Xen: Ineffective Zenbleed workaround allows data leak on AMD CPUs
Summary
A regression in the Xen hypervisor's AMD-specific Zenbleed workaround (XSA-433) inverts a condition, so the workaround is skipped when it is needed. An attacker in a co-resident guest or on the same host could read data from other contexts on the same CPU core, such as AES-NI key material or REP-MOVS data. The issue only applies to AMD Zen2 systems running outdated microcode, according to the vendor advisory.
Affected
- Xen 4.19.4 and later, 4.20.3 and later, 4.21.1 and later, and 4.22
- Not affected: Xen 4.18, 4.19.3 and earlier, 4.20.2 and earlier, and 4.21.0
- Only AMD Zen2 systems with out-of-date microcode (per the vendor advisory)
- Systems with updated firmware or amd-ucode from linux-firmware are not vulnerable
Fixed in
- Upstream fixed Xen release: Unknown.
- Patch: xsa515.patch from XSA-515
- Debian trixie: 4.20.4+39-gf1e1321c87-0+deb13u1
- Alpine: 4.19.6-r2, 4.20.4-r2, 4.21.2-r2
- SUSE: SUSE-SU-2026-4942-1
Workaround
- Disable AVX: boot Xen with cpuid=no-avx on the command line, or set cpuid="host:avx=0" in the vm.cfg of all untrusted VMs
- The vendor advisory states this has a significant performance impact and is not recommended where microcode or patches can be deployed
- Changing AVX exposure is a guest-visible configuration change
Exploitation
- No evidence of exploitation in the wild found (KEV: No)
- No public proof of concept found; this is not the same as none existing
- EPSS is about 0.0035 (very low)
- CISA SSVC: exploitation none, automatable yes, technical impact total
Our recommendation
- Inventory Xen hosts, their versions and CPU vendor and generation
- Apply the vendor or distribution update to affected hosts
- Where patching is delayed on affected AMD Zen2 hosts, consider the AVX mitigation after checking guest impact
- Note that NVD rates this 9.8 while SUSE rates it 7.0 with a local vector; use your own exposure assessment
- No action needed for non-Xen hypervisors or non-AMD hosts
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Xen Security Advisory XSA-515 xenbits.xenproject.org
- Xen Security Advisory XSA-433 (Zenbleed) xenbits.xen.org
- Debian security tracker: xen security-tracker.debian.org
- Alpine OSV entry osv.dev
- SUSE advisory SUSE-SU-2026-4942-1 linuxsecurity.com
- Xen fix commit github.com
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 30 Oct 2026, 19:04.
Original description (NVD)
The workaround for XSA-433 (Zenbleed, CVE-2023-20593) was later re-worked to better integrate with other manipulations of the same MSR. In this rework an inverted conditional was introduced, resulting in the workaround to not be applied when needed and vice versa. For more details, see: http://xenbits.xen.org/xsa/advisory-433.html
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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