high CVSS 7.8

CVE-2021-4439·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: isdn: cpai: check ctr->cnr to avoid array index out of bound The cmtp_add_connection() would add a cmtp session to a controller and run a kernel thread to process cmtp. __module_get(THIS_MODULE); session->task = kthread_run(cmtp_session, session, "kcmtpd_ctr_%d", session->num); During this process, the kernel thread would call detach_capi_ctr() to detach a register controller. if the controller was not attached yet, detach_capi_ctr() would trigger an array-index-out-bounds bug. [ 46.866069][ T6479] UBSAN: array-index-out-of-bounds in drivers/isdn/capi/kcapi.c:483:21 [ 46.867196][ T6479] index -1 is out of range for type 'capi_ctr *[32]' [ 46.867982][ T6479] CPU: 1 PID: 6479 Comm: kcmtpd_ctr_0 Not tainted 5.15.0-rc2+ #8 [ 46.869002][ T6479] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014 [ 46.870107][ T6479] Call Trace: [ 46.870473][ T6479] dump_stack_lvl+0x57/0x7d [ 46.870974][ T6479] ubsan_epilogue+0x5/0x40 [ 46.871458][ T6479] __ubsan_handle_out_of_bounds.cold+0x43/0x48 [ 46.872135][ T6479] detach_capi_ctr+0x64/0xc0 [ 46.872639][ T6479] cmtp_session+0x5c8/0x5d0 [ 46.873131][ T6479] ? __init_waitqueue_head+0x60/0x60 [ 46.873712][ T6479] ? cmtp_add_msgpart+0x120/0x120 [ 46.874256][ T6479] kthread+0x147/0x170 [ 46.874709][ T6479] ? set_kthread_struct+0x40/0x40 [ 46.875248][ T6479] ret_from_fork+0x1f/0x30 [ 46.875773][ T6479]

Severity
high
Software
Kernel
Fixed in
5.14.15
Published
2024-06-20

Affected versions

From: 5.11

Until: 5.14.15

Fixed in: 5.14.15

How to fix this CVE

Update your Linux kernel to version 5.14.15 or later to address an array bounds checking vulnerability in the ISDN CAPI subsystem. This vulnerability allows local attackers with low privileges to trigger out-of-bounds memory access when a CMTP controller is improperly detached. Apply the kernel update immediately and reboot to activate the patched version.

sudo dnf update kernel kernel-devel && sudo reboot

Defensia detects this vulnerability

How to check if you are affected

  1. Check your kernel version: uname -r — compare against 5.14.15; versions 5.11 through 5.14.14 are vulnerable
  2. Verify if ISDN/CAPI modules are loaded: lsmod | grep -E 'isdn|capi|cmtp' — if output appears, the vulnerable subsystem is active
  3. Search kernel logs for UBSAN errors: dmesg | grep -i 'array-index-out-of-bounds' or journalctl -u kernel | grep 'detach_capi_ctr' — exploitation attempts will show UBSAN array bounds warnings
  4. After patching, confirm the new kernel: uname -r — verify version is 5.14.15 or later

FAQ

What is CVE-2021-4439?

CVE-2021-4439 is a memory safety bug in the Linux kernel's ISDN CAPI (Common ISDN API) subsystem where improper controller detachment can trigger an out-of-bounds array access, allowing local privilege escalation.

Is CVE-2021-4439 being actively exploited?

No, CVE-2021-4439 is not listed in the CISA KEV catalog and no public exploits are known, though the vulnerability is easily triggered by local attackers with standard user privileges.

What versions of Kernel are affected by CVE-2021-4439?

Linux kernel versions 5.11 through 5.14.14 are vulnerable; version 5.14.15 and later contain the fix.

How do I check if my server is vulnerable to CVE-2021-4439?

Run uname -r and compare the output to 5.14.15; if your version is between 5.11 and 5.14.14 (inclusive), your system is vulnerable.

Does Defensia detect CVE-2021-4439?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Kernel is installed on a monitored server, CVE-2021-4439 will appear in your dashboard with remediation steps.

Related Kernel CVEs

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CVE-2026-64439CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: crypto: krb5 - filter out async aead implementations at alloc krb5_aead_encrypt(), krb5_aead_decrypt() in rfc3961_simplified.c and rfc8009_encrypt(), rfc8009_decrypt() in rfc8009_aes2.c set a NULL completion callback and treat any negative return from crypto_aead_{encrypt,decrypt}() as terminal, falling through to kfree_sensitive(buffer). When the encrypt_name resolves to an async AEAD instance the request returns -EINPROGRESS, the buffer is freed while the backend's worker still holds a pointer, and the worker dereferences the freed slab on completion. KASAN report under UML+SLUB with a synthetic async aead backend bound to krb5->encrypt_name: BUG: KASAN: slab-use-after-free in t5_stub_complete+0x7d/0xc7 The helpers were written synchronously, so filter the async instances out at allocation time instead of plumbing crypto_wait_req() through every call site. Reachable via net/rxrpc/rxgk.c, fs/afs/cm_security.c and net/ceph/crypto.c on systems with an async AEAD provider bound to the krb5 enctype name.
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References

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