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CVE Record

CVE-2021-3594: An invalid pointer initialization issue was found in the SLiRP networking implementation of QEMU.

An invalid pointer initialization issue was found in the SLiRP networking implementation of QEMU. The flaw exists in the udp_input() function and could occur while processing a udp packet that is smaller than the size of the 'udphdr' structure. This issue may lead to out-of-bounds read access or indirect host memory disclosure to the guest. The highest threat from this vulnerability is to data confidentiality. This flaw affects libslirp versions prior to 4.6.0.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

CVE-2021-3594 is a QEMU/libslirp networking flaw that could let a guest VM indirectly read host memory. The cited record emphasizes confidentiality impact, not system takeover. Business urgency is highest for environments running QEMU with SLiRP/user-mode networking, especially where untrusted or multi-tenant guests exist.

Executive priority

Treat this as a targeted confidentiality risk for virtualization environments, not a broad internet-facing emergency based on the provided evidence. Patch in normal vulnerability cycles, faster for multi-tenant or untrusted guest hosts.

Technical view

The flaw is an invalid pointer initialization issue in libslirp's udp_input() handling. A UDP packet smaller than the udphdr structure can trigger out-of-bounds read access or indirect host memory disclosure to the guest. The bundle states libslirp versions before 4.6.0 are affected.

Likely exposure

Exposure is likely where QEMU relies on libslirp/SLiRP networking, including distro-packaged QEMU/libslirp installations referenced by Fedora, Gentoo, and Debian advisories. The bundle does not prove exposure for all QEMU deployments or non-SLiRP networking modes.

Exploitation context

The source bundle does not show CISA KEV listing or active exploitation evidence. The described attack context is a crafted small UDP packet processed by SLiRP, with potential host memory disclosure to a guest.

Researcher notes

Evidence is strongest for the root cause, affected libslirp versions before 4.6.0, and confidentiality impact. The bundle lacks CVSS scoring, exploit maturity details, and exact fixed downstream package versions, so validation should rely on vendor advisories.

Mitigation direction

  • Inventory QEMU and libslirp versions across virtualization hosts and images.
  • Update libslirp to 4.6.0 or later where vendor guidance confirms applicability.
  • Apply relevant Fedora, Gentoo, Debian, Red Hat, or NetApp security updates.
  • Prioritize hosts running untrusted guests or shared virtualization workloads.
  • Check vendor advisories for product-specific fixed package versions.

Validation and detection

  • Confirm installed libslirp packages are not older than the fixed vendor version.
  • Identify QEMU workloads using SLiRP or user-mode networking.
  • Review vendor advisory status for each affected operating system or appliance.
  • Document whether untrusted guest workloads can reach the vulnerable networking path.
  • Re-test package inventory after patching or image rebuilds.
Prepared
Confidence
medium
Sources
9

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

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CVE-2021-3594 mapping review

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
0Timeline events
0ADP providers
8Source links

CVSS and timeline data

No CVSS vectors or timeline events were available in the normalized CVE source material.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
n/aQEMUlibslirp 4.6.0Listed
Weakness

CWE details

CWE-824

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CWE-824 · source CWE mapping

Access of Uninitialized Pointer

Access of Uninitialized Pointer represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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