Symlink guard bypass in unarchive module allows planting symlinks during extraction

BBOT's unarchive module rejects archives containing symlink entries before extraction, but for zip and 7z archives it failed to detect symlinks whose listing carries a DOS-attribute prefix before the unix mode, as produced by legacy versions of p7zip. Such an archive, downloaded and extracted during a scan (for example via filedownload), bypassed the guard and caused an attacker-controlled symlink to be written into the extraction directory. The effect is limited to planting the symlink (its target is not written through), and only hosts using such a legacy p7zip build are affected; current mainline 7-Zip is not.