CVE-2025-21647
Published: 19 January 2025
Summary
CVE-2025-21647 is a high-severity Out-of-bounds Read (CWE-125) vulnerability in Linux Linux Kernel. Its CVSS base score is 7.1 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Privilege Escalation (T1068); ranked at the 6.5th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 SI-16 (Memory Protection) and SI-2 (Flaw Remediation).
Deeper analysis
CVE-2025-21647 is a vulnerability in the Linux kernel's sch_cake queue discipline, part of the scheduling subsystem. It stems from an underflow in the per-host bulk flow fairness counters, which could lead to an out-of-bounds memory access. Although a prior logic error fix had been applied, syzbot fuzzing still triggered the issue, prompting the addition of bounds checks via helper functions to prevent such accesses during increments and decrements.
A local attacker with low privileges can exploit this vulnerability with low complexity and no user interaction required. Successful exploitation grants high-impact confidentiality by enabling out-of-bounds reads (CWE-125) and high-impact availability disruption through memory corruption or denial of service, while integrity remains unaffected. The CVSS v3.1 base score is 7.1 (AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H).
Mitigation involves applying upstream kernel patches, as detailed in the referenced stable commits, which introduce bounds-checking helpers for all per-host bulk flow counter accesses, consolidate flow quantum calculations, and enhance code readability to prevent future logic errors. These changes introduce minor user-visible effects, such as slight variations in maximum packet sizes for sparse flows, but do not significantly alter behavior.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2025-2596
Vulnerability details
In the Linux kernel, the following vulnerability has been resolved: sched: sch_cake: add bounds checks to host bulk flow fairness counts Even though we fixed a logic error in the commit cited below, syzbot still managed to trigger an underflow…
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of the per-host bulk flow counters, leading to an out of bounds memory access. To avoid any such logic errors causing out of bounds memory accesses, this commit factors out all accesses to the per-host bulk flow counters to a series of helpers that perform bounds-checking before any increments and decrements. This also has the benefit of improving readability by moving the conditional checks for the flow mode into these helpers, instead of having them spread out throughout the code (which was the cause of the original logic error). As part of this change, the flow quantum calculation is consolidated into a helper function, which means that the dithering applied to the ost load scaling is now applied both in the DRR rotation and when a sparse flow's quantum is first initiated. The only user-visible effect of this is that the maximum packet size that can be sent while a flow stays sparse will now vary with +/- one byte in some cases. This should not make a noticeable difference in practice, and thus it's not worth complicating the code to preserve the old behaviour.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Local kernel memory corruption and OOB read vulnerability exploitable by low-privileged users, directly enabling exploitation for privilege escalation (T1068) and endpoint DoS via system exploitation (T1499.004).
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Directly mandates timely remediation of the kernel flaw through patching to add bounds checks preventing underflow and out-of-bounds access in sch_cake.
Implements memory protection mechanisms that safeguard against out-of-bounds memory reads and corruption exploited via the sch_cake counter underflow.
Requires secure error handling to mitigate underflow conditions in per-host bulk flow counters that lead to out-of-bounds access.