CVE-2020-16120 Unprivileged overlay + shiftfs read access
Unprivileged users can read restricted files via overlayfs copy-up in user namespaces, requiring unprivileged user namespaces to be enabled.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2020-16120?
CVE-2020-16120 is a medium-severity vulnerability in Canonical Ubuntu Linux, published on 10 Feb 2021. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
Overlayfs did not properly perform permission checking when copying up files in an overlayfs and could be exploited from within a user namespace, if, for example, unprivileged user namespaces were allowed. It was possible to have a file not readable by an unprivileged user to be copied to a mountpoint controlled by the user, like a removable device. This was introduced in kernel version 4.19 by commit d1d04ef ("ovl: stack file ops"). This was fixed in kernel version 5.8 by commits 56230d9 ("ovl: verify permissions in ovl_path_open()"), 48bd024 ("ovl: switch to mounter creds in readdir") and 05acefb ("ovl: check permission to open real file"). Additionally, commits 130fdbc ("ovl: pass correct flags for opening real directory") and 292f902 ("ovl: call secutiry hook in ovl_real_ioctl()") in kernel 5.8 might also be desired or necessary. These additional commits introduced a regression in overlay mounts within user namespaces which prevented access to files with ownership outside of the user namespace. This regression was mitigated by subsequent commit b6650da ("ovl: do not fail because of O_NOATIMEi") in kernel 5.11.
Weakness: CWE-266
Which versions are affected by CVE-2020-16120?
It affects 2 products.
Products include Canonical Ubuntu Linux, Linux kernel.
How do I fix CVE-2020-16120?
Apply the vendor's patch.
Is CVE-2020-16120 being exploited?
There is no known exploitation of CVE-2020-16120 in the wild and no public exploit code.
Other vulnerabilities in Canonical Ubuntu Linux
Sources
Also known as AZL-6522, DEBIAN-CVE-2020-16120, GHSA-q6jw-34cj-733v, UBUNTU-CVE-2020-16120.