Linux kernel vulnerability in network tunnel PMTUD code can allow remote kernel compromise across virtually all deployments using Linux kernels.
CVE-2026-63994
Linux kernel use-after-free in tunnel PMTUD code allowing remote kernel compromise.
Is CVE-2026-63994 being exploited?
Not confirmed. CVE-2026-63994 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.51% probability of exploitation in the next 30 days.
How severe is CVE-2026-63994?
CVE-2026-63994 is rated Critical. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-63994?
Yes. A fix has been recorded for CVE-2026-63994. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-63994 affect?
CVE-2026-63994 affects Linux kernel (iptunnel_pmtud_build_icmp / iptunnel_pmtud_build_icmpv6), Major Linux distributions' kernels (vendor-specific affected versions). Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-63994?
Apply vendor/kernel patch or update to the fixed kernel immediately.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Remote use-after-free in iptunnel PMTUD ICMP/ICMPv6 handling; can cause kernel memory corruption leading to RCE, privilege escalation, or denial-of-service.
Immediate action required
- affected>= 4cb47a8644cc9eb8ec81190a50e79e6530d0297f and < 95b6d772bfe788331d9742d73eaa12e113b2adc4
- affected>= 4cb47a8644cc9eb8ec81190a50e79e6530d0297f and < 7254aef4d1a7e18e887af9010e2f2dc34806789b
- affected>= 4cb47a8644cc9eb8ec81190a50e79e6530d0297f and < bf8b3f34c37c162357138e7c0942723b8b94fed1
- affected>= 4cb47a8644cc9eb8ec81190a50e79e6530d0297f and < 76cd9398a0470257ab765bdf5f358a2af2e17934
- affected>= 4cb47a8644cc9eb8ec81190a50e79e6530d0297f and < 50750d86a2e5266aba0c295483b3397843198b11
- affected>= 4cb47a8644cc9eb8ec81190a50e79e6530d0297f and < 6dff77899b9e9fe5d854abda3a98ad04e7229ef7
- affected>= 4cb47a8644cc9eb8ec81190a50e79e6530d0297f and < f3f204541f280a6ecb04503a0d6794d93990ca43
- affected>= 4cb47a8644cc9eb8ec81190a50e79e6530d0297f and < b4bc94353050b1fa7b702bd4c6600710dd926cff
- affected5.9
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.