---
id: "CVE-2026-99203"
kind: "vulnerability"
aliases: ["CVE-2026-99203"]
title: "Linux kernel: failed fasync enable on FIFO leaves freed-file signal use-after-free"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["linux-kernel", "runtime", "self-hosted"]
not_affected_if: ["unpatched-kernel-not-present"]
published_at: "2026-10-28T11:17:41+00:00"
first_seen_at: "2026-10-28T11:32:37+00:00"
updated_at: "2026-10-28T11:33:58+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-99203", "first_seen_at": "2026-10-28T11:32:37+00:00"}]
---

# Linux kernel: failed fasync enable on FIFO leaves freed-file signal use-after-free

A race in the Linux kernel's pipe code lets a repeated fasync enable on an O_RDWR FIFO, under memory pressure, drop the reader registration while the writer registration stays. A later pipe notification can then signal a freed file, which an unprivileged local user can trigger.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- unknown

## Fix

- Update the Linux kernel to a release that includes the upstream fix for CVE-2026-99203 (the sources do not name the fixed version)

## Timeline

- 2026-10-28T11:32:37+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-99203

## From the sources

In the Linux kernel, the following vulnerability has been resolved:

pipe: preserve existing fasync readers on writer setup failure

F_SETFL and FIOASYNC decide whether to invoke ->fasync() before
pipe_fasync() takes pipe->mutex. Two enables of one O_RDWR FIFO can
therefore both enter the callback.

fasync_helper() allocates before checking for an existing entry. If the
first enable installs both entries, a second enable can update the
reader entry and then fail the allocation for the writer entry. The
error path removes the pre-existing reader entry, clearing FASYNC while
the writer entry remains registered. __fput() then skips ->fasync(), so a
later pipe notification can dereference the freed file or its f_owner.
An unprivileged memory-pressure reproducer triggers a KASAN
slab-use-after-free in send_sigio() through this path.

Keep the reader helper's result and undo the registration only if this
call added it. The pipe mutex serializes changes to both lists, so a
failed new registration still removes its reader, while a failed
duplicate preserves the existing registrations and FASYNC. Successful
duplicate enables continue to update fa_fd as before.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_