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CVE Record

CVE-2021-4439: isdn: cpai: check ctr->cnr to avoid array index out of bound

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]

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysisunknown

Security readout for executives and security teams

Plain-English summary

CVE-2021-4439 is a Linux kernel bug in the ISDN CAPI/CMTP code. Under a specific controller state, the kernel can access an array with an invalid index. The public record does not provide CVSS, impact detail, or evidence of active exploitation, so urgency depends on whether affected ISDN CAPI functionality is present or enabled.

Executive priority

Handle through standard kernel patch governance unless ISDN CAPI/CMTP is used in production or exposed through legacy telephony workflows. The lack of CVSS and exploitation evidence reduces prioritization confidence, but kernel memory-safety bugs should not be ignored.

Technical view

The issue occurs when cmtp_add_connection starts a CMTP kernel thread and detach_capi_ctr is called before a controller is attached. This can produce an array-index-out-of-bounds condition in drivers/isdn/capi/kcapi.c, with the provided trace showing index -1 into a capi_ctr array of 32 entries.

Likely exposure

Exposure is most plausible on Linux systems running affected kernels with ISDN CAPI/CMTP support built in or loadable. Systems without this kernel functionality enabled appear less likely to be reachable through this bug, but the provided sources do not define exact attack prerequisites.

Exploitation context

The source bundle does not report active exploitation, and KEV is false. It also does not describe attacker location, privileges, or a complete trigger path. Treat this as a kernel memory-safety defect with incomplete public exploitation context.

Researcher notes

The public record identifies the failing code path and stable kernel commits, but omits CVSS, CWE, exploitability, and impact detail. Avoid assuming remote code execution or privilege escalation without vendor confirmation. Focus analysis on affected kernel branches and whether CMTP/CAPI paths are reachable.

Mitigation direction

  • Apply Linux vendor kernel updates that include the referenced stable fixes.
  • Check distribution advisories for the exact fixed kernel package version.
  • Disable or remove ISDN CAPI/CMTP support if it is not required.
  • Restrict loading of unnecessary ISDN-related kernel modules where operationally acceptable.

Validation and detection

  • Inventory Linux kernel versions across servers, appliances, and embedded systems.
  • Check whether ISDN CAPI/CMTP support is enabled, built in, or loadable.
  • Confirm vendor package changelogs include the CVE or referenced stable commits.
  • Prioritize validation on systems using legacy telephony or ISDN-related components.
Prepared
Confidence
medium
Sources
6

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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CVE-2021-4439 mapping review

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
3Timeline events
2ADP providers
9Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: noTechnical Impact: partial

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
CVECVE Program Container

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2unaffected
LinuxLinux2.6.12, 0, 4.4.290, 4.9.288, 4.14.253, 4.19.214, 5.4.156, 5.10.76, 5.14.15, 5.15affected
Weakness

CWE details

No CWE listed

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