CyberSecBulletin
CVE-2026-63939NVD status: Analyzed

Linux: Heap-based Buffer Overflow

Published Updated By the CyberSecBulletin desk

CVE-2026-63939 is a critical-severity heap-based buffer overflow vulnerability in Linux, with a CVSS 3.1 base score of 9.3 out of 10. It can be exploited with local access to the system by an unauthenticated attacker, without any action from a user. The record lists versions 6ca9400d36005ffdca25f80186bea781c7e1dc4c, 9f0a9e780f02c02d025a190f1885e1d1d73b87bd, 6644565527c4c5f507088b1c9ddf72de47790b68 as the first without the flaw, so updating Linux to one of those releases or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.2% chance of exploitation within 30 days, which is low compared with other CVEs.

Patch this week

Critical severity: a successful attack would have severe impact.

What to do

  1. Update Linux to 6ca9400d36005ffdca25f80186bea781c7e1dc4c or 9f0a9e780f02c02d025a190f1885e1d1d73b87bd or 6644565527c4c5f507088b1c9ddf72de47790b68 or 5867d7e202e09f037cefe77f7af4413c7c0fa088, whichever matches your release line, or a later version.
  2. Update Linux Kernel to 6.12.93 or 6.18.35 or 7.0.12, whichever matches your release line, or a later version.

Affected products and versions

VendorProductAffected versionsFixed in
LinuxLinux9b54e248d2644be71cb394eb85f31ad99e023a05 up to, but not including, 6ca9400d36005ffdca25f80186bea781c7e1dc4c; 9b54e248d2644be71cb394eb85f31ad99e023a05 up to, but not including, 9f0a9e780f02c02d025a190f1885e1d1d73b87bd; 9b54e248d2644be71cb394eb85f31ad99e023a05 up to, but not including, 6644565527c4c5f507088b1c9ddf72de47790b68; 9b54e248d2644be71cb394eb85f31ad99e023a05 up to, but not including, 5867d7e202e09f037cefe77f7af4413c7c0fa088; 6.116ca9400d36005ffdca25f80186bea781c7e1dc4c, 9f0a9e780f02c02d025a190f1885e1d1d73b87bd, 6644565527c4c5f507088b1c9ddf72de47790b68, 5867d7e202e09f037cefe77f7af4413c7c0fa088
LinuxLinux Kernel6.11 up to, but not including, 6.12.93; 6.13 up to, but not including, 6.18.35; 6.19 up to, but not including, 7.0.12; 7.16.12.93, 6.18.35, 7.0.12

Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.

How the severity score breaks down

Attack vector
LocalThe attacker needs local access or must trick a user into running something.
Attack complexity
LowNo special conditions are needed; the attack works reliably.
Privileges required
NoneNo account or login is needed.
User interaction
NoneNo action by a victim is required.
Scope
ChangedThe attack can reach beyond the vulnerable component into other systems.
Confidentiality
HighAll data handled by the component can be read.
Integrity
HighData or code can be modified at will.
Availability
HighThe component can be taken down completely.

Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H. Scored by CNA.

Official description

In the Linux kernel, the following vulnerability has been resolved: KVM: SEV: Compute the correct max length of the in-GHCB scratch area When setting the length of the GHCB scratch area, and the area is in the GHCB shared buffer, set the effective length of the scratch area to the max possible size given the start of the guest-provided pointer, and the end of the shared buffer. The code was "fine" when first introduced, as KVM doesn't consult the length of the buffer when emulating MMIO, because the passed in @len always specifies the *max* size required. But for PSC requests, the incoming @len is just the minimum length (to process the header), and KVM needs to know the full size of the scratch area to avoid buffer overflows (spoiler alert). Opportunistically rename @len => @min_len to better reflect its role.

Source: the CVE record as published by its numbering authority, via NVD and CVE.org.

The underlying weakness: Heap-based Buffer Overflow

The product writes past the end of a buffer that was allocated on the heap.

How developers prevent this class of flaw

  • Pre-design: Use a language or compiler that performs automatic bounds checking.
  • Use an abstraction library to abstract away risky APIs. Not a complete solution.
  • Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking. D3-SFCV (Stack Frame Canary Validation) from D3FEND discusses canary-based detection in detail.

More Heap-based Buffer Overflow vulnerabilities (CWE-122)

Timeline

  1. CVE record published.
  2. CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: total).
  3. Record last changed materially (score, affected products, status or references).

References

Recent Heap-based Buffer Overflow vulnerabilities

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