<PAGE>

EXHIBIT 31(2)

                            CERTIFICATION PURSUANT TO
             RULE 13A-14(A) OF THE SECURITIES EXCHANGE ACT OF 1934,
                             AS ADOPTED PURSUANT TO
                  SECTION 302 OF THE SARBANES-OXLEY ACT OF 2002

I, John F. Corcoran, certify that:

1.  I have reviewed this quarterly report on Form 10-Q of CNA Surety
    Corporation;

2.  Based on my knowledge, this quarterly report does not contain any untrue
    statement of a material fact or omit to state a material fact necessary to
    make the statements made, in light of the circumstances under which such
    statements were made, not misleading with respect to the period covered by
    this report;

3.  Based on my knowledge, the financial statements, and other financial
    information included in this report, fairly present in all material respects
    the financial condition, results of operations and cash flows of the
    registrant as of, and for, the periods presented in this report;

4.  The registrant's other certifying officer and I are responsible for
    establishing and maintaining disclosure controls and procedures (as defined
    in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal control over
    financial reporting (as defined in Exchange Act Rules 13a-15(f) and
    15d-15(f) for the registrant and have:

    c)  designed such disclosure controls and procedures, or caused such
        disclosure controls and procedures to be designed under our supervision,
        to ensure that material information relating to the registrant,
        including its consolidated subsidiaries, is made known to us by others
        within those entities, particularly during the period in which this
        report is being prepared;

    d)  designed such internal control over financial reporting, or caused such
        internal control over financial reporting to be designed under our
        supervision, to provide reasonable assurance regarding the reliability
        of financial reporting and the preparation of financial statements for
        external purposes in accordance with generally accepted accounting
        principles;

    c)  evaluated the effectiveness of the registrant's disclosure controls and
        procedures and presented in this report our conclusions about the
        effectiveness of the disclosure controls and procedures, as of the end
        of the period covered by this quarterly report based on such evaluation;
        and

    d)  disclosed in this quarterly report any change in the registrant's
        internal control over financial reporting that occurred during the
        registrant's most recent fiscal quarter (the registrant's fourth fiscal
        quarter in the case of an annual report) that has materially affected,
        or is reasonably likely to materially affect, the registrant's internal
        control over financial reporting; and

5.  The registrant's other certifying officer and I have disclosed, based on our
    most recent evaluation of internal control over financial reporting, to the
    registrant's auditors and the audit committee of the registrant's board of
    directors (or persons performing the equivalent functions):

    a)  all significant deficiencies and material weaknesses in the design or
        operation of internal control over financial reporting which are
        reasonably likely to adversely affect the registrant's ability to
        record, process, summarize and report financial information; and

    b)  any fraud, whether or not material, that involves management or other
        employees who have a significant role in the registrant's internal
        control over financial reporting.

Date: May 2, 2005              /s/ John F. Corcoran
                               -------------------------------------------------
                               John F. Corcoran
                               Senior Vice President and Chief Financial Officer