CyberSecBulletin
CVE-2025-21991NVD status: Modified

Linux: Improper Validation of Array Index

Published Updated By the CyberSecBulletin desk

CVE-2025-21991 is a high-severity improper validation of array index 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 d509c4731090ebd9bbdb72c70a2d70003ae81f4f, 985a536e04bbfffb1770df43c6470f635a6b1073, 18b5d857c6496b78ead2fd10001b81ae32d30cac 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 0.22% 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 d509c4731090ebd9bbdb72c70a2d70003ae81f4f or 985a536e04bbfffb1770df43c6470f635a6b1073 or 18b5d857c6496b78ead2fd10001b81ae32d30cac or ec52240622c4d218d0240079b7c1d3ec2328a9f4, whichever matches your release line, or a later version.
  2. Update Linux Kernel to 4.15 or 4.20 or 5.5 or 5.11, whichever matches your release line, or a later version.

Affected products and versions

VendorProductAffected versionsFixed in
LinuxLinux979e197968a1e8f09bf0d706801dba4432f85ab3 up to, but not including, d509c4731090ebd9bbdb72c70a2d70003ae81f4f; 44a44b57e88f311c1415be1f567c50050913c149 up to, but not including, 985a536e04bbfffb1770df43c6470f635a6b1073; be2710deaed3ab1402379a2ede30a3754fe6767a up to, but not including, 18b5d857c6496b78ead2fd10001b81ae32d30cac; d576547f489c935b9897d4acf8beee3325dea8a5 up to, but not including, ec52240622c4d218d0240079b7c1d3ec2328a9f4; 7ff6edf4fef38ab404ee7861f257e28eaaeed35f up to, but not including, e686349cc19e800dac8971929089ba5ff59abfb0; 7ff6edf4fef38ab404ee7861f257e28eaaeed35f up to, but not including, 488ffc0cac38f203979f83634236ee53251ce593; 7ff6edf4fef38ab404ee7861f257e28eaaeed35f up to, but not including, 5ac295dfccb5b015493f86694fa13a0dde4d3665; 7ff6edf4fef38ab404ee7861f257e28eaaeed35f up to, but not including, e3e89178a9f4a80092578af3ff3c8478f9187d59; and 11 more rangesd509c4731090ebd9bbdb72c70a2d70003ae81f4f, 985a536e04bbfffb1770df43c6470f635a6b1073, 18b5d857c6496b78ead2fd10001b81ae32d30cac, ec52240622c4d218d0240079b7c1d3ec2328a9f4, e686349cc19e800dac8971929089ba5ff59abfb0, 488ffc0cac38f203979f83634236ee53251ce593
LinuxLinux Kernel4.14.308 up to, but not including, 4.15; 4.19.276 up to, but not including, 4.20; 5.4.235 up to, but not including, 5.5; 5.10.173 up to, but not including, 5.11; 5.15.99 up to, but not including, 5.16; 6.1.16 up to, but not including, 6.1.132; 6.2.3 up to, but not including, 6.6.84; 6.7 up to, but not including, 6.12.20; and 2 more ranges4.15, 4.20, 5.5, 5.11, 5.16, 6.1.132

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: x86/microcode/AMD: Fix out-of-bounds on systems with CPU-less NUMA nodes Currently, load_microcode_amd() iterates over all NUMA nodes, retrieves their CPU masks and unconditionally accesses per-CPU data for the first CPU of each mask. According to Documentation/admin-guide/mm/numaperf.rst: "Some memory may share the same node as a CPU, and others are provided as memory only nodes." Therefore, some node CPU masks may be empty and wouldn't have a "first CPU". On a machine with far memory (and therefore CPU-less NUMA nodes): - cpumask_of_node(nid) is 0 - cpumask_first(0) is CONFIG_NR_CPUS - cpu_data(CONFIG_NR_CPUS) accesses the cpu_info per-CPU array at an index that is 1 out of bounds This does not have any security implications since flashing microcode is a privileged operation but I believe this has reliability implications by potentially corrupting memory while flashing a microcode update. When booting with CONFIG_UBSAN_BOUNDS=y on an AMD machine that flashes a microcode update. I get the following splat: UBSAN: array-index-out-of-bounds in arch/x86/kernel/cpu/microcode/amd.c:X:Y index 512 is out of range for type 'unsigned long[512]' [...] Call Trace: dump_stack __ubsan_handle_out_of_bounds load_microcode_amd request_microcode_amd reload_store kernfs_fop_write_iter vfs_write ksys_write do_syscall_64 entry_SYSCALL_64_after_hwframe Change the loop to go over only NUMA nodes which have CPUs before determining whether the first CPU on the respective node needs microcode update. [ bp: Massage commit message, fix typo. ]

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

The underlying weakness: Improper Validation of Array Index

The product uses untrusted input when calculating or using an array index, but the product does not validate or incorrectly validates the index to ensure the index references a valid position within the array.

How developers prevent this class of flaw

  • Use an input validation framework such as Struts or the OWASP ESAPI Validation API. Note that using a framework does not automatically address all input validation problems; be mindful of weaknesses that could arise from misusing the framework itself (CWE-1173).
  • Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, Ada allows the programmer to constrain the values of a variable and languages such as Java and Ruby will allow the programmer to handle exceptions when an out-of-bounds index is accessed.
  • Use a CPU and operating system that offers Data Execution Protection (using hardware NX or XD bits) or the equivalent techniques that simulate this feature in software, such as PaX. These techniques ensure that any instruction executed is exclusively at a memory address that is part of the code segment. For more information on these techniques see D3-PSEP (Process Segment Execution Prevention) from D3FEND.

More Improper Validation of Array Index vulnerabilities (CWE-129)

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 Improper Validation of Array Index vulnerabilities

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