Because of two recent USCISinterpretive changes, Musillo Unkenholt LLC (MU Law) advises that STEM OPTworkers do not work at third-party worksites under their SETM OPT workauthorization until the USCIS issues better and clearer guidance on theissue. 
Working and training at third-partyworksites is probably legal.  However, byworking at the third-party worksite the STEM OPT trainee puts himself at risk forenormous negative immigration consequences, including a ten-year bar fromentering or living in the United States.
USCIS’ APPARENT THIRD-PARTY WORKSITE PROHIBITION
Earlier this year, and without anywarning or notice, the USCISchanged its webpage to include this key change:
thetraining experience may not take place at the place of business or worksite ofthe employer’s clients or customers because ICE would lack authority to visitsuch sites.
The USCIS’ justification for thethird-party worksite prohibition is, apparently, because ICE (ImmigrationCustoms Enforcement) would lack the authority to investigate at the third-partyworksite. 
Curiously, ICE has not produced anyguidance on this point.  ICE likely wouldbe surprised to learn that they do not have the authority to investigate aworksite where it believed immigration fraud was being committed.
The website change alone isprobably not good law.  There has been noregulatory change.  There has been nonotice and comment period, something required for regulatory change.  The “ICE lacks investigative authority”justification for the prohibition against third-party worksites is weak.
If the only thing that USCIS haddone was updated their webpage, then MU Law’s position might be that STEMworkers could continue to work at third-party worksites, provided that theother qualifications of the program were being met, however, this is not theonly change for F-1 students.
F-1 STUDENTS NOW ACCRUE UNLAWFUL PRESENCE FOR FAILURE TO MAINTAINSTATUS
A brand new USCIS policy, effectiveAugust 9, 2018, says that F-1students including STEM OPTs will now accrue “unlawful presence”“the day after he or she engages in an unauthorized activity.”  Accordingly, after August 9, 2018, the USCISis expected to find that STEM OPTs working at third-party worksites areengaging in “unauthorized activity” and are therefore “unlawfully present”.
Even worse, a USCIS official can retroactivelyfind that an F-1 student engaged in “unauthorized activity”.  A number of leadinguniversity administrators have made this point directly to USCISDirector James Cissna.
This is a massive change inlong-standing USCIS policy.  Under theprior interpretation, an F-1 student or OPT did not accrue unlawful presenceuntil an immigration judge said so.  Engaging in “unauthorized activity” meant thatan F-1 worker “failed to maintain status,” which is a lesser finding.
The distinction between “failing tomaintain status” and “unlawful presence” is enormous:
  • When someone fails to “maintainstatus” they must immediately leave the US but can ordinarily immediatelyreenter the US. 
  • When someone is “unlawfullypresent” for more than 180 days, they must immediately leave the US and are barredfrom reentering the US for 3 years. When someone is “unlawfully present” for more than 365 days, they mustimmediately leave the US and are barred from reentering the US for 10 years. 

Consider this hypotheticalscenario:

August 9, 2018 – STEM OPT continuesto work at a third-party worksite

April 1, 2019 – STEM OPT workerfiles an H-1B cap petition

May 1, 2019 – STEM OPT worker’sH-1B cap cases is selected in the H-1B lottery

August 10, 2019 – STEM OPT workerreceives an RFE from USCIS asking for proof that he has only engaged inauthorized activity.

September 20, 2019 –  H-1B is denied.  USCIS finds that STEM OPT worker’sthird-party work was “unauthorized activity”. USCIS also finds that the STEM OPT worker was “unlawfully present” fromAugust 9, 2018 until September 20, 2019, a period of more than 365 days.  Consequently, the STEM OPT worker mustimmediately leave the US and cannot reenter the US for 10 years.
CONCLUSION

At this time, MU Law recommendsthat STEM OPT workers are not placed at third party worksites unless comprehensiveanalysis is done regarding the viability of the assignment.  STEM OPT workers at third party worksites runthe risk of 3 and 10 year bars from reentry into the US.  It is our hope that USCIS provides greatlyclarity on these points and engages the public on the issue, rather thancreating law by fiat.