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 Don McCauley 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 agent 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, is 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/ Sumedh S. Thakar
                                                   -----------------------------

                                        Print Name: Sumedh S. Thakar
                                                    ----------------------------

                                        Dated: August 15, 2012
                                               ---------------------------------