Todd P Headley Form 3 QUALYS / Todd P Headley ownership change

                                   Exhibit 24
                                   ----------

             LIMITED POWER OF ATTORNEY - SECURITIES LAW COMPLIANCE

      The undersigned, as an officer or director of Qualys, Inc. (the
"Corporation"), hereby constitutes and appoints Nancy Hsiang and Bruce Posey,
each the undersigned's true and lawful attorney-in-fact and agent to complete
and execute such Forms 144, Form ID, Forms 3, 4 and 5 and other forms as such
attorney shall in his or her discretion determine to be required or advisable
pursuant to Rule 144 promulgated under the Securities Act of 1933, as amended,
Section 16 of the Securities Exchange Act of 1934, as amended, and the rules and
regulations promulgated thereunder, or any successor laws and regulations, as a
consequence of the undersigned's ownership, acquisition or disposition of
securities of the Corporation, and to do all acts necessary in order to file
such forms with the Securities and Exchange Commission, any securities exchange
or national association, the Corporation and such other person or agency as the
attorney shall deem appropriate.

      The undersigned hereby ratifies and confirms all that said attorneys-in-
fact and agents shall do or cause to be done by virtue hereof.  The undersigned
acknowledges that the foregoing attorneys-in-fact, in serving in such capacity
at the request of the undersigned, are not assuming, nor is the Company
assuming, any of the undersigned's responsibilities to comply with Section 16 of
the Securities Exchange Act of 1934 (as amended).

      This Limited 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
Corporation unless earlier revoked by the undersigned in a writing delivered to
the foregoing attorneys-in-fact.

      This Limited Power of Attorney is executed at Redwood City, CA, as of the
date set forth below.

                                         Signature:  /s/ Todd P. Headley
                                                    ----------------------------
                                         Print Name:  Todd P. Headley
                                                      --------------------------
                                         Dated:  February 4, 2016
                                                --------------------------------


Witness:  /s/ Bruce K. Posey
         ----------------------------
Print Name:  Bruce K. Posey
            -------------------------
Dated:  February 4, 2016
       ------------------------------