This issue looks similar to  bug 802968 , hence assigning to rharrison@ for more inputs.

rharrison@ Since this issue has reproducible steps assigning it to you, could you please take a look in to this issue? If this issue is dupe of 802968 please go ahead merge in to it.

Thanks!

https://pdfium-review.googlesource.com/c/pdfium/+/29750

The following revision refers to this bug:
  https://pdfium.googlesource.com/pdfium/+/3faa382d4feb3c272333f20e16106a5bbc472990

commit 3faa382d4feb3c272333f20e16106a5bbc472990
Author: Dan Sinclair <dsinclair@chromium.org>
Date: Wed Apr 04 15:49:01 2018

Decrease XML parser buffer size

This CL decreases the m_iXMLPlaneSize in the CFX_XMLSyntaxParser from
32*1024 to 1024 bytes. This plane size is the size of the chunks we read
the XML file in. The issue with having a large plane size is inside the
CFX_SeekableStreamProxy::ReadString we allocate a vector of the
min(planeSize, remaining_bytes). This means, we could potentially be
allocating 32*1024 bytes to read in a single character.

For the test in the attached bug the runtime went from 12s to 25ms.

Bug:  chromium:813195 
Change-Id: Id57e619b7a118d45e3fed491ad81e86a4b1cacb8
Reviewed-on: https://pdfium-review.googlesource.com/29750
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>

[modify] https://crrev.com/3faa382d4feb3c272333f20e16106a5bbc472990/core/fxcrt/xml/cfx_xmlsyntaxparser.cpp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/330797ce54c871eb0b9232119a8ce876025c50df

commit 330797ce54c871eb0b9232119a8ce876025c50df
Author: pdfium-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com <pdfium-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Date: Wed Apr 04 19:57:36 2018

Roll src/third_party/pdfium/ 28bb2f2ff..dc566b0a7 (4 commits)

https://pdfium.googlesource.com/pdfium.git/+log/28bb2f2ffe75..dc566b0a76f9

$ git log 28bb2f2ff..dc566b0a7 --date=short --no-merges --format='%ad %ae %s'
2018-04-04 hnakashima Fix XFA caret blinking only while mouse moves.
2018-04-04 thestig Parse "yy" correctly for AFDate_FormatEx().
2018-04-04 dsinclair Decrease XML parser buffer size
2018-04-04 dsinclair [fwl] Restrict setting cursor outside text bounds

Created with:
  roll-dep src/third_party/pdfium
BUG= chromium:828561 , chromium:436572 , chromium:813195 


The AutoRoll server is located here: https://pdfium-roll.skia.org

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.


TBR=dsinclair@chromium.org

Change-Id: I2515beeac588ea2d53d3caa6d92bae0165da4824
Reviewed-on: https://chromium-review.googlesource.com/996232
Reviewed-by: pdfium-chromium-autoroll <pdfium-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Commit-Queue: pdfium-chromium-autoroll <pdfium-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#548171}
[modify] https://crrev.com/330797ce54c871eb0b9232119a8ce876025c50df/DEPS

ClusterFuzz has detected this issue as fixed in range 548153:548173.

Detailed report: https://clusterfuzz.com/testcase?key=5466962436816896

Fuzzer: libFuzzer_pdf_xml_fuzzer
Job Type: libfuzzer_chrome_asan_debug
Platform Id: linux

Crash Type: Timeout (exceeds 25 secs)
Crash Address: 
Crash State:
  pdf_xml_fuzzer
  
Sanitizer: address (ASAN)

Regressed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_asan_debug&range=498912:498966
Fixed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_asan_debug&range=548153:548173

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=5466962436816896

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 5466962436816896 is verified as fixed, so closing issue as verified.

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