# CVE-2021-28687: Null dereference in xen > A guest VM can trigger a crash in the Xen toolstack during soft reboot due to uninitialized data, causing denial of service. - **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`) - **Exploited in the wild:** Not known - **Login needed:** Any signed-in user - **Attackable:** Local or same network only - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 11 Jun 2021 - **Updated:** 3 Aug 2024 - **Also known as:** ALPINE-CVE-2021-28687, DEBIAN-CVE-2021-28687, GHSA-f8g3-gxq4-xqv7 ## What is CVE-2021-28687? CVE-2021-28687 is a medium-severity vulnerability in Xen Xen, published on 11 Jun 2021. It can only be exploited from the same machine or local network, with no action from the victim. ### Official description HVM soft-reset crashes toolstack libxl requires all data structures passed across its public interface to be initialized before use and disposed of afterwards by calling a specific set of functions. Many internal data structures also require this initialize / dispose discipline, but not all of them. When the "soft reset" feature was implemented, the libxl__domain_suspend_state structure didn't require any initialization or disposal. At some point later, an initialization function was introduced for the structure; but the "soft reset" path wasn't refactored to call the initialization function. When a guest nwo initiates a "soft reboot", uninitialized data structure leads to an assert() when later code finds the structure in an unexpected state. The effect of this is to crash the process monitoring the guest. How this affects the system depends on the structure of the toolstack. For xl, this will have no security-relevant effect: every VM has its own independent monitoring process, which contains no state. The domain in question will hang in a crashed state, but can be destroyed by `xl destroy` just like any other non-cooperating domain. For daemon-based toolstacks linked against libxl, such as libvirt, this will crash the toolstack, losing the state of any in-progress operations (localized DoS), and preventing further administrator operations unless the daemon is configured to restart automatically (system-wide DoS). If crashes "leak" resources, then repeated crashes could use up resources, also causing a system-wide DoS. Weakness: CWE-909 (Missing Initialization of Resource) ## Which versions are affected by CVE-2021-28687? It affects 1 product. Products: Xen Xen. ## How do I fix CVE-2021-28687? Apply the vendor's patch. ## Is CVE-2021-28687 being exploited? There is no known exploitation of CVE-2021-28687 in the wild and no public exploit code. ## Other vulnerabilities in Xen Xen - [CVE-2017-10912](https://insights.sbomify.com/CVE-2017-10912/): Privilege escalation in n/a (critical 10.0) - [CVE-2017-10918](https://insights.sbomify.com/CVE-2017-10918/): Memory corruption in n/a (critical 10.0) - [CVE-2017-10920](https://insights.sbomify.com/CVE-2017-10920/): Memory corruption in n/a (critical 10.0) - [CVE-2017-10921](https://insights.sbomify.com/CVE-2017-10921/): Memory corruption in n/a (critical 10.0) - [CVE-2025-27466](https://insights.sbomify.com/CVE-2025-27466/): Mutiple vulnerabilities in the Viridian interface (critical 9.8) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2021-28687 and fix it if it is affected. What it is: CVE-2021-28687 is a medium-severity vulnerability in Xen Xen, published on 11 Jun 2021. It can only be exploited from the same machine or local network, with no action from the victim. Exploitation: There is no known exploitation of CVE-2021-28687 in the wild and no public exploit code. Affected product: Xen Xen. No package-level version data is available, so check the vendor advisory linked from the reference page. Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2021-28687/ (Markdown: https://insights.sbomify.com/CVE-2021-28687/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2021-28687 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-28687 - OSV: https://osv.dev/vulnerability/CVE-2021-28687 --- From sbomify Insights: https://insights.sbomify.com/CVE-2021-28687/ Summaries are generated and may contain mistakes; the official description above is the record.