Microsoft has bundled five critical bulletins into a mammoth April Patch Tuesday release.

The release contains a total of eight bulletins, a few of which address vulnerabilities already under attack. Among these are critical bulletins affecting Microsoft Office Excel and text converters for WordPad and Office.

MS09-09 touches on two memory corruption vulnerabilities in Excel tied to the way the program parses the Excel spreadsheet file format. One of these flaws is already on the radar of hackers and is being targeted by a Trojan making the rounds on the Web. Both of these flaws affect multiple versions of the product but are only rated "critical" for Excel 2000. The bulletin's severity is lowered to "important" for other Excel versions because later editions generate prompts that require additional user interaction for the exploits to work.

The bulletin covering the WordPad and Office text converters touches on four issues, including two bugs Microsoft says are being targeted in the wild. According to Microsoft, hackers have been targeting a vulnerability in the way the text converters in WordPad and Office process memory when a user opens a specially crafted Word 6 file with malformed data. Attackers have also had their eyes on a vulnerability in the way WordPad processes memory when parsing specially crafted Word 97 documents.

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