Raw vector
CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:HSummary
CVE-2025-21655 is a medium-severity Use After Free (CWE-416) vulnerability in Linux Linux Kernel. Its CVSS base score is 4.7 (Medium).
Operationally, ranked at the 13th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to SI-16 (Memory Protection) and SI-2 (Flaw Remediation) — see the control section below for these in your framework.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2025-2604
Vulnerability Data
In the Linux kernel, the following vulnerability has been resolved: io_uring/eventfd: ensure io_eventfd_signal() defers another RCU period io_eventfd_do_signal() is invoked from an RCU callback, but when dropping the reference to the io_ev_fd, it calls io_eventfd_free() directly if the refcount drops…
more
to zero. This isn't correct, as any potential freeing of the io_ev_fd should be deferred another RCU grace period. Just call io_eventfd_put() rather than open-code the dec-and-test and free, which will correctly defer it another RCU grace period.
- CWE(s)
Related Threats
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
Mitigating Controls (NIST 800-53 r5) AI
Memory Protection directly mitigates the CWE-416 use-after-free by enforcing bounds and lifetime checks on kernel objects such as io_ev_fd during RCU grace periods.
Flaw Remediation requires prompt application of the kernel patch that replaces the direct io_eventfd_free call with io_eventfd_put to ensure proper RCU deferral.
Process Isolation limits the blast radius of a UAF in io_uring/eventfd by confining the affected RCU callback context within the kernel's isolated address spaces.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Secure SDLC practices directly incorporate memory-safety tooling and reviews that prevent most use-after-free defects.
Vulnerability identification processes can discover use-after-free issues via scanning or analysis but do not prevent their introduction.
Routine patching removes known use-after-free instances after they have been introduced in released software.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Security testing in development can detect use-after-free bugs before release.
Secure SDLC mandates memory-safety practices that reduce use-after-free defects.
Application security requirements can specify memory-management rules that mitigate use-after-free.
Secure architecture principles include memory-safety design choices that limit use-after-free exposure.
Secure coding standards directly prescribe avoidance of use-after-free patterns.
Change-management processes help ensure memory-safety fixes are deployed consistently.