CVE-2026-71227: libkcapi Denial of Service Vulnerability

CVE-2026-71227 Overview

A flaw exists in libkcapi, the userspace library for the Linux Kernel Crypto API. The vulnerability affects applications using the Asynchronous Input/Output (AIO) interface. A local attacker can influence an application that reuses an AIO-enabled handle after a prior completion error. Under this condition, the _kcapi_aio_read_all() function can enter a non-terminating wait loop. The result is a persistent denial of service that renders the affected application or thread unresponsive. The issue is tracked under [CWE-835] (Loop with Unreachable Exit Condition).

Critical Impact
Local attackers can trigger an infinite loop in _kcapi_aio_read_all(), causing persistent denial of service in applications relying on the libkcapi AIO interface.

Affected Products

libkcapi (Linux Kernel Crypto API userspace library)
Applications using the libkcapi AIO interface
Linux distributions packaging vulnerable versions of libkcapi (see Red Hat CVE-2026-71227 Advisory)

Discovery Timeline

2026-08-05 - CVE CVE-2026-71227 published to NVD
2026-08-05 - Last updated in NVD database

Technical Details for CVE-2026-71227

Vulnerability Analysis

The defect resides in _kcapi_aio_read_all(), a routine that waits for asynchronous cryptographic I/O operations to complete. The function iterates while pending operations remain outstanding. When an AIO-enabled handle is reused after a prior completion error, the internal accounting of pending operations is left inconsistent. The wait loop then never observes the terminating condition and continues indefinitely.

Because the loop blocks in kernel-side event waiting, the calling thread stops making forward progress. Applications that rely on libkcapi for cryptographic operations become unresponsive. The impact is limited to availability; there is no memory corruption, information disclosure, or integrity violation. Exploitation requires the ability to influence how a local application drives its AIO handles.

Root Cause

The root cause is a Loop with Unreachable Exit Condition [CWE-835] in _kcapi_aio_read_all(). Reusing an AIO handle following a prior error path leaves the outstanding-operation counter in a state that the wait loop cannot reconcile. The function has no bounded timeout or error-based exit path to recover from this inconsistent state.

Attack Vector

The attack vector is local. An attacker must supply input or otherwise influence an application that uses the libkcapi AIO interface into triggering a completion error on an AIO handle, and then reusing that same handle. No privileges or user interaction are required beyond that local interaction with the vulnerable application. Successful triggering hangs the affected application or thread and can be repeated to sustain the denial of service.

No public proof-of-concept exploit code is available at this time. For technical background, refer to the Red Hat Bug Report #2462867.

Detection Methods for CVE-2026-71227

Indicators of Compromise

Application threads stuck in _kcapi_aio_read_all() visible in stack traces or /proc/<pid>/stack.
Processes linked against libkcapi showing 100% wall-clock blocking with no I/O progress.
Repeated hangs of the same service after cryptographic errors are logged.

Detection Strategies

Inspect running processes with gdb, perf, or bpftrace for threads blocked inside libkcapi AIO wait paths.
Correlate application-level cryptographic error events with subsequent unresponsiveness of the same process.
Query installed package versions of libkcapi across the fleet and flag hosts running versions predating the vendor fix.

Monitoring Recommendations

Alert on service watchdog timeouts and health-check failures for daemons that consume libkcapi.
Track CPU and thread-state metrics for processes that use the Kernel Crypto API to identify long-lived uninterruptible or waiting states.
Centralize syslog and application logs to detect patterns of crypto errors immediately followed by process hangs.

How to Mitigate CVE-2026-71227

Immediate Actions Required

Inventory all systems that ship or use libkcapi, especially services invoking the AIO interface.
Apply vendor-supplied updates as soon as they are available from your Linux distribution.
Restart affected services after patching to clear any threads already stuck in the wait loop.

Patch Information

Refer to the Red Hat CVE-2026-71227 Advisory for authoritative fix status and package updates. Track the corresponding Red Hat Bug Report #2462867 for upstream progress. Apply distribution updates through the standard package manager once patched builds are published.

Workarounds

Where feasible, modify applications to avoid reusing an AIO-enabled libkcapi handle after any completion error, and instead close and reinitialize the handle.
Restrict local access to systems running services that depend on libkcapi AIO to reduce the pool of potential attackers.
Implement service-level watchdogs that terminate and restart hung processes to limit denial-of-service duration.

bash

# Check installed libkcapi version on RPM-based systems
rpm -q libkcapi

# Check on Debian-based systems
dpkg -l | grep libkcapi

# Update once a fixed package is published
sudo dnf update libkcapi   # RHEL/Fedora
sudo apt update && sudo apt install --only-upgrade libkcapi   # Debian/Ubuntu