VulnerabilityPublished 28 Oct 2026

CVE-2026-99121: Linux kernel · ipv4: avoid divide by zero in fib_rebalance

Source GCVE Database

Authoritative record:CVE-2026-99121

Not known to be exploited · fixed in d5520e815c4b1427104ce1428ee23821f2aa824d : update Linux kernel when convenient.

Data as of 28 Oct 2026, 14:13 UTC

Severity

Medium

Rated Medium by Threat Radar's AI analysis · CVSS not yet scored by NVD

Patch: Available – fixed in d5520e815c4b1427104ce1428ee23821f2aa824d, 5046d2880fec7d49ebed2fd2866747ee1d06ad71, 5dac5c7ba8f5f3fb4bf29f2a2c3cb552f5b83c8f

Exploitation evidence

Disclosed: evidence found.
CVE published 28 Oct 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-6q9r-cqw9-q9q4

What to do

Upgrade Linux kernel to d5520e815c4b1427104ce1428ee23821f2aa824d, 5046d2880fec7d49ebed2fd2866747ee1d06ad71 or 5dac5c7ba8f5f3fb4bf29f2a2c3cb552f5b83c8f.

Affected software

Product	Affected	Fixed in
LinuxLinux	Affected ≥ 0e884c78ee19e902f300ed147083c28a0c6302f0, < d5520e815c4b1427104ce1428ee23821f2aa824d	Fixed in d5520e815c4b1427104ce1428ee23821f2aa824d
LinuxLinux	Affected ≥ 0e884c78ee19e902f300ed147083c28a0c6302f0, < 5046d2880fec7d49ebed2fd2866747ee1d06ad71	Fixed in 5046d2880fec7d49ebed2fd2866747ee1d06ad71
LinuxLinux	Affected ≥ 0e884c78ee19e902f300ed147083c28a0c6302f0, < 5dac5c7ba8f5f3fb4bf29f2a2c3cb552f5b83c8f	Fixed in 5dac5c7ba8f5f3fb4bf29f2a2c3cb552f5b83c8f
LinuxLinux	Affected =4.4	Fixed in–

AI-suggested

A fix has been implemented in the Linux kernel to address this issue by adjusting the fib_rebalance logic. Users should apply the official kernel updates containing this patch once available. No additional mitigation steps are indicated.

Description

A divide-by-zero vulnerability was identified and resolved in the Linux kernel's IPv4 fib_rebalance function. The issue occurs when concurrent changes cause the first pass of nexthop weight calculation to return zero, while the second pass still sees eligible nexthops, leading to division by zero. The fix sets nexthop upper bounds to -1 when the total is zero, avoiding the division and aligning with a similar IPv6 fix.

References

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