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===========================================
skipfish - web application security scanner
===========================================
http://code.google.com/p/skipfish/
* Written and maintained by Michal Zalewski <lcamtuf@google.com>.
* Copyright 2009, 2010 Google Inc, rights reserved.
* Released under terms and conditions of the Apache License, version 2.0.
--------------------
1. What is skipfish?
--------------------
Skipfish is an active web application security reconnaissance tool. It prepares
an interactive sitemap for the targeted site by carrying out a recursive crawl
and dictionary-based probes. The resulting map is then annotated with the
output from a number of active (but hopefully non-disruptive) security checks.
The final report generated by the tool is meant to serve as a foundation for
professional web application security assessments.
Why should I bother with this particular tool?
A number of commercial and open source tools with analogous functionality is
readily available (e.g., Nikto, Nessus); stick to the one that suits you best.
That said, skipfish tries to address some of the common problems associated
with web security scanners. Specific advantages include:
* High performance: 500+ requests per second against responsive Internet
targets, 2000+ requests per second on LAN / MAN networks, and 7000+ requests
against local instances has been observed, with a very modest CPU, network,
and memory footprint. This can be attributed to:
- Multiplexing single-thread, fully asynchronous network I/O and data
processing model that eliminates memory management, scheduling, and IPC
inefficiencies present in some multi-threaded clients.
- Advanced HTTP/1.1 features such as range requests, content
compression, and keep-alive connections, as well as forced response size
limiting, to keep network-level overhead in check.
- Smart response caching and advanced server behavior heuristics are
used to minimize unnecessary traffic.
- Performance-oriented, pure C implementation, including a custom
HTTP stack.
* Ease of use: skipfish is highly adaptive and reliable. The scanner
features:
- Heuristic recognition of obscure path- and query-based parameter
handling schemes.
- Graceful handling of multi-framework sites where certain paths obey
a completely different semantics, or are subject to different filtering
rules.
- Automatic wordlist construction based on site content analysis.
- Probabilistic scanning features to allow periodic, time-bound
assessments of arbitrarily complex sites.
* Well-designed security checks: the tool is meant to provide accurate and
meaningful results:
- Three-step differential probes are preferred to signature checks
for detecting vulnerabilities.
- Ratproxy-style logic is used to spot subtle security problems:
cross-site request forgery, cross-site script inclusion, mixed content,
issues MIME- and charset mismatches, incorrect caching directive, etc.
- Bundled security checks are designed to handle tricky scenarios:
stored XSS (path, parameters, headers), blind SQL or XML injection, or
blind shell injection.
- Report post-processing drastically reduces the noise caused by any
remaining false positives or server gimmicks by identifying repetitive
patterns.
That said, skipfish is not a silver bullet, and may be unsuitable for certain
purposes. For example, it does not satisfy most of the requirements outlined in
WASC Web Application Security Scanner Evaluation Criteria (some of them on
purpose, some out of necessity); and unlike most other projects of this type,
it does not come with an extensive database of known vulnerabilities for
banner-type checks.
-----------------------------------------------------
2. Most curious! What specific tests are implemented?
-----------------------------------------------------
A rough list of the security checks offered by the tool is outlined below.
* High risk flaws (potentially leading to system compromise):
- Server-side SQL injection (including blind vectors, numerical
parameters).
- Explicit SQL-like syntax in GET or POST parameters.
- Server-side shell command injection (including blind vectors).
- Server-side XML / XPath injection (including blind vectors).
- Format string vulnerabilities.
- Integer overflow vulnerabilities.
* Medium risk flaws (potentially leading to data compromise):
- Stored and reflected XSS vectors in document body (minimal JS XSS
support present).
- Stored and reflected XSS vectors via HTTP redirects.
- Stored and reflected XSS vectors via HTTP header splitting.
- Directory traversal (including constrained vectors).
- Assorted file POIs (server-side sources, configs, etc).
- Attacker-supplied script and CSS inclusion vectors (stored and
reflected).
- External untrusted script and CSS inclusion vectors.
- Mixed content problems on script and CSS resources (optional).
- Incorrect or missing MIME types on renderables.
- Generic MIME types on renderables.
- Incorrect or missing charsets on renderables.
- Conflicting MIME / charset info on renderables.
- Bad caching directives on cookie setting responses.
* Low risk issues (limited impact or low specificity):
- Directory listing bypass vectors.
- Redirection to attacker-supplied URLs (stored and reflected).
- Attacker-supplied embedded content (stored and reflected).
- External untrusted embedded content.
- Mixed content on non-scriptable subresources (optional).
- HTTP credentials in URLs.
- Expired or not-yet-valid SSL certificates.
- HTML forms with no XSRF protection.
- Self-signed SSL certificates.
- SSL certificate host name mismatches.
- Bad caching directives on less sensitive content.
* Internal warnings:
- Failed resource fetch attempts.
- Exceeded crawl limits.
- Failed 404 behavior checks.
- IPS filtering detected.
- Unexpected response variations.
- Seemingly misclassified crawl nodes.
* Non-specific informational entries:
- General SSL certificate information.
- Significantly changing HTTP cookies.
- Changing Server, Via, or X-... headers.
- New 404 signatures.
- Resources that cannot be accessed.
- Resources requiring HTTP authentication.
- Broken links.
- Server errors.
- All external links not classified otherwise (optional).
- All external e-mails (optional).
- All external URL redirectors (optional).
- Links to unknown protocols.
- Form fields that could not be autocompleted.
- All HTML forms detected.
- Password entry forms (for external brute-force).
- Numerical file names (for external brute-force).
- User-supplied links otherwise rendered on a page.
- Incorrect or missing MIME type on less significant content.
- Generic MIME type on less significant content.
- Incorrect or missing charset on less significant content.
- Conflicting MIME / charset information on less significant content.
- OGNL-like parameter passing conventions.
Along with a list of identified issues, skipfish also provides summary
overviews of document types and issue types found; and an interactive sitemap,
with nodes discovered through brute-force denoted in a distinctive way.
-----------------------------------------------------------
3. All right, I want to try it out. What do I need to know?
-----------------------------------------------------------
First and foremost, please do not be evil. Use skipfish only against services
you own, or have a permission to test.
Keep in mind that all types of security testing can be disruptive. Although the
scanner is designed not to carry out disruptive malicious attacks, it may
accidentally interfere with the operations of the site. You must accept the
risk, and plan accordingly. Run the scanner against test instances where
feasible, and be prepared to deal with the consequences if things go wrong.
Also note that the tool is meant to be used by security professionals, and is
experimental in nature. It may return false positives or miss obvious security
problems - and even when it operates perfectly, it is simply not meant to be a
point-and-click application. Do not rely on its output at face value.
How to run the scanner?
To compile it, simply unpack the archive and try make. Chances are, you will
need to install libidn first.
Next, you need to copy the desired dictionary file from dictionaries/ to
skipfish.wl. Please read dictionaries/README-FIRST carefully to make the right
choice. This step has a profound impact on the quality of scan results later on.
Once you have the dictionary selected, you can try:
$ ./skipfish -o output_dir http://www.example.com/some/starting/path.txt
Note that you can provide more than one starting URL if so desired; all of them
will be crawled.
In the example above, skipfish will scan the entire www.example.com (including
services on other ports, if linked to from the main page), and write a report
to output_dir/index.html. You can then view this report with your favorite
browser (JavaScript must be enabled). The index.html file is static; actual
results are stored as a hierarchy of JSON files, suitable for machine
processing if needs be.
Some sites may require authentication; for simple HTTP credentials, you can try:
$ ./skipfish -A user:pass ...other parameters...
Alternatively, if the site relies on HTTP cookies instead, log in in your
browser or using a simple curl script, and then provide skipfish with a session
cookie:
$ ./skipfish -C name=val ...other parameters...
Other session cookies may be passed the same way, one per each -C option.
Certain URLs on the site may log out your session; you can combat this in two
ways: by using the -N option, which causes the scanner to reject attempts to
set or delete cookies; or with the -X parameter, which prevents matching URLs
from being fetched:
$ ./skipfish -X /logout/logout.aspx ...other parameters...
The -X option is also useful for speeding up your scans by excluding /icons/,
/doc/, /manuals/, and other standard, mundane locations along these lines. In
general, you can use -X, plus -I (only spider URLs matching a substring) and -S
(ignore links on pages where a substring appears in response body) to limit the
scope of a scan any way you like - including restricting it only to a specific
protocol and port:
$ ./skipfish -I http://example.com:1234/ ...other parameters...
Another useful scoping option is -D - allowing you to specify additional hosts
or domains to consider in-scope for the test. By default, all hosts appearing
in the command-line URLs are added to the list - but you can use -D to broaden
these rules, for example:
$ ./skipfish -D test2.example.com -o output-dir http://test1.example.com/
...or, for a domain wildcard match, use:
$ ./skipfish -D .example.com -o output-dir http://test1.example.com/
In some cases, you do not want to actually crawl a third-party domain, but you
trust the owner of that domain enough not to worry about cross-domain content
inclusion from that location. To suppress warnings, you can use the -B option,
for example:
$ ./skipfish -B .google-analytics.com -B .googleapis.com ...other parameters...
By default, skipfish sends minimalistic HTTP headers to reduce the amount of
data exchanged over the wire; some sites examine User-Agent strings or header
ordering to reject unsupported clients, however. In such a case, you can use -b
ie or -b ffox to mimic one of the two popular browsers.
When it comes to customizing your HTTP requests, you can also use the -H option
to insert any additional, non-standard headers; or -F to define a custom
mapping between a host and an IP (bypassing the resolver). The latter feature
is particularly useful for not-yet-launched or legacy services.
Some sites may be too big to scan in a reasonable timeframe. If the site
features well-defined tarpits - for example, 100,000 nearly identical user
profiles as a part of a social network - these specific locations can be
excluded with -X or -S. In other cases, you may need to resort to other
settings: -d limits crawl depth to a specified number of subdirectories; -c
limits the number of children per directory; and -r limits the total number of
requests to send in a scan.
An interesting option is available for repeated assessments: -p. By specifying
a percentage between 1 and 100%, it is possible to tell the crawler to follow
fewer than 100% of all links, and try fewer than 100% of all dictionary
entries. This - naturally - limits the completeness of a scan, but unlike most
other settings, it does so in a balanced, non-deterministic manner. It is
extremely useful when you are setting up time-bound, but periodic assessments
of your infrastructure. Another related option is -q, which sets the initial
random seed for the crawler to a specified value. This can be used to exactly
reproduce a previous scan to compare results. Randomness is relied upon most
heavily in the -p mode, but also for making a couple of other scan management
decisions elsewhere.
Some particularly complex (or broken) services may involve a very high number
of identical or nearly identical pages. Although these occurrences are by
default grayed out in the report, they still use up some screen estate and take
a while to process on JavaScript level. In such extreme cases, you may use the
-Q option to suppress reporting of duplicate nodes altogether, before the
report is written. This may give you a less comprehensive understanding of how
the site is organized, but has no impact on test coverage.
In certain quick assessments, you might also have no interest in paying any
particular attention to the desired functionality of the site - hoping to
explore non-linked secrets only. In such a case, you may specify -P to inhibit
all HTML parsing. This limits the coverage and takes away the ability for the
scanner to learn new keywords by looking at the HTML, but speeds up the test
dramatically. Another similarly crippling option that reduces the risk of
persistent effects of a scan is -O, which inhibits all form parsing and
submission steps.
By default, skipfish complains loudly about all MIME or character set
mismatches on renderable documents, and classifies many of them as "medium
risk"; this is because, if any user-controlled content is returned, the
situation could lead to cross-site scripting attacks in certain browsers. On
some poorly designed and maintained sites, this may contribute too much noise;
if so, you may use -J to mark these issues as "low risk" unless the scanner can
explicitly sees its own user input being echoed back on the resulting page.
This may miss many subtle attack vectors, though.
Some sites that handle sensitive user data care about SSL - and about getting
it right. Skipfish may optionally assist you in figuring out problematic mixed
content scenarios - use the -M option to enable this. The scanner will complain
about situations such as http:// scripts being loaded on https:// pages - but
will disregard non-risk scenarios such as images.
Likewise, certain pedantic sites may care about cases where caching is
restricted on HTTP/1.1 level, but no explicit HTTP/1.0 caching directive is
given on specifying -E in the command-line causes skipfish to log all such
cases carefully.
Lastly, in some assessments that involve self-contained sites without extensive
user content, the auditor may care about any external e-mails or HTTP links
seen, even if they have no immediate security impact. Use the -U option to have
these logged.
Dictionary management is a special topic, and - as mentioned - is covered in
more detail in dictionaries/README-FIRST. Please read that file before
proceeding. Some of the relevant options include -W to specify a custom
wordlist, -L to suppress auto-learning, -V to suppress dictionary updates, -G
to limit the keyword guess jar size, -R to drop old dictionary entries, and -Y
to inhibit expensive $keyword.$extension fuzzing.
Skipfish also features a form auto-completion mechanism in order to maximize
scan coverage. The values should be non-malicious, as they are not meant to
implement security checks - but rather, to get past input validation logic. You
can define additional rules, or override existing ones, with the -T option (-T
form_field_name=field_value, e.g. -T login=test123 -T password=test321 -
although note that -C and -A are a much better method of logging in).
There is also a handful of performance-related options. Use -g to set the
maximum number of connections to maintain, globally, to all targets (it is
sensible to keep this under 50 or so to avoid overwhelming the TCP/IP stack on
your system or on the nearby NAT / firewall devices); and -m to set the per-IP
limit (experiment a bit: 2-4 is usually good for localhost, 4-8 for local
networks, 10-20 for external targets, 30+ for really lagged or non-keep-alive
hosts). You can also use -w to set the I/O timeout (i.e., skipfish will wait
only so long for an individual read or write), and -t to set the total request
timeout, to account for really slow or really fast sites.
Lastly, -f controls the maximum number of consecutive HTTP errors you are
willing to see before aborting the scan; and -s sets the maximum length of a
response to fetch and parse (longer responses will be truncated).
--------------------------------
4. But seriously, how to run it?
--------------------------------
A standard, authenticated scan of a well-designed and self-contained site
(warns about all external links, e-mails, mixed content, and caching header
issues):
$ ./skipfish -MEU -C "AuthCookie=value" -X /logout.aspx -o output_dir \
http://www.example.com/
Five-connection crawl, but no brute-force; pretending to be MSIE and caring
less about ambiguous MIME or character set mismatches:
$ ./skipfish -m 5 -LVJ -W /dev/null -o output_dir -b ie http://www.example.com/
Brute force only (no HTML link extraction), trusting links within example.com
and timing out after 5 seconds:
$ ./skipfish -B .example.com -O -o output_dir -t 5 http://www.example.com/
For a short list of all command-line options, try ./skipfish -h.
----------------------------------------------------
5. How to interpret and address the issues reported?
----------------------------------------------------
Most of the problems reported by skipfish should self-explanatory, assuming you
have a good gasp of the fundamentals of web security. If you need a quick
refresher on some of the more complicated topics, such as MIME sniffing, you
may enjoy our comprehensive Browser Security Handbook as a starting point:
http://code.google.com/p/browsersec/
If you still need assistance, there are several organizations that put a
considerable effort into documenting and explaining many of the common web
security threats, and advising the public on how to address them. I encourage
you to refer to the materials published by OWASP and Web Application Security
Consortium, amongst others:
* http://www.owasp.org/index.php/Category:Principle
* http://www.owasp.org/index.php/Category:OWASP_Guide_Project
* http://www.webappsec.org/projects/articles/
Although I am happy to diagnose problems with the scanner itself, I regrettably
cannot offer any assistance with the inner wokings of third-party web
applications.
---------------------------------------
6. Known limitations / feature wishlist
---------------------------------------
Below is a list of features currently missing in skipfish. If you wish to
improve the tool by contributing code in one of these areas, please let me know:
* Buffer overflow checks: after careful consideration, I suspect there is
no reliable way to test for buffer overflows remotely. Much like the actual
fault condition we are looking for, proper buffer size checks may also
result in uncaught exceptions, 500 messages, etc. I would love to be proved
wrong, though.
* Fully-fledged JavaScript XSS detection: several rudimentary checks are
present in the code, but there is no proper script engine to evaluate
expressions and DOM access built in.
* Variable length encoding character consumption / injection bugs: these
problems seem to be largely addressed on browser level at this point, so
they were much lower priority at the time of this writing.
* Security checks and link extraction for third-party, plugin-based content
(Flash, Java, PDF, etc).
* Password brute-force and numerical filename brute-force probes.
* Search engine integration (vhosts, starting paths).
* VIEWSTATE decoding.
* NTLM and digest authentication.
* Proxy support: somewhat incompatible with performance control features
currently employed by skipfish; but in the long run, should be provided as
a last-resort option.
* Scan resume option.
* Standalone installation (make install) support.
* Config file support.
-------------------------------------
7. Oy! Something went horribly wrong!
-------------------------------------
There is no web crawler so good that there wouldn't be a web framework to one
day set it on fire. If you encounter what appears to be bad behavior (e.g., a
scan that takes forever and generates too many requests, completely bogus nodes
in scan output, or outright crashes), please recompile the scanner with:
$ make clean debug
...and re-run it this way:
$ ./skipfish [...previous options...] 2>logfile.txt
You can then inspect logfile.txt to get an idea what went wrong; if it looks
like a scanner problem, please scrub any sensitive information from the log
file and send it to the author.
If the scanner crashed, please recompile it as indicated above, and then type:
$ ulimit -c unlimited
$ ./skipfish [...previous options...] 2>logfile.txt
$ gdb --batch -ex back ./skipfish core
...and be sure to send the author the output of that last command as well.
-----------------------
8. Credits and feedback
-----------------------
Skipfish is made possible thanks to the contributions of, and valuable feedback
from, Google's information security engineering team.
If you have any bug reports, questions, suggestions, or concerns regarding the
application, the author can be reached at lcamtuf@google.com.
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