Linux Kernel: NULL Pointer Dereference (CVE-2026-99174)
Published Last modified Sources: Debian, Msrc Update Guide, CVE.org, BSI
What is affected, what to do
In the Linux kernel, the following vulnerability has been resolved: ext4: free the sbi after a grace period ext4_get_link() follows a symlink that keeps its target in a data block in rcu pathwalk mode with ext4_getblk(EXT4_GET_BLOCKS_CACHED_NOWAIT). That reads s_mount_state in the ASSERT() of ext4_getblk() and again in ext4_es_lookup_extent(), and the lookup bumps the extent status hit or miss counter, all through sb->s_fs_info. ext4_put_super() destroys those counters, clears sb->s_fs_info and frees the sbi without waiting for a grace period.
Update Linux Kernel to version 6.1.191, Linux Kernel to version 6.6.161, Linux Kernel to version 6.12.115.
- Affected
- Linux Kernel
lt 5.19..6.1.191 - Linux Kernel
lt 6.2..6.6.161 - Linux Kernel
lt 6.7..6.12.115 - Linux Kernel
lt 6.13..6.18.59 - Linux Kernel
lt 6.19..7.2.13
- Linux Kernel
- Fixed in
- Linux Kernel
6.1.191 - Linux Kernel
6.6.161 - Linux Kernel
6.12.115 - Linux Kernel
6.18.59 - Linux Kernel
7.2.13 - Linux Kernel
7.3
- Linux Kernel
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Metrics
7.1
Priority
XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.
Source: Vendor (CNA), CVSS v3
7.1
CVSS v3
Severity High on the scale from 0 to 10.
Source: Debian, Msrc Update Guide, CVE.org, BSI
0.2 %
EPSS score
Low — model estimates < 1% exploitation likelihood.
Source: FIRST EPSS, percentile 7.5
Show all metrics
- Severity
- High
Priority (0–325)
The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?
Read in glossary- 65.95 Methodology
Exploit maturity
An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.
Read in glossary- no public PoC known
XIEM Effective CVSS
XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.
Read in glossary- 7.1 Methodology
CVSS v3
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 7.1
EPSS score
The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.
Read in glossary- 0.2 %
- Low — model estimates < 1% exploitation likelihood.
EPSS percentile
The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.
Read in glossary- 7.5
- Low — this CVE sits in the lower 10% of all CVEs scored today.
KEV status
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- —
- Published
Detection rules
No NEOSEC detection rule is available for this vulnerability yet.
Classification under NIS2 and CRA
Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.
NIS2
NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.
Read in glossary- High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.
CRA
The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.
Read in glossary- From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001
ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.
Read in glossary- Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.
Description
In the Linux kernel, the following vulnerability has been resolved: ext4: free the sbi after a grace period ext4_get_link() follows a symlink that keeps its target in a data block in rcu pathwalk mode with ext4_getblk(EXT4_GET_BLOCKS_CACHED_NOWAIT). That reads s_mount_state in the ASSERT() of ext4_getblk() and again in ext4_es_lookup_extent(), and the lookup bumps the extent status hit or miss counter, all through sb->s_fs_info. ext4_put_super() destroys those counters, clears sb->s_fs_info and frees the sbi without waiting for a grace period. A pathwalk that is still in ext4_get_link() when the last reference to the mount goes away then dereferences a NULL s_fs_info, the freed sbi or the destroyed percpu counters. With a pathwalk held up in ext4_get_link() while the last reference goes away: KASAN: null-ptr-deref in range [0x00000000000000a8-0x00000000000000af] RIP: 0010:ext4_getblk+0xf0/0x640 Call Trace: <TASK> ext4_get_link+0x26c/0x410 pick_link+0xf96/0x1780 step_into_slowpath+0x9b8/0x1250 path_lookupat+0x484/0xb00 filename_lookup+0x1f1/0x5b0 vfs_statx+0xd7/0x3b0 vfs_fstatat+0x4d/0x90 __do_sys_newfstatat+0xa0/0x100 do_syscall_64+0x10e/0x5e0 entry_SYSCALL_64_after_hwframe+0x77/0x7f So far that has been hard to hit. umount(2) waits for a grace period in namespace_unlock() before it drops the mount, so only a mount whose last reference goes away elsewhere, like the close of a file or a chdir out of a lazily unmounted directory, can race a walker. Keep sb->s_fs_info and free the sbi together with the extent status counters after a grace period. Split the counters out of ext4_es_unregister_shrinker() for that. The shrinker still gets wasted in ext4_put_super(), the counters go with the sbi.
Source: Debian Security Advisories, Microsoft Security Update Guide, cvelistv5, NVD (NIST), BSI WID Portal, BSI CSAF
Affected products
The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.
| Affected versions | Fixed in | ||
|---|---|---|---|
| Linux | Linux |
|
|
Vendors and products
Affected operating systems
| Linux | debian | linux | trixie |
References & sources
- git.kernel.orgstable/c/80d5db1686b05fad6f384be017714297d828a714
- git.kernel.orgstable/c/87d29b8b849738d52fe820d495fa0aa2d5a1ec82
- git.kernel.orgstable/c/c17b44266a1f9fcb49b894d4611d1254bf1aa313
- git.kernel.orgstable/c/4ce0e27ea610663a9cdc04e3bca38559147428d4
- git.kernel.orgstable/c/b4b0598fad51a978fcbda9d55eef6b5bd3cbe7d8
- git.kernel.orgstable/c/cf3bd5824c985b350d150bcd76698ee5e993fcba
Changes to this CVE
Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).
Record modifiedLinux
Show 4 original changes
Individual changes of this event as published by NVD Type Action Before After Affected versions (publisher) added —Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/99xxx/CVE-2026-99174.json">CVE-2026-99174</a>Reference added —https://git.kernel.org/stable/c/4ce0e27ea610663a9cdc04e3bca38559147428d4Reference added —https://git.kernel.org/stable/c/b4b0598fad51a978fcbda9d55eef6b5bd3cbe7d8Reference added —https://git.kernel.org/stable/c/cf3bd5824c985b350d150bcd76698ee5e993fcbaSource identifier: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
ReceivedLinux
Description added or changed
Show 5 original changes
Individual changes of this event as published by NVD Type Action Before After Description added —In the Linux kernel, the following vulnerability has been resolved: ext4: free the sbi after a grace period ext4_get_link() follows a symlink that keeps its target in a data block in rcu pathwalk mode with ext4_getblk(EXT4_GET_BLOCKS_CACHED_NOWAIT). That reads s_mount_state in the ASSERT() of ext4_getblk() and again in ext4_es_lookup_extent(), and the lookup bumps the extent status hit or miss counter, all through sb->s_fs_info. ext4_put_super() destroys those counters, clears sb->s_fs_info and frees the sbi without waiting for a grace period. A pathwalk that is still in ext4_get_link() when the last reference to the mount goes away then dereferences a NULL s_fs_info, the freed sbi or the destroyed percpu counters. With a pathwalk held up in ext4_get_link() while the last reference goes away: KASAN: null-ptr-deref in range [0x00000000000000a8-0x00000000000000af] RIP: 0010:ext4_getblk+0xf0/0x640 Call Trace: <TASK> ext4_get_link+0x26c/0x410 pick_link+0xf96/0x1780 step_into_slowpath+0x9b8/0x1250 path_lookupat+0x484/0xb00 filename_lookup+0x1f1/0x5b0 vfs_statx+0xd7/0x3b0 vfs_fstatat+0x4d/0x90 __do_sys_newfstatat+0xa0/0x100 do_syscall_64+0x10e/0x5e0 entry_SYSCALL_64_after_hwframe+0x77/0x7f So far that has been hard to hit. umount(2) waits for a grace period in namespace_unlock() before it drops the mount, so only a mount whose last reference goes away elsewhere, like the close of a file or a chdir out of a lazily unmounted directory, can race a walker. Keep sb->s_fs_info and free the sbi together with the extent status counters after a grace period. Split the counters out of ext4_es_unregister_shrinker() for that. The shrinker still gets wasted in ext4_put_super(), the counters go with the sbi.Affected versions (publisher) added —New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/99xxx/CVE-2026-99174.json">CVE-2026-99174</a>Reference added —https://git.kernel.org/stable/c/80d5db1686b05fad6f384be017714297d828a714Reference added —https://git.kernel.org/stable/c/87d29b8b849738d52fe820d495fa0aa2d5a1ec82Reference added —https://git.kernel.org/stable/c/c17b44266a1f9fcb49b894d4611d1254bf1aa313Source identifier: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
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<a href="https://intel.neosec.eu/en/cve/CVE-2026-99174"><img src="https://intel.neosec.eu/badge/cve/CVE-2026-99174.svg?lang=en" alt="CVE-2026-99174 – NEOSEC Intel" height="20"></a>