Evidence used
- No CISA KEV confirmation is currently recorded.
- Exploitation requires an existing local or physical foothold with privileges.
- EPSS is 0.36% for the current model date.
BlackTreeCVE IntelligenceFull vulnerability report · 2021
CVE-2021-28689High confidence
x86: Speculative vulnerabilities with bare (non-shim) 32-bit PV guests 32-bit x86 PV guest kernels run in ring 1
Xen · Xen
5.5MediumCVSS 3.1
Scheduled
Fix not verifiedMedium technical severity with no CISA KEV confirmation; remediate through the normal risk-based patch cycle unless local exposure raises the priority.
R
Open reassessment dashboard →Operational reassessment
Published severity in operational context
Published severityMedium→Operational priority:Low, lowered one band.downgradedsince 3 Aug 2024
Compensating controls
- Restrict local access and enforce least privilege on affected hosts.
- Monitor vendor guidance and exploitation sources for a material change.
Verification
- Confirm that the asset runs Xen Xen and falls inside the recorded affected range.
- Recheck the vendor advisory before scheduling a change because no verified fixed version is currently retained.
- Validate exposure, authentication requirements and compensating controls in the actual environment.
- Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Distribution package intelligence
5 package statesRelease-specific package status
Debian, ubuntu findings are scoped to the named distribution, release and source package. An absent finding does not mean a package is unaffected.
Repository candidate not checked
A published vendor fix does not prove that a matching update is enabled and installable on a particular asset. Confirm the local package candidate before scheduling remediation.
| Distribution release | Source package | Vendor state | Fixed version | Evidence |
|---|---|---|---|---|
| Debian trixietrixie · source | xen | Affected, no fix publishedDebian currently tracks this release as open. | Not published in this feed | Debian Security Tracker ↗Source updated 5 Oct 2026 |
| Debian bookwormbookworm · source | xen | Affected, no fix publishedDebian currently tracks this release as open. | Not published in this feed | Debian Security Tracker ↗Source updated 5 Oct 2026 |
| Debian forkyforky · source | xen | Affected, no fix publishedDebian currently tracks this release as open. | Not published in this feed | Debian Security Tracker ↗Source updated 5 Oct 2026 |
| Debian sidsid · source | xen | Affected, no fix publishedDebian currently tracks this release as open. | Not published in this feed | Debian Security Tracker ↗Source updated 5 Oct 2026 |
| Ubuntu 24.04 LTSnoble · standard archive | xen | Under evaluationCanonical reports that the package might be affected and still needs evaluation or fixing. | Not published in this feed | Canonical Ubuntu Security ↗Source updated 5 Oct 2026 |
Medium technical severity with no CISA KEV confirmation; remediate through the normal risk-based patch cycle unless local exposure raises the priority.
Fix not verified01
What, why and how
x86: Speculative vulnerabilities with bare (non-shim) 32-bit PV guests 32-bit x86 PV guest kernels run in ring 1. At the time when Xen was developed, this area of the i386 architecture was rarely used, which is why Xen was able to use it to implement paravirtualisation, Xen's novel approach to virtualization. In AMD64, Xen had to use a different implementation approach, so Xen does not use ring 1 to support 64-bit guests. With the focus now being on 64-bit systems, and the availability of explicit hardware support for virtualization, fixing speculation issues in ring 1 is not a priority for processor companies. Indirect Branch Restricted Speculation (IBRS) is an architectural x86 extension put together to combat speculative execution sidechannel attacks, including Spectre v2. It was retrofitted in microcode to existing CPUs. For more details on Spectre v2, see: http://xenbits.xen.org/xsa/advisory-254.html However, IBRS does not architecturally protect ring 0 from predictions learnt in ring 1. For more details, see: https://software.intel.com/security-software-guidance/deep-dives/deep-dive-indirect-branch-restricted-speculation Similar situations may exist with other mitigations for other kinds of speculative execution attacks. The situation is quite likely to be similar for speculative execution attacks which have yet to be discovered, disclosed, or mitigated.
What
x86: Speculative vulnerabilities with bare (non-shim) 32-bit PV guests 32-bit x86 PV guest kernels run in ring 1. At the time when Xen was developed, this area of the i386 architecture was rarely used, which is why Xen was able to use it to implement paravirtualisation, Xen's novel approach to virtualization. In AMD64, Xen had to use a different implementation approach, so Xen does not use ring 1 to support 64-bit guests. With the focus now being on 64-bit systems, and the availability of explicit hardware support for virtualization, fixing speculation issues in ring 1 is not a priority for processor companies. Indirect Branch Restricted Speculation (IBRS) is an architectural x86 extension put together to combat speculative execution sidechannel attacks, including Spectre v2. It was retrofitted in microcode to existing CPUs. For more details on Spectre v2, see: http://xenbits.xen.org/xsa/advisory-254.html However, IBRS does not architecturally protect ring 0 from predictions learnt in ring 1. For more details, see: https://software.intel.com/security-software-guidance/deep-dives/deep-dive-indirect-branch-restricted-speculation Similar situations may exist with other mitigations for other kinds of speculative execution attacks. The situation is quite likely to be similar for speculative execution attacks which have yet to be discovered, disclosed, or mitigated.
Why
The product stores, transfers, or shares a resource that contains sensitive information, but it does not properly remove that information before the product makes the resource available to unauthorized actors.
How
An attacker operating through local access may attempt exploitation with low privileges. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.
What
x86: Speculative vulnerabilities with bare (non-shim) 32-bit PV guests 32-bit x86 PV guest kernels run in ring 1. At the time when Xen was developed, this area of the i386 architecture was rarely used, which is why Xen was able to use it to implement paravirtualisation, Xen's novel approach to virtualization. In AMD64, Xen had to use a different implementation approach, so Xen does not use ring 1 to support 64-bit guests. With the focus now being on 64-bit systems, and the availability of explicit hardware support for virtualization, fixing speculation issues in ring 1 is not a priority for processor companies. Indirect Branch Restricted Speculation (IBRS) is an architectural x86 extension put together to combat speculative execution sidechannel attacks, including Spectre v2. It was retrofitted in microcode to existing CPUs. For more details on Spectre v2, see: http://xenbits.xen.org/xsa/advisory-254.html However, IBRS does not architecturally protect ring 0 from predictions learnt in ring 1. For more details, see: https://software.intel.com/security-software-guidance/deep-dives/deep-dive-indirect-branch-restricted-speculation Similar situations may exist with other mitigations for other kinds of speculative execution attacks. The situation is quite likely to be similar for speculative execution attacks which have yet to be discovered, disclosed, or mitigated.
Why
The product stores, transfers, or shares a resource that contains sensitive information, but it does not properly remove that information before the product makes the resource available to unauthorized actors.
How
An attacker operating through local access may attempt exploitation with low privileges. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.
02
Exploit reality and attack path
CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.
Observed exploitation No confirmed evidence
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.No CISA KEV match was present at the last successful refresh. This means no confirmation from that source, not proof of no exploitation.
Public PoC / exploit material None recorded
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.No exploit-tagged reference or CISA SSVC proof-of-concept state is currently recorded. Research may still exist outside the structured feeds.
- Likely attack path
- local access → Improper Removal of Sensitive Information Before Storage or Transfer → cause the confidentiality, integrity or availability impact described by the vendor
- Attack surface
- Local
- Privileges required
- Low: a basic authenticated account is required
- User interaction
- None
- Attack complexity
- Low: no specialised conditions are recorded
- Security boundary
- Unchanged: impact remains within the vulnerable component's security authority
- Weakness
?
CWE means Common Weakness Enumeration: a standard category for the underlying weakness. - CWE-212 ↗
CWE-212: Improper Removal of Sensitive Information Before Storage or Transfer. The product stores, transfers, or shares a resource that contains sensitive information, but it does not properly remove that information before the product makes the resource available to unauthorized actors.
- CVSS vector
?
CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity. CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:NCommon Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.
AVLocalAttack vector: The attacker needs local access to the vulnerable system.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.PRLowPrivileges required: The attacker needs basic user-level privileges.UINoneUser interaction: No action by another user is required.SUnchangedScope: The security impact remains within the vulnerable component's authority.CHighConfidentiality impact: A successful attack can cause a major loss.INoneIntegrity impact: No direct loss is represented by this metric.ANoneAvailability impact: No direct loss is represented by this metric.- Post-exploitation / living off the land
- The issue can support a local privilege or sandbox boundary transition; normal system utilities may then be available in the gained context.
03
Patch and workaround
Operational remediation based on structured source evidence.
- Status
?
Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change. - Fix not verified
- Affected
- Xen: unspecified < 4.12
- Fixed
- No fixed version is explicitly recorded in the structured CVE data.
- Action
- No verified patch reference is present in the current structured sources. Check the vendor advisory before making a change.
- Workaround
- No verified workaround is recorded. Limit untrusted access and use least privilege until authoritative guidance is available.
04
Evidence and provenance
Published 11 Jun 2021 · Last source change 3 Aug 2024, 21:47 UTC · CWE-212 · Improper Removal of Sensitive Information Before Storage or Transfer
CVE recordCVE.org · 5.1
CVSS sourceNIST NVD
EPSS source FIRST · tracked since 2026-08-14
?
The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.European sourceENISA EUVD · EUVD-2021-15346
Product sourceCNA
Remediation sourceCVE/CNA references
CWE sourceNIST NVD
NVD statusNVD modified after enrichment
Core structured fields are present and their contributing authorities are shown above.
Material change intelligence
View recent updates ↗What changed after publication
No material field changes have been recorded since change tracking began. Routine source refreshes and cosmetic edits are intentionally excluded.
Technical terms and abbreviations used in this report
- CVE
- Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
- CVSS
- Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
- EPSS
- Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
- CWE
- Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
- CNA
- CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
- CISA ADP
- Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
- NVD
- National Vulnerability Database: NIST's enrichment service for CVE records.
- CERT / CSIRT
- A computer security incident response team that publishes warnings or coordinates incident response.
- PoC
- Proof of concept: public material that demonstrates or helps reproduce exploitation.
- CSAF
- Common Security Advisory Framework: a machine-readable format for security advisories.
- LoTL
- Living off the land: abuse of legitimate tools or system functions during an attack.