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fuzzitdev / jsfuzz, Hacker News

fuzzitdev / jsfuzz, Hacker News



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Jsfuzz is coverage-guidedfuzzerfor testing javascript / nodejs packages.

Fuzzing for safe languages ​​like nodejs is a powerful strategy for finding bugs like unhandled exceptions, logic bugs, security bugs that arise from both logic bugs and Denial-of-Service caused by hangs and excessive memory usage.

Fuzzing can be seen as a powerful and efficient strategy in real-world software in addition to classic unit-tests.


Fuzz Target

The first step is to implement the following function (also called a fuzz target):

functionfuzz(buf) {   //call your package with buf}Module.exports={     fuzz };

Features of the fuzz target:

  • Jsfuzz will call the fuzz target in an infinite loop with random data (according to the coverage guided algorithm) passed to (buf) (in a separate process).
  • The function must catch and ignore any expected exceptions that arise when passing invalid input to the tested package.
  • The fuzz target must call the test function / library with wither the passed buffer or a transformation on the test buffer if the structure is different or from different type.
  • Fuzz functions can also implement application level checks to catch application / logical bugs – For example: decode the buffer with the testable library encode it again and check that both results are equal. To communicate the results the result / bug the function should throw an exception.
  • jsfuzz will report any unhandled exceptions as crashes as well as inputs that hit the memory limit specified to jsfuzz or hangs / they run more the the specified timeout limit per testcase.

Here is an example of a simple fuzz function forjpeg-jsmodule.

constJPEG=require(' jpeg-js'');functionfuzz(buf) {     try{         JPEG.decode(buf);     }catch(e) {         //Those are "valid" exceptions. we can't catch them in one line as        //jpeg-js doesn't export / inherit from one exception class / style.        if(e.message.indexOf('JPEG'')!==-1||            e.message.indexOf('length octect'')!==-(1)||            e.message.indexOf('Failed to'')!==-(1)||            e.message.indexOf('DecoderBuffer')!==-1|            e.message.indexOf('invalid table spec')!==-1||            e.message.indexOf('SOI not found)!==-1'')) {         }else{             Throwe;         }     } }Module.exports={     fuzz };


The next step is to download js-fuzz and the your fuzzer

npm i -g jsfuzz jsfuzz ./examples/jpeg/fuzz.js corpus#Output:#0 READ units: 0#1 NEW cov: 61 corp: 0 exec / s: 1 rss: 23.  (MB)#23320 PULSE cov: 61 corp: 1 exec / s: 10614 RSS: 35 .3 MB#96022 NEW COV: 70 corp: 1 EXEC / S: 11320 rss: 129.  (MB)#96971 NEW COV: 78 corp: 2 exec / s: 10784 rss: 129.  (MB)#97046 NEW COV: 79 corp: 3 exec / s: 9375 rss: 129.  (MB)#97081 NEW COV: 81 corp: 4 exec / s: 11666 rss:  (*************************************************************************************************************************** (MB)#97195 NEW COV: 93 corp: 5 exec / s: 9500 rss: 129.  (MB)#97216 NEW COV: 97 corp: 6 exec / s: 10500 rss: 129. *************************************************************************************************************************** (MB)#97238 NEW cov: 102 corp: 7 exec / s: 11000 rss: 129.  MB#97303 NEW cov: 108 corp: 8 exec / s: 10833 rss: 129. 96 MB#97857 PULSE COV: 108 corp: 9 exec / s: 225 rss: 129.  MB#97857 PULSE COV: 108 corp: 9 exec / s: 0 rss: 940. 97 MB#97857 PULSE COV: 108 corp: 9 exec / s: 0 rss: 1566. 01 MB#97857 PULSE COV: 108 corp: 9 exec / s: 0 rss: 2053. 49 MBMEMORY OOM: exceeded 2048 MB. Killing worker Worker killed crash was written to crash - 819587841 E3C 275338593 B0D  (b) ************************************************************************************************ (D)  e2abf3be8cfc 781 D 2688 d crash (hex)=ffd8ffc 09 dfdb0ffff0e 5296 bd7fbbc4f 9579096 bd7fbbfc0e  (d)  ffff 36 fa  (BF)  ffaf  (a) ********************************************************************************************************************************************* (f)  F5e  (C4F)  bd7fbb  (B)  E8C 018 FDA 5212

This example quickly finds an infinite hang which takes all the memory injpeg-js.


Jsfuzz will generate and test various inputs in an infinite loop.corpusis optional directory and will be used to save the generated testcases so later runs can be started from the same point and provided as seed corpus.

JsFuzz can also start with an empty directory (i.e no seed corpus) though some valid test-cases in the seed corpus may speed up the fuzzing substantially.

jsfuzz tries to mimic some of the arguments and output style fromlibFuzzer.

More fuzz targets examples (for real and popular libraries) are located under the examples directory and bugs that were found using those targets are listed in the trophies section.

Credits & Acknowledgments

jsfuzz logic is heavily based ongo-fuzzoriginally developed byDmitry Vyukov’s. Which is in turn heavily based onMichal ZalewskiAFL.

A previous take on that was done by [] with a bit different design, coverage and interface but it looks like it is currently unmaintained.

For coverage jsfuzz is usingistanbuljsinstrumentation and coverage library.


Contributions are welcome! 🙂 There are still a lot of things to improve, test and features to add. We will slowly post those in the issues section. Before doing any major contribution please open an issue so we can discuss and help guide the process before any unnecessary work is done.


Feel free to add bugs that you found with jsfuzz to this list via pull-request


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