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Trustwave Knowledge Base

Home » Knowledgebase » Legacy Products » Secure Web Gateway » Security Policies » FAQ: Unable to Dowload Latest Adobe Reader

FAQ: Unable to Dowload Latest Adobe Reader

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  • Question
    Why do I receive a "network error" when trying to download the latest version of Adobe Reader using the Adobe Download Manager?

  • Answer
    The installer for Adobe Reader is not downloaded directly.  Rather, when a user goes to Adobe's website and attempts to download Adobe Reader, the Adobe Download Manager is downloaded.  With Internet Explorer, the download manager is implemented as an ActiveX Control, and it might initially be blocked by a Vital Security Web Appliance due to the fact that it performs network and disk operations. Given a typical configuration, this block will be logged, allowing the administrator to easily modify the policy and allow the control.

    Once it is installed, the Adobe Download Manager operates as a web client application. In other words, like a browser, it uses HTTP to retrieve the latest version of Adobe Reader from Adobe's website. Consequently, Vital Security Web Appliances will scan any downloads initiated by the Adobe Download Manager.

    In order to keep the connection with the web client alive while the download is being scanned, Vital Security Web Appliances "trickle" data to the download manager. Simply put, when Adobe Download Manager requests a file from Adobe's update site, the Vital Security Web Appliance will slowly send the site's response back to the download manager. This gives the Vital Security Web Appliance time to scan the requested download and verify that it is safe.  Once the appliance finishes scanning the download, the remaining content will immediately be sent to the download manager.

    Although most web clients accept this behavior, the Adobe Download Manager appears to require that files be downloaded at a faster rate. Therefore, even though the Vital Security Web Appliance successfully trickles some data, the download manager closes the connection before the scan is complete. No events will be logged on the Vital Security Appliance because the appliance did not block the download. Instead, the transaction was aborted by the Adobe Download Manager itself.  In this case, users might receive the following message:

    "The download has been interrupted. A network error occurred. Resume download?"



    In order for the Adobe Download Manager to operate properly, it must receive the requested file in real-time.  Therefore, scanning must be disabled for its downloads. Vital Security Web Appliances with VSOS 8.4.3 and Hot Fix M08 (or higher) allow administrators to disable scanning on specific sites by adding such URLs to the built-in Trusted Sites URL list. This list is invoked by the "Allow Trusted Sites" rule in the Default Security Policy.

    It is important that the rule that allows sites on the Trusted Sites list appears near the top of the policy, before any rules that would require in-depth scanning of the download. For example, in the Default Security Policy, the only rules that preceed "Allow Trusted Sites" are the "Allow Streaming" rule and two block rules that analyze the requested URL string. None of these rules require extensive scanning of the downloaded file.

    Since downloading the latest version of Acrobat Reader involves transactions with multiple adobe.com URLs, including www.adobe.com and ardownload.adobe.com, the simplest way to allow the download is to add the following entry to the Trusted Sites list:
    *.adobe.com/*

  • Software Version
    8.4.3

    • This article applies to:
    NG 1000
    NG 5000
    NG 6000
    NG 8000
    This article was previously published as:
    Finjan KB 1568

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