CVE-2021-47555

MEDIUMNVD 5.55.5—
EchelonGraph scoreMEDIUM confidence

This medium-severity CVE scores 5.5 under NVD CVSS v3. EPSS exploit probability: 0.2%, top 89% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
5.5
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS: 0%CVSS: 5.5Exploit: NoneExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

In the Linux kernel, the following vulnerability has been resolved:

net: vlan: fix underflow for the real_dev refcnt

Inject error before dev_hold(real_dev) in register_vlan_dev(), and execute the following testcase:

ip link add dev dummy1 type dummy ip link add name dummy1.100 link dummy1 type vlan id 100 ip link del dev dummy1

When the dummy netdevice is removed, we will get a WARNING as following:

======================================================================= refcount_t: decrement hit 0; leaking memory. WARNING: CPU: 2 PID: 0 at lib/refcount.c:31 refcount_warn_saturate+0xbf/0x1e0

and an endless loop of:

======================================================================= unregister_netdevice: waiting for dummy1 to become free. Usage count = -1073741824

That is because dev_put(real_dev) in vlan_dev_free() be called without dev_hold(real_dev) in register_vlan_dev(). It makes the refcnt of real_dev underflow.

Move the dev_hold(real_dev) to vlan_dev_init() which is the call-back of ndo_init(). That makes dev_hold() and dev_put() for vlan's real_dev symmetrical.

CVSS v3
5.5
EG Score
5.5(medium)
EG Risk
—
EPSS
11.1%
KEV
Not listed

Published

May 24, 2024

Last Modified

September 18, 2025

References (8)

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 11× in last 7d / 49× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-07-22 22:53 UTCEG score recompute
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Frequently asked(5)

What is CVE-2021-47555?
CVE-2021-47555 is a medium vulnerability published on May 24, 2024. In the Linux kernel, the following vulnerability has been resolved: net: vlan: fix underflow for the real_dev refcnt Inject error before devhold(realdev) in registervlandev(), and execute the following testcase: ip link add dev dummy1 type dummy ip link add name dummy1.100 link dummy1 type vlan id…
When was CVE-2021-47555 disclosed?
CVE-2021-47555 was first published in the National Vulnerability Database on May 24, 2024, with the most recent update on September 18, 2025. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-47555 actively exploited?
CVE-2021-47555 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 11.1% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2021-47555?
CVE-2021-47555 has a CVSS v3 base score of 5.5 (NVD).
How do I remediate CVE-2021-47555?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-47555, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

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