Adobe has patched its ubiquitous Acrobat Reader application against two bugs that could allow an attacker to take over a user's system via a malicious pdf file attached to an email message. The bugs affect Windows, Mac OS X and Unix.

Adobe released the fix in version 6.0.3 of both Acrobat and Acrobat Reader for Windows and Mac OS X. All the updates are available from Adobe's Web site.

Security research company iDefense warned of the bug affecting Windows and Mac in an advisory published on the Bugtraq mailing list late on Tuesday. The problem is a format string vulnerability in version 6.0.2 of Adobe Reader, allowing users to craft a special .etd file that could cause an invalid memory access and allow for the execution of malicious code with the privileges of the user. Reader uses .etd files in handling eBooks.

The bug could be exploited by an email containing either a malicious pdf file or a link to such a file, according to iDefense. The company said earlier versions of Reader 6 could be vulnerable, and said the bug is likely to also affect Adobe Acrobat, the application used to create pdf files.

IDefense said users could also work around the problem by deleting the file "C:\Program Files\Adobe\Acrobat 6.0\Reader\plug_ins\eBook.api", which makes Reader and Acrobat unable to handle eBooks.

A similar bug affects Unix. A boundary error in the "mailListIsPdf()" function, which checks to see whether a document in an email is a PDF file, unsafely copies user supplied data into a fixed sized buffer, according to iDefense.

This could allow an attacker to cause a buffer overflow and execute malicious code, the company said. Adobe has fixed the bug in Acrobat Reader version 5.0.9 for Unix, available on its site. iDefense said previous versions of Reader 5 are likely to also be affected. In its advisory, iDefense included a shell script patch users can apply for additional protection.

Separately, developers have warned of bugs in Ethereal (a popular network protocol analyser) that could allow an attacker to take over a system.

Ethereal claims to be one of the most popular tools for network software and protocol development, troubleshooting and analysis. The bugs can make the application hang, crash or otherwise disrupt a system, and may also allow for malicious code execution, Ethereal's developers said.

"It may be possible to make Ethereal crash or run arbitrary code by injecting a purposefully malformed packet onto the wire or by convincing someone to read a malformed packet trace file," the project said in a Wednesday advisory.

The bugs affect versions 0.9.0 up to and including 0.10.7, and are fixed in version 0.10.8. Secunia, which publishes an independent security database, said the problems were "highly critical."