VulnerabilityPublished 19 Jul 2026

CVE-2026-63981: Linux kernel · net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow

Source GCVE Database

Not known to be exploited.

Data as of 26 Oct 2026, 14:05 UTC

Severity

High

Rated by Threat Radar · No CVSS score from NVD or the CNA

EPSS: 0.2% chance of exploitation in 30 days – higher than 13% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

Exploitation evidence

Disclosed: evidence found.
CVE published 19 Jul 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

ghsa-v39p-wrrc-5j75

What to do

AI-suggested

The described fix involves incrementing the recursion counter before the blockcast call and decrementing it after, ensuring the recursion limit is enforced. Until a patch is applied, avoid configurations that share TC egress blocks with mirred blockcast rules between devices, and restrict unprivileged user access to user namespaces with CAP_NET_ADMIN capabilities.

Description

A vulnerability in the Linux kernel's traffic control mirred action (act_mirred) allows a recursion bypass that leads to a stack overflow. The issue arises because the recursion counter is not incremented when using blockcast, enabling unbounded recursion between devices sharing a mirred blockcast rule. This can cause a kernel stack overflow and panic. The vulnerability is exploitable by unprivileged users with user namespace capabilities that grant CAP_NET_ADMIN in a new network namespace.

References

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