Until the NFS component of kernel 2.6.9 was released on 19 October, 2004, Sophos Anti-Virus for Linux was unable to intercept files on NFS servers using kernels 2.6 to 2.6.8 in kernel mode. This issue only affects On-Access scanning.
What to do
Please upgrade your operating system to use the NFS component of kernel 2.6.9 or above.
Until you upgrade, you can make Sophos Anti-Virus check these files by using a scheduled/on-demand scan.
This problem was caused by incomplete implementation of filesystem objects in the 2.6.8 and earlier 2.6 series kernels. The kernels do not provide the Sophos on-access scanner with sufficient information to be able to scan the file, and therefore it is skipped.
Note that SUSE Linux is unaffected because SUSE have backported a fix to their 2.6.8 kernels.