • Leadership Job Roles
      • NICE Framework
        • Security Provisionals
        • Operate and Maintain
        • Oversee and Govern
        • Protect and Defend
        • Analyze
        • Collect and Operate
        • Investigate
        • Industrial Control Systems
    • GIAC Certifications
    • Training Events & Summits
      • Events Overview
      • Event Locations
        • Asia
        • Australia & New Zealand
        • Latin America
        • Mainland Europe
        • Middle East & Africa
        • Scandinavia
        • United Kingdom & Ireland
        • United States & Canada
      • Summits
    • OnDemand
    • Get Started in Cyber
      • Overview
      • Degree and Certificate Programs
      • Scholarships
    • Cyber Ranges
  • Manage Your Team
    Manage Your Team

    Build a world-class cyber team with our workforce development programs

    • Overview
    • Why Work with SANS
    • Group Purchasing
    • Build Your Team
      • Team Development
      • Assessments
      • Private Training
      • Hire Cyber Professionals
      • By Industry
        • Health Care
        • Industrial Control Systems Security
        • Military
    • Leadership Training
  • Security Awareness
    Security Awareness

    Increase your staff’s cyber awareness, help them change their behaviors, and reduce your organizational risk

    • Overview
    • Products & Services
      • Security Awareness Training
        • EndUser Training
        • Phishing Platform
      • Specialized
        • Developer Training
        • ICS Engineer Training
        • NERC CIP Training
        • IT Administrator
      • Risk Assessments
        • Knowledge Assessment
        • Culture Assessment
        • Behavioral Risk Assessment
    • OUCH! Newsletter
    • Career Development
      • Overview
      • Training & Courses
      • Professional Credential
    • Blog
    • Partners
    • Reports & Case Studies
  • Resources
    Resources

    Enhance your skills with access to thousands of free resources, 150+ instructor-developed tools, and the latest cybersecurity news and analysis

    • Overview
    • Webcasts
    • Free Cybersecurity Events
      • Free Events Overview
      • Summits
      • Solutions Forums
      • Community Nights
    • Content
      • Newsletters
        • NewsBites
        • @RISK
        • OUCH! Newsletter
      • Blog
      • Podcasts
      • Summit Presentations
      • Posters & Cheat Sheets
    • Research
      • White Papers
      • Security Policies
    • Tools
    • Focus Areas
      • Cyber Defense
      • Cloud Security
      • Digital Forensics & Incident Response
      • Industrial Control Systems
      • Cyber Security Leadership
      • Offensive Operations
  • Get Involved
    Get Involved

    Help keep the cyber community one step ahead of threats. Join the SANS community or begin your journey of becoming a SANS Certified Instructor today.

    • Overview
    • Join the Community
    • Work Study
    • Teach for SANS
    • CISO Network
    • Partnerships
    • Sponsorship Opportunities
  • About
    About

    Learn more about how SANS empowers and educates current and future cybersecurity practitioners with knowledge and skills

    • SANS
      • Overview
      • Our Founder
      • Awards
    • Instructors
      • Our Instructors
      • Full Instructor List
    • Mission
      • Our Mission
      • Diversity
      • Scholarships
    • Contact
      • Contact Customer Service
      • Contact Sales
      • Press & Media Enquiries
    • Frequent Asked Questions
    • Customer Reviews
    • Press
    • Careers
  • Contact Sales
  • SANS Sites
    • GIAC Security Certifications
    • Internet Storm Center
    • SANS Technology Institute
    • Security Awareness Training
  • Search
  • Log In
  • Join
    • Account Dashboard
    • Log Out
  1. Home >
  2. Blog >
  3. Digital Forensic SIFTing: String Searching and File Carving using srch_strings_wrap
Dave Lassalle

Digital Forensic SIFTing: String Searching and File Carving using srch_strings_wrap

December 21, 2011

The latest version of the SIFT 2.12 contains a few scripts I wrote, and Rob asked me to write a post for the blog going over their functionality. The scripts add on to the functionality provided by The Sleuth Kit's srch_strings to provide additional information on string matches and automatically carve out matching files or blocks. The scripts are located in /usr/local/bin and are as follows:

  • srch_strings_blk
  • srch_strings_pipe
  • srch_strings_wrap.sh (the initial version of the perl script srch_strings_wrap)
  • srch_strings_wrap (note that in SIFT 2.12, this file's permissions will need to be changed to 755; there are also a couple bugs in the SIFT version: one affecting auto-carving and the other grepping, so be sure to download the latest version)

While they can be found in SIFT, you can also get them from my GitHub repository: https://github.com/superponible/Search-Strings-Extension. Read on for more information on how to use these scripts.

Background

I recently took SANS FOR508 with Rob Lee in Las Vegas. For those familiar with the class, one of the areas covered is string searching through an image by using srch_strings from the Sleuth Kit to obtain the byte offset of a matching string. This is done using the "-t d" option:

ssw01srch_strings.png

Then, after obtaining the block size of the filesystem using fsstat, we figure out which block each of these strings is in. For example, this is an image of a filesystem with 1024 byte blocks, so divide each byte offset by 1024, and drop the remainder to get the block:

Block  String
 7     vmlinuz-2.2.14-5.0
 7     System.map-2.2.14-5.0smp
 7     module-info-2.2.14-5.0
 256   lost+found
 256   kernel.h

During class, I got tired of opening the calculator to figure out these blocks, so I came up with a little one liner to do everything at once:

ssw02oneliner.png

Eventually, I got tired of typing that out and turned it into a script when I got back home. That script turned into the three initial shell scripts I created, which I'll give a brief overview of below.

The Initial Scripts

If you're used to running srch_strings and just wanted to get the block for all the matches, you could use srch_strings_pipe:

ssw03ss_pipe.png

The srch_strings_blk command is very similar, but instead runs against the output of srch_strings which has been previously dumped to a file. With this command, I built in a check for the blocksize so it doesn't need to be specified. Instead, you can point to the original image the strings dump is from:

ssw04ss_blk.png

The last original script srch_strings_wrap.sh is a rudimentary version of the current srch_strings_wrap. It accepts the -b argument to specify the blocksize. If that is not given, it will behave exactly like srch_strings. If the -b is given, it will print out the block as the previous two scripts do:

ssw05ss_wrap_orig.png

These scripts write to temporary files and definitely aren't the best or most efficient. They can be found in my GitHub and in SIFT and were what I initially came up with and presented to Rob. He put me in touch with Hal Pomeranz, who had been talking about doing something similar. After a few emails about some additional functionality, I switched the code over to perl and created what became the current srch_strings_wrap.

The Old Way

Before going into the examples with srch_strings_wrap, I wanted to briefly show all the manual steps it accomplishes with one command. In the image I'm working with, I will search for the string ADVISORY, which has two matches in the image. There are several commands needed to extract all the information we want and carve the file out:

  • fs_stat to determine the block size
  • srch_strings with grep to find the matches
  • divide the byte offset by the block size to determine the block number
  • blkstat to check the allocation status of the block
  • ifind to find the inode using the block number
  • istat to check the allocation status of the inode
  • ffind to find the filename for the inode
  • icat to carve the file using icat
  • list the carved file
ssw06manualway.png

These 8 commands (not counting the final ls) are combined into one by using srch_strings_wrap.

The New Way

By using "-d" (enable additional features and determine block size), -g (grep for ADVISORY), and "-A" (autocarve), we can accomplish the 8 steps above in one command. This is even better because the commands before only found the matches and carved out the one matching file. This srch_strings_wrap command will print the data and carve both matching files.

ssw07basic_ssw_with_grep_and_carve.png

srch_strings_wrap — Overview

Currently, the command line options include:

If no special options are given (such as "-d" or "-b <blocksize>"), srch_strings_wrap can be used instead of srch_strings and will output the same results.

The blocksize of the filesystem can be specified (-b) or automatically determined from the image (-d). Multiple filesystem images can be given as arguments, but only one full disk image can be specified. The output can be grouped by file/inode/block (-O) or printed out line by line (default). It supports custom delimiters (-F) and can output to CSV (-C). Output can be written, if desired with a header (-H), to a file (-w), to standard out (default), or not at all (-N). Grep terms can be passed on the command line (-g) or in a dirty word file (-G), with case insensitivity (-i).

If full lookups to the filename layer are not needed, the level can be specified to decrease runtime: byte (-l0, no different from "srch_strings -t d"), block (-l1), inode (-l2), and filename (-l3, the default). There is an option to autocarve (-A) which will carve out all matching strings at the highest level available.

And if multiple grep searches will be conducted, "srch_strings -a -t d fs.img > output.asc" can be run on an image to capture all the strings and save the output to a file, then -P can be used to accept the output of that file piped in ("cat output.asc | srch_strings_wrap -P -g REGEX -I fs.img"). This way the initial time consuming dump of strings only has to be run once.

All of this information is printed in the help menu (-h).

srch_strings_wrap — Examples

In showing the new way to do searching above, I gave a basic version of using the srch_strings_wrap command. Now I'll give some more examples showing all the different command line options.

As I said in the overview, if you just supply the same command line options as you would to srch_strings, srch_strings_wrap will give the same output:

ssw08noarguments.png

If you know the blocksize you can specify with the -b option or use -d and it will be determined from fsstat. These basic versions of the command will print all string matches with the additional information:

ssw09_with-dashb_or_d.png

There are a few different output options. To write STDOUT to a file, use "-w file". To suppress STDOUT, use "-N". To print a header line, use "-H", which for the output above would be:

ssw10withheader1.png

The default delimiter is the tab character, but it can be changed with "-F delim" where delim is 1 or more characters to use. Alternatively, "-C" can be used to print in CSV format, which will put quotes around the string and escape any quotes within the string.

ssw11csv.png

The default output takes the srch_strings output and prepends the additional columns. Another option is to use "-O" to print in a more human readable format that will group all the hits within a single file or inode, if it was found, or the block if not.

ssw12dashO.png

All these commands are using the default "level" of 3 which tries to go all the way from the byte offset (level 0) to the block (level 1) to the inode (level 2) to the filename (level 3). The "-l #" option can be used to specify a custom level if going all the way to the filename layer is not needed. The output will be adjusted accordingly and the command should run faster at lower levels. Note that "level 0? is essentially the same as the basic srch_strings output. Here's an example of only going to level 1:

ssw13level1.png

The -A option can be used to automatically carve any matches into a folder. The default folder is in the current directory and is called ssw_output, but it can be changed with the "-D path" option. Note that I used -N to suppress STDOUT.

ssw14autocarve.png

The file named DIRECTORY_FILE is, in this case, the root directory on the filesystem. The directory structure within ssw_output is "image_name/partition_number/" followed by [root] for allocated files, [deleted] for deleted files and [filename_unknown] when the filename couldn't be found. If an unallocated block is carved, then the top level will either be [unallocated] for a data block or [metadata] for a metadata block.

All of these examples so far have assumed that the image is a partition or filesystem image. This is the default, but a full disk image can be given as well. The mmls command is used to pick the partitions and their offsets. The partition number will be prepended in the output and "00? will be used if it's just a filesystem image.

ssw15fulldisk.png

Autocarving the whole image may match on many files, so srch_strings_wrap accepts "-g regex" or "-G file" where regex is a grep regex or file contains a list of regexes to pass to "grep -f". Case insensitivity can be specified with "-i". The grep commands can be used with or without the "-A" option.

ssw16grep.png

The last option is "-P" which, rather than an image, accepts the output of a previously run srch_strings command. This would be useful if you wanted to run srch_strings on an entire image just once, then wanted to run multiple different "grep" searches on those results. The precomputed file is cat'd in via the pipeline. The "-I image" is also required where image is the image file that srch_strings was run against:

ssw17precompute.png

Additional Links

Here are some links if you're interested in keeping up with the latest on this tool:

  • A page on my blog with all posts related to srch_strings_wrap: http://blog.superponible.com/srch_strings_wrap/
  • My GitHub repository for these tools: https://github.com/superponible/Search-Strings-Extension)

If you have any ideas for future updates or find any bugs, let me know either on Twitter or send email to dave {at} superponible {dot} com.

Dave Lassalle has over 8 years of experience in Information Security. He is currently a Senior Cyber Security Engineer for a government contractor in New Orleans, LA, focusing on SIEM analysis, forensics, and incident response. You can follow Dave on Twitter @superponible.

Share:
TwitterLinkedInFacebook
Copy url Url was copied to clipboard
Subscribe to SANS Newsletters
Receive curated news, vulnerabilities, & security awareness tips
United States
Canada
United Kingdom
Spain
Belgium
Denmark
Norway
Netherlands
Australia
India
Japan
Singapore
Afghanistan
Aland Islands
Albania
Algeria
American Samoa
Andorra
Angola
Anguilla
Antarctica
Antigua and Barbuda
Argentina
Armenia
Aruba
Austria
Azerbaijan
Bahamas
Bahrain
Bangladesh
Barbados
Belarus
Belize
Benin
Bermuda
Bhutan
Bolivia
Bonaire, Sint Eustatius, and Saba
Bosnia And Herzegovina
Botswana
Bouvet Island
Brazil
British Indian Ocean Territory
Brunei Darussalam
Bulgaria
Burkina Faso
Burundi
Cambodia
Cameroon
Cape Verde
Cayman Islands
Central African Republic
Chad
Chile
China
Christmas Island
Cocos (Keeling) Islands
Colombia
Comoros
Cook Islands
Costa Rica
Croatia (Local Name: Hrvatska)
Curacao
Cyprus
Czech Republic
Democratic Republic of the Congo
Djibouti
Dominica
Dominican Republic
East Timor
East Timor
Ecuador
Egypt
El Salvador
Equatorial Guinea
Eritrea
Estonia
Ethiopia
Falkland Islands (Malvinas)
Faroe Islands
Fiji
Finland
France
French Guiana
French Polynesia
French Southern Territories
Gabon
Gambia
Georgia
Germany
Ghana
Gibraltar
Greece
Greenland
Grenada
Guadeloupe
Guam
Guatemala
Guernsey
Guinea
Guinea-Bissau
Guyana
Haiti
Heard And McDonald Islands
Honduras
Hong Kong
Hungary
Iceland
Indonesia
Iraq
Ireland
Isle of Man
Israel
Italy
Jamaica
Jersey
Jordan
Kazakhstan
Kenya
Kiribati
Korea, Republic Of
Kosovo
Kuwait
Kyrgyzstan
Lao People's Democratic Republic
Latvia
Lebanon
Lesotho
Liberia
Liechtenstein
Lithuania
Luxembourg
Macau
Macedonia
Madagascar
Malawi
Malaysia
Maldives
Mali
Malta
Marshall Islands
Martinique
Mauritania
Mauritius
Mayotte
Mexico
Micronesia, Federated States Of
Moldova, Republic Of
Monaco
Mongolia
Montenegro
Montserrat
Morocco
Mozambique
Myanmar
Namibia
Nauru
Nepal
Netherlands Antilles
New Caledonia
New Zealand
Nicaragua
Niger
Nigeria
Niue
Norfolk Island
Northern Mariana Islands
Oman
Pakistan
Palau
Palestine
Panama
Papua New Guinea
Paraguay
Peru
Philippines
Pitcairn
Poland
Portugal
Puerto Rico
Qatar
Reunion
Romania
Russian Federation
Rwanda
Saint Bartholemy
Saint Kitts And Nevis
Saint Lucia
Saint Martin
Saint Vincent And The Grenadines
Samoa
San Marino
Sao Tome And Principe
Saudi Arabia
Senegal
Serbia
Seychelles
Sierra Leone
Sint Maarten
Slovakia
Slovenia
Solomon Islands
South Africa
South Georgia and the South Sandwich Islands
South Sudan
Sri Lanka
St. Helena
St. Pierre And Miquelon
Suriname
Svalbard And Jan Mayen Islands
Swaziland
Sweden
Switzerland
Taiwan
Tajikistan
Tanzania
Thailand
Togo
Tokelau
Tonga
Trinidad And Tobago
Tunisia
Turkey
Turkmenistan
Turks And Caicos Islands
Tuvalu
Uganda
Ukraine
United Arab Emirates
United States Minor Outlying Islands
Uruguay
Uzbekistan
Vanuatu
Vatican City
Venezuela
Vietnam
Virgin Islands (British)
Virgin Islands (U.S.)
Wallis And Futuna Islands
Western Sahara
Yemen
Yugoslavia
Zambia
Zimbabwe

By providing this information, you agree to the processing of your personal data by SANS as described in our Privacy Policy.

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.

Tags:
  • Digital Forensics and Incident Response

Related Content

Blog
Top_10_Summit_Talks_2022.png
Cybersecurity Insights, Digital Forensics and Incident Response, Cyber Defense, Cloud Security, Open-Source Intelligence (OSINT), Security Management, Legal, and Audit, Security Awareness
December 5, 2022
Top 10 SANS Summits Talks of 2022
This year, SANS hosted 13 Summits with 246 talks. Here were the top-rated talks of the year.
370x370-person-placeholder.png
Alison Kim
read more
Blog
FOR577.png
Digital Forensics and Incident Response
September 22, 2022
NEW SANS DFIR COURSE IN DEVELOPMENT | FOR577: LINUX Incident Response & Analysis
FOR577: Linux Incident Response & Analysis course teaches how Linux systems work and how to respond and investigate attacks effectively.
Viv_Ross_370x370.png
Viviana Ross
read more
Blog
Untitled_design-43.png
Digital Forensics and Incident Response, Cybersecurity and IT Essentials, Industrial Control Systems Security, Purple Team, Open-Source Intelligence (OSINT), Penetration Testing and Red Teaming, Cyber Defense, Cloud Security, Security Management, Legal, and Audit
December 8, 2021
Good News: SANS Virtual Summits Will Remain FREE for the Community in 2022
They’re virtual. They’re global. They’re free.
370x370-person-placeholder.png
Emily Blades
read more
  • Register to Learn
  • Courses
  • Certifications
  • Degree Programs
  • Cyber Ranges
  • Job Tools
  • Security Policy Project
  • Posters & Cheat Sheets
  • White Papers
  • Focus Areas
  • Cyber Defense
  • Cloud Security
  • Cybersecurity Leadership
  • Digital Forensics
  • Industrial Control Systems
  • Offensive Operations
Subscribe to SANS Newsletters
Receive curated news, vulnerabilities, & security awareness tips
United States
Canada
United Kingdom
Spain
Belgium
Denmark
Norway
Netherlands
Australia
India
Japan
Singapore
Afghanistan
Aland Islands
Albania
Algeria
American Samoa
Andorra
Angola
Anguilla
Antarctica
Antigua and Barbuda
Argentina
Armenia
Aruba
Austria
Azerbaijan
Bahamas
Bahrain
Bangladesh
Barbados
Belarus
Belize
Benin
Bermuda
Bhutan
Bolivia
Bonaire, Sint Eustatius, and Saba
Bosnia And Herzegovina
Botswana
Bouvet Island
Brazil
British Indian Ocean Territory
Brunei Darussalam
Bulgaria
Burkina Faso
Burundi
Cambodia
Cameroon
Cape Verde
Cayman Islands
Central African Republic
Chad
Chile
China
Christmas Island
Cocos (Keeling) Islands
Colombia
Comoros
Cook Islands
Costa Rica
Croatia (Local Name: Hrvatska)
Curacao
Cyprus
Czech Republic
Democratic Republic of the Congo
Djibouti
Dominica
Dominican Republic
East Timor
East Timor
Ecuador
Egypt
El Salvador
Equatorial Guinea
Eritrea
Estonia
Ethiopia
Falkland Islands (Malvinas)
Faroe Islands
Fiji
Finland
France
French Guiana
French Polynesia
French Southern Territories
Gabon
Gambia
Georgia
Germany
Ghana
Gibraltar
Greece
Greenland
Grenada
Guadeloupe
Guam
Guatemala
Guernsey
Guinea
Guinea-Bissau
Guyana
Haiti
Heard And McDonald Islands
Honduras
Hong Kong
Hungary
Iceland
Indonesia
Iraq
Ireland
Isle of Man
Israel
Italy
Jamaica
Jersey
Jordan
Kazakhstan
Kenya
Kiribati
Korea, Republic Of
Kosovo
Kuwait
Kyrgyzstan
Lao People's Democratic Republic
Latvia
Lebanon
Lesotho
Liberia
Liechtenstein
Lithuania
Luxembourg
Macau
Macedonia
Madagascar
Malawi
Malaysia
Maldives
Mali
Malta
Marshall Islands
Martinique
Mauritania
Mauritius
Mayotte
Mexico
Micronesia, Federated States Of
Moldova, Republic Of
Monaco
Mongolia
Montenegro
Montserrat
Morocco
Mozambique
Myanmar
Namibia
Nauru
Nepal
Netherlands Antilles
New Caledonia
New Zealand
Nicaragua
Niger
Nigeria
Niue
Norfolk Island
Northern Mariana Islands
Oman
Pakistan
Palau
Palestine
Panama
Papua New Guinea
Paraguay
Peru
Philippines
Pitcairn
Poland
Portugal
Puerto Rico
Qatar
Reunion
Romania
Russian Federation
Rwanda
Saint Bartholemy
Saint Kitts And Nevis
Saint Lucia
Saint Martin
Saint Vincent And The Grenadines
Samoa
San Marino
Sao Tome And Principe
Saudi Arabia
Senegal
Serbia
Seychelles
Sierra Leone
Sint Maarten
Slovakia
Slovenia
Solomon Islands
South Africa
South Georgia and the South Sandwich Islands
South Sudan
Sri Lanka
St. Helena
St. Pierre And Miquelon
Suriname
Svalbard And Jan Mayen Islands
Swaziland
Sweden
Switzerland
Taiwan
Tajikistan
Tanzania
Thailand
Togo
Tokelau
Tonga
Trinidad And Tobago
Tunisia
Turkey
Turkmenistan
Turks And Caicos Islands
Tuvalu
Uganda
Ukraine
United Arab Emirates
United States Minor Outlying Islands
Uruguay
Uzbekistan
Vanuatu
Vatican City
Venezuela
Vietnam
Virgin Islands (British)
Virgin Islands (U.S.)
Wallis And Futuna Islands
Western Sahara
Yemen
Yugoslavia
Zambia
Zimbabwe

By providing this information, you agree to the processing of your personal data by SANS as described in our Privacy Policy.

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
  • © 2023 SANS™ Institute
  • Privacy Policy
  • Contact
  • Careers
  • Twitter
  • Facebook
  • Youtube
  • LinkedIn