EXHIBIT 24.2

           LIMITED POWER OF ATTORNEY FOR SECTION 16 REPORTING OBLIGATIONS

    Know all by these presents, that the undersigned hereby makes, constitutes
and appoints each of Rene Russo and Mike Gray, signing singly and each acting
individually, as the undersigned's true and lawful attorney-in-fact with full
power and authority as hereinafter described to:

    (1)  execute for and on behalf of the undersigned, Forms 3, 4, and 5
(including any amendments thereto) in accordance with Section 16(a) of the
Securities Exchange Act of 1934 and the rules thereunder (the "Exchange Act");

    (2)  do and perform any and all acts for and on behalf of the undersigned
which may be necessary or desirable to prepare, complete and execute any such
Form 3, 4, or 5, prepare, complete and execute any amendment or amendments
thereto, and timely deliver and file such form with the United States Securities
and Exchange Commission (the "SEC") and any stock exchange or similar authority,
including without limitation the filing of a Form ID or any other application
materials to enable the undersigned to gain or maintain access to the Electronic
Data Gathering, Analysis and Retrieval system of the SEC;

    (3)  seek or obtain, as the undersigned's representative and on the
undersigned's behalf, information regarding transactions in the securities of
Arsanis, Inc. (the "Company") from any third party, including brokers, employee
benefit plan administrators and trustees, and the undersigned hereby authorizes
any such person to release any such information to such attorney-in-fact and
approves and ratifies any such release of information; and

    (4)  take any other action of any type whatsoever in connection with the
foregoing which, in the opinion of such attorney-in-fact, may be of benefit to,
in the best interest of, or legally required by, the undersigned, it being
understood that the documents executed by such attorney-in-fact on behalf of the
undersigned pursuant to this Power of Attorney shall be in such form and shall
contain such terms and conditions as such attorney-in-fact may approve in such
attorney-in-fact's discretion.

    The undersigned hereby grants to each such attorney-in-fact full power and
authority to do and perform any and every act and thing whatsoever requisite,
necessary, or proper to be done in the exercise of any of the rights and powers
herein granted, as fully to all intents and purposes as the undersigned might or
could do if personally present, with full power of substitution or revocation,
hereby ratifying and confirming all that such attorney-in-fact, or such
attorney-in-fact's substitute or substitutes, shall lawfully do or cause to be
done by virtue of this power of attorney and the rights and powers herein
granted.  The undersigned acknowledges that the foregoing attorneys-in-fact, in
serving in such capacity at the request of the undersigned, are not assuming nor
relieving, nor is the Company assuming nor relieving, any of the undersigned's
responsibilities to comply with Section 16 of the Exchange Act.  The undersigned
acknowledges that neither the Company nor the foregoing attorneys-in-fact assume
(i) any liability for the undersigned's responsibility to comply with the
requirement of the Exchange Act, (ii) any liability of the undersigned for any
failure to comply with such requirements, or (iii) any obligation or liability
of the undersigned for profit disgorgement under Section 16(b) of the Exchange
Act.

    This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file Forms 3, 4, and 5 with respect to the
undersigned's holdings of and transactions in securities issued by the Company,
unless earlier revoked by the undersigned in a signed writing delivered to the
foregoing attorneys-in-fact.

    IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to be
executed as of this 13th day of November, 2017.

                                        SVLSF VI, LLC

                                        By: /s/ Denise W. Marks
                                            ------------------------------------
                                        Name: Denise W. Marks
                                        Title: SVLSF VI, LLC, Member