Seems like this can happen if the stream is too short to even contain a bom.

The way this is used, this doesn't leak any useful information.  We mask off parts of it, and then just set one of 3 values based on remainder.  

CL at https://pdfium-review.googlesource.com/4591

ClusterFuzz has detected this issue as fixed in range 467695:467736.

Detailed report: https://clusterfuzz.com/testcase?key=5323719200473088

Fuzzer: libfuzzer_pdfium_xfa_fuzzer
Job Type: libfuzzer_chrome_msan
Platform Id: linux

Crash Type: Use-of-uninitialized-value
Crash Address: 
Crash State:
  CFX_SeekableStreamProxy::CFX_SeekableStreamProxy
  CFX_RetainPtr<CFX_SeekableStreamProxy> pdfium::MakeRetain<CFX_SeekableStreamProx
  CXFA_SimpleParser::StartParse
  
Sanitizer: memory (MSAN)

Recommended Security Severity: Medium

Regressed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_msan&range=467663:467695
Fixed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_msan&range=467695:467736

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=5323719200473088


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.

This bug has been closed for more than 14 weeks. Removing security view restrictions.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot