CVE-2019-18421 Race condition in n/a

What is CVE-2019-18421?

CVE-2019-18421 is a high-severity vulnerability in Debian Debian Linux, published on 31 Oct 2019. Any signed-in user can exploit it over the network, with no action from the victim.

Official description

An issue was discovered in Xen through 4.12.x allowing x86 PV guest OS users to gain host OS privileges by leveraging race conditions in pagetable promotion and demotion operations. There are issues with restartable PV type change operations. To avoid using shadow pagetables for PV guests, Xen exposes the actual hardware pagetables to the guest. In order to prevent the guest from modifying these page tables directly, Xen keeps track of how pages are used using a type system; pages must be "promoted" before being used as a pagetable, and "demoted" before being used for any other type. Xen also allows for "recursive" promotions: i.e., an operating system promoting a page to an L4 pagetable may end up causing pages to be promoted to L3s, which may in turn cause pages to be promoted to L2s, and so on. These operations may take an arbitrarily large amount of time, and so must be re-startable. Unfortunately, making recursive pagetable promotion and demotion operations restartable is incredibly complicated, and the code contains several races which, if triggered, can cause Xen to drop or retain extra type counts, potentially allowing guests to get write access to in-use pagetables. A malicious PV guest administrator may be able to escalate their privilege to that of the host. All x86 systems with untrusted PV guests are vulnerable. HVM and PVH guests cannot exercise this vulnerability.

Weakness: CWE-362

Which versions are affected by CVE-2019-18421?

It affects 4 products.

Products include Debian Debian Linux, Fedoraproject Fedora, Opensuse Leap, Xen Xen.

How do I fix CVE-2019-18421?

Apply the vendor's patch.

Is CVE-2019-18421 being exploited?

There is no known exploitation of CVE-2019-18421 in the wild and no public exploit code.

Other vulnerabilities in Debian Debian Linux

• CVE-2015-3306 ProFTPD Improper Access Control Exploited Critical 10.0
• CVE-2025-49113 RoundCube Webmail Deserialization of Untrusted Data Exploited Critical 9.9
• CVE-2026-24061 GNU InetUtils Argument Injection Exploited Critical 9.8
• CVE-2025-39682 Linux Kernel Improper Check for Unusual or Exceptional Conditions Exploited Critical 9.8
• CVE-2024-9680 Mozilla Firefox Use-After-Free Exploited Critical 9.8

All Debian Debian Linux vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as ALPINE-CVE-2019-18421, DEBIAN-CVE-2019-18421, GHSA-7g44-3rf3-p4xf, UBUNTU-CVE-2019-18421.