With reference to the  Issue 723513 , assigning to its owner

jshin@ Could you please look into it

From  bug 723513  comment 3 and comment 4:

Testcase has an invalid UTF-8. 

𧻓㌛蟟m 

followed by 0xE8. 

Given that only valid UTF-8 or UTF-16 is fed to the API in question in Chromium/Blink, the risk woouldn't be that high.  Anyway, the ICU code in question needs to be hardened. 
Comment 4 by jshin@chromium.org, Jun 13
Delete comment ⚐
The byte sequences of the minimized test string :
ef ab 97 e3 8c 9b e8 9f  9f 6d e8

3 characters (of 3 bytes in UTF-8) followed by 'm' and followed by 0xE8. 

This one (unlike  bug 723513 ) has a valid UTF-32 input:

29 00 02 00 00 33 00 00 00 c4 02 00

U+20029, U+3300, U+2C400

An upstream bug filed with a standalone test program. 

http://www.icu-project.org/trac/ticket/13549 

Upstream bug is now assigned. 

The input for  bug 723513  also leads to the same assertion failure (after being converted to UTF-8 with UnicodeString::fromUTF8(). Invalid sequences are converted to U+FFFD). 

 Issue 723513  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/deps/icu.git/+/aff99f5c22aded55ee29753ce049e61570294967

commit aff99f5c22aded55ee29753ce049e61570294967
Author: Jungshik Shin <jshin@chromium.org>
Date: Thu Apr 12 20:51:41 2018

Cherry pick 3 patches from the upstream

* Fix the undefined behavior in decimal number parsing

      http://bugs.icu-project.org/trac/changeset/40950

* Fix the handling of non-BMP characters in CJK breakiterator

      http://www.icu-project.org/trac/changeset/40949

* Limit the recursion depth of UnicodeSet pattern

      http://bugs.icu-project.org/trac/changeset/40979

TBR=inferno@chromium.org
Bug:  chromium:799850 ,  chromium:796807 ,  chromium:796752 
Test: See the bugs.
Change-Id: I1a8909371b601f36faca911039b10d36c7a92c85
Reviewed-on: https://chromium-review.googlesource.com/1009001
Reviewed-by: Jungshik Shin <jshin@chromium.org>

[modify] https://crrev.com/aff99f5c22aded55ee29753ce049e61570294967/README.chromium
[add] https://crrev.com/aff99f5c22aded55ee29753ce049e61570294967/patches/cjkdict_nonbmp.patch
[add] https://crrev.com/aff99f5c22aded55ee29753ce049e61570294967/patches/number_ub.patch
[add] https://crrev.com/aff99f5c22aded55ee29753ce049e61570294967/patches/uset_depth.patch
[modify] https://crrev.com/aff99f5c22aded55ee29753ce049e61570294967/source/common/dictbe.cpp
[modify] https://crrev.com/aff99f5c22aded55ee29753ce049e61570294967/source/common/unicode/uniset.h
[modify] https://crrev.com/aff99f5c22aded55ee29753ce049e61570294967/source/common/uniset_closure.cpp
[modify] https://crrev.com/aff99f5c22aded55ee29753ce049e61570294967/source/common/uniset_props.cpp
[modify] https://crrev.com/aff99f5c22aded55ee29753ce049e61570294967/source/i18n/decNumber.cpp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bcb59cd6dd0e451d00d47b3d4a42a146dd307add

commit bcb59cd6dd0e451d00d47b3d4a42a146dd307add
Author: Jungshik Shin <jshin@chromium.org>
Date: Fri Apr 13 00:33:00 2018

Roll ICU to aff99f5

There are two changes:

 https://chromium.googlesource.com/chromium/deps/icu.git/+log/d888fd2..aff99f5

$ git log d888fd2..aff99f5 --date=short --no-merges --format='%ad %ae %s'
  2018-04-11 jshin@chromium.org Cherry pick 3 patches from the upstream
  2018-04-10 jshin@chromium.org Update IANA tzdb to 2018d and apply a fix
                                for long word selection

TBR=mark@chromium.org,inferno@chromium.org

Bug:  chromium:799850 ,  chromium:796807 ,  chromium:796752 
Bug: chromium:829144, chromium:473288
Test: See the two ICU cls above.
Change-Id: I0adf27e01c0349bd00d4916567bdc0bc70483439
Reviewed-on: https://chromium-review.googlesource.com/1011238
Reviewed-by: Jungshik Shin <jshin@chromium.org>
Commit-Queue: Jungshik Shin <jshin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550435}
[modify] https://crrev.com/bcb59cd6dd0e451d00d47b3d4a42a146dd307add/DEPS

ClusterFuzz has detected this issue as fixed in range 550400:550448.

Detailed report: https://clusterfuzz.com/testcase?key=5938875993423872

Fuzzer: libFuzzer_icu_break_iterator_utf32_fuzzer
Job Type: libfuzzer_chrome_asan_debug
Platform Id: linux

Crash Type: ASSERT
Crash Address: 
Crash State:
  prev.elementAti(t_boundary.elementAti(numBreaks - 1)) == 0
  icu_60::CjkBreakEngine::divideUpDictionaryRange
  icu_60::DictionaryBreakEngine::findBreaks
  
Sanitizer: address (ASAN)

Regressed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_asan_debug&range=525358:525394
Fixed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_asan_debug&range=550400:550448

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=5938875993423872

See https://chromium.googlesource.com/chromium/src/+/master/testing/libfuzzer/reference.md for more information.

If you suspect that the result above is incorrect, try re-doing that job on the test case report page.

ClusterFuzz testcase 5938875993423872 is verified as fixed, so closing issue as verified.

If this is incorrect, please add ClusterFuzz-Wrong label and re-open the issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bcb59cd6dd0e451d00d47b3d4a42a146dd307add

commit bcb59cd6dd0e451d00d47b3d4a42a146dd307add
Author: Jungshik Shin <jshin@chromium.org>
Date: Fri Apr 13 00:33:00 2018

Roll ICU to aff99f5

There are two changes:

 https://chromium.googlesource.com/chromium/deps/icu.git/+log/d888fd2..aff99f5

$ git log d888fd2..aff99f5 --date=short --no-merges --format='%ad %ae %s'
  2018-04-11 jshin@chromium.org Cherry pick 3 patches from the upstream
  2018-04-10 jshin@chromium.org Update IANA tzdb to 2018d and apply a fix
                                for long word selection

TBR=mark@chromium.org,inferno@chromium.org

Bug:  chromium:799850 ,  chromium:796807 ,  chromium:796752 
Bug: chromium:829144, chromium:473288
Test: See the two ICU cls above.
Change-Id: I0adf27e01c0349bd00d4916567bdc0bc70483439
Reviewed-on: https://chromium-review.googlesource.com/1011238
Reviewed-by: Jungshik Shin <jshin@chromium.org>
Commit-Queue: Jungshik Shin <jshin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550435}
[modify] https://crrev.com/bcb59cd6dd0e451d00d47b3d4a42a146dd307add/DEPS

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/tools/buildspec/+/82e8c77b1e704e509f1a8aefa4f5e3380943405d

commit 82e8c77b1e704e509f1a8aefa4f5e3380943405d
Author: Jungshik Shin <jungshik@google.com>
Date: Wed Apr 25 16:57:19 2018