Predator and CL could not provide any possible suspects.
Using Code Search for the file, "cfx_bytestring.cpp" assigning to the concern owner.

Suspecting Commit#
https://pdfium.googlesource.com/pdfium.git/+/e6c67cd64a80bd67c587ad3ffb380fe1b7574d1f

@hnakashima -- Could you please look into the issue, kindly re-assign if this is not related to your changes.
Thank You.

The following revision refers to this bug:
  https://pdfium.googlesource.com/pdfium/+/dce09b18b48837d8006694b9dc3b2d026e5e7869

commit dce09b18b48837d8006694b9dc3b2d026e5e7869
Author: Henrique Nakashima <hnakashima@chromium.org>
Date: Fri Sep 01 16:30:01 2017

Fix integer overflow in Buffer_itoa when passing INT_MIN.

Bug:  chromium:760034 
Change-Id: Id0862749b1454e065de4de7d746a27e78ac58e30
Reviewed-on: https://pdfium-review.googlesource.com/12730
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>

[modify] https://crrev.com/dce09b18b48837d8006694b9dc3b2d026e5e7869/core/fpdfapi/parser/cpdf_number.cpp
[modify] https://crrev.com/dce09b18b48837d8006694b9dc3b2d026e5e7869/core/fxcrt/cfx_bytestring.h
[modify] https://crrev.com/dce09b18b48837d8006694b9dc3b2d026e5e7869/core/fxcrt/cfx_bytestring_unittest.cpp
[modify] https://crrev.com/dce09b18b48837d8006694b9dc3b2d026e5e7869/xfa/fxfa/fm2js/cxfa_fm2jscontext.cpp
[modify] https://crrev.com/dce09b18b48837d8006694b9dc3b2d026e5e7869/core/fxcrt/cfx_bytestring.cpp

ClusterFuzz has detected this issue as fixed in range 499325:499364.

Detailed report: https://clusterfuzz.com/testcase?key=5080440062083072

Fuzzer: libFuzzer_pdfium_xfa_fuzzer
Job Type: libfuzzer_chrome_ubsan
Platform Id: linux

Crash Type: Integer-overflow
Crash Address: 
Crash State:
  Buffer_itoa
  CFX_ByteString::FormatInteger
  CXFA_FM2JSContext::GenerateSomExpression
  
Sanitizer: undefined (UBSAN)

Regressed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_ubsan&range=462191:462244
Fixed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_ubsan&range=499325:499364

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=5080440062083072

See https://chromium.googlesource.com/chromium/src/+/master/testing/libfuzzer/reproducing.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 5080440062083072 is verified as fixed, so closing issue as verified.

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