CyberSecBulletin
CVE-2025-39821NVD status: Modified

Linux: Out-of-bounds Write

Published Updated By the CyberSecBulletin desk

CVE-2025-39821 is a high-severity out-of-bounds write vulnerability in Linux, with a CVSS 3.1 base score of 7.8 out of 10. It can be exploited with local access to the system by an attacker with a low-privileged account, without any action from a user. The record lists versions d689135aa9c5e4e0eab5a92bbe35dab0c8d6677f, b64fdd422a85025b5e91ead794db9d3ef970e369 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 0.16% chance of exploitation within 30 days, which is low compared with other CVEs.

Next patch cycle

High severity with no evidence of exploitation so far.

What to do

  1. Update Linux to d689135aa9c5e4e0eab5a92bbe35dab0c8d6677f or b64fdd422a85025b5e91ead794db9d3ef970e369, whichever matches your release line, or a later version.
  2. Update Linux Kernel to 6.16.5 or a later version.

Affected products and versions

VendorProductAffected versionsFixed in
LinuxLinux9734e25fbf5ae68eb04234b2cd14a4b36ab89141 up to, but not including, d689135aa9c5e4e0eab5a92bbe35dab0c8d6677f; 9734e25fbf5ae68eb04234b2cd14a4b36ab89141 up to, but not including, b64fdd422a85025b5e91ead794db9d3ef970e369; 6.16d689135aa9c5e4e0eab5a92bbe35dab0c8d6677f, b64fdd422a85025b5e91ead794db9d3ef970e369
LinuxLinux Kernel6.16 up to, but not including, 6.16.5; 6.176.16.5

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
LowA basic user account is enough.
User interaction
NoneNo action by a victim is required.
Scope
UnchangedImpact stays within the vulnerable component.
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:L/UI:N/S:U/C:H/I:H/A:H. Scored by NVD.

Scoring systemScoreSeverityScored by
CVSS 3.17.8HighNVD
CVSS 3.17.8HighCISA (ADP)

Official description

In the Linux kernel, the following vulnerability has been resolved: perf: Avoid undefined behavior from stopping/starting inactive events Calling pmu->start()/stop() on perf events in PERF_EVENT_STATE_OFF can leave event->hw.idx at -1. When PMU drivers later attempt to use this negative index as a shift exponent in bitwise operations, it leads to UBSAN shift-out-of-bounds reports. The issue is a logical flaw in how event groups handle throttling when some members are intentionally disabled. Based on the analysis and the reproducer provided by Mark Rutland (this issue on both arm64 and x86-64). The scenario unfolds as follows: 1. A group leader event is configured with a very aggressive sampling period (e.g., sample_period = 1). This causes frequent interrupts and triggers the throttling mechanism. 2. A child event in the same group is created in a disabled state (.disabled = 1). This event remains in PERF_EVENT_STATE_OFF. Since it hasn't been scheduled onto the PMU, its event->hw.idx remains initialized at -1. 3. When throttling occurs, perf_event_throttle_group() and later perf_event_unthrottle_group() iterate through all siblings, including the disabled child event. 4. perf_event_throttle()/unthrottle() are called on this inactive child event, which then call event->pmu->start()/stop(). 5. The PMU driver receives the event with hw.idx == -1 and attempts to use it as a shift exponent. e.g., in macros like PMCNTENSET(idx), leading to the UBSAN report. The throttling mechanism attempts to start/stop events that are not actively scheduled on the hardware. Move the state check into perf_event_throttle()/perf_event_unthrottle() so that inactive events are skipped entirely. This ensures only active events with a valid hw.idx are processed, preventing undefined behavior and silencing UBSAN warnings. The corrected check ensures true before proceeding with PMU operations. The problem can be reproduced with the syzkaller reproducer:

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

The underlying weakness: Out-of-bounds Write

The product writes data past the end, or before the beginning, of the intended buffer.

How developers prevent this class of flaw

  • Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, many languages that perform their own memory management, such as Java and Perl, are not subject to buffer overflows. Other languages, such as Ada and C#, typically provide overflow protection, but the protection can be disabled by the programmer. Be wary that a language's interface to native code may still be subject to overflows, even if the language itself is theoretically safe.
  • Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. Examples include the Safe C String Library (SafeStr) by Messier and Viega, and the Strsafe.h library from Microsoft. These libraries provide safer versions of overflow-prone string-handling functions.
  • 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 Out-of-bounds Write vulnerabilities (CWE-787)

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 Out-of-bounds Write vulnerabilities

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