LiveActive security incident?Get immediate response
CVE Record

CVE-2019-15791: Reference count underflow in shiftfs

In shiftfs, a non-upstream patch to the Linux kernel included in the Ubuntu 5.0 and 5.3 kernel series, shiftfs_btrfs_ioctl_fd_replace() installs an fd referencing a file from the lower filesystem without taking an additional reference to that file. After the btrfs ioctl completes this fd is closed, which then puts a reference to that file, leading to a refcount underflow.

HighCVSS 7.1Not KEV-listedUpdated
Discuss exposureBack to CVEs
Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

This Ubuntu-specific kernel issue can let a local low-privileged user disrupt or corrupt kernel-managed state on affected Ubuntu 5.0 or 5.3 kernel series systems. It is not described as remotely reachable, and the bundle does not show active exploitation.

Executive priority

Prioritize remediation for affected Ubuntu servers where multiple users or workloads can execute locally. The business risk is service disruption or system integrity impact, while remote exposure is not supported by the provided sources.

Technical view

shiftfs_btrfs_ioctl_fd_replace() installs a file descriptor for a lower-filesystem file without taking an extra reference. When the btrfs ioctl completes, closing that fd drops a reference that was never acquired, causing a reference count underflow. CVSS rates integrity and availability impact as high.

Likely exposure

Exposure is most likely on Ubuntu systems running the affected Ubuntu 5.0 or 5.3 kernel series with shiftfs present. The attack vector is local and requires low privileges, so systems with untrusted local users deserve priority review.

Exploitation context

The source bundle marks KEV as false and provides no cited evidence of active exploitation. Treat this as a local privilege-context kernel vulnerability with potentially serious integrity and availability impact, not as a confirmed internet-scale exploited issue.

Researcher notes

Evidence supports CWE-672-style resource lifetime mishandling in Ubuntu’s non-upstream shiftfs patch. The key condition is a missing file reference before fd installation and a later close that decrements the reference count. The bundle does not provide exploit primitives or fixed package versions.

Mitigation direction

  • Review Ubuntu USN-4183-1 and USN-4184-1 for vendor-approved kernel updates.
  • Apply the relevant Ubuntu kernel security updates for affected systems.
  • Reboot systems after kernel update so the fixed kernel is active.
  • Restrict untrusted local access until affected kernels are updated.
  • Monitor Ubuntu guidance for any additional affected kernel packages.

Validation and detection

  • Inventory Ubuntu hosts running 5.0 or 5.3 kernel series.
  • Confirm whether shiftfs is present on those systems.
  • Compare installed kernel packages against the referenced Ubuntu Security Notices.
  • After patching, verify the running kernel changed, not just the installed package.
  • Prioritize validation on shared hosts with low-privileged user access.
Prepared
Confidence
high
Sources
5

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

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · low confidence lookup

CWE-672: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cve · low confidence lookup

CVE-2019-15791 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

Open ATT&CK lookup
Vulnerability profileCVE Program record
Severity
High
CVSS
7.1 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H

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.

1CVSS vectors
0Timeline events
0ADP providers
4Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.1CVSS 3.1HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H1.85.2Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

7.1High
CVSS 3.1 vector shape for CVE-2019-15791Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
UbuntuShiftfs in the Linux kernel5.3.0-11.12, 5.0 kernelListed
Weakness

CWE details

CWE-672

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-672 · source CWE mapping

Operation on a Resource after Expiration or Release

Operation on a Resource after Expiration or Release represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs