CVE-2026-63994Network attack vector

Kernel memory disclosure via stale tunnel headers

Published Jul 19, 2026 · Updated Jul 19, 2026

Use-after-free in Linux Kernel tunnel PMTU handling allows remote attackers to disclose kernel memory or crash hosts via tunneled traffic. The iptunnel_pmtud_build_icmp() and iptunnel_pmtud_build_icmpv6() paths retain IP-header pointers across skb_cow(), which can reallocate and free the referenced packet head. Reachability requires oversized DF-marked traffic through a bridged VXLAN or GENEVE overlay that enters PMTU ICMP generation; no victim-host account or user action is required.

CVSS severity9.8
Critical
EPSS probability0.51%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Use-after-free in Linux Kernel tunnel PMTU handling allows remote attackers to disclose kernel memory or crash hosts via tunneled traffic. The iptunnel_pmtud_build_icmp() and iptunnel_pmtud_build_icmpv6() paths retain IP-header pointers across skb_cow(), which can reallocate and free the referenced packet head. Reachability requires oversized DF-marked traffic through a bridged VXLAN or GENEVE overlay that enters PMTU ICMP generation; no victim-host account or user action is required.

The record

CVE
CVE-2026-63994
Published
Jul 19, 2026
Updated
Jul 19, 2026
Vendor
The Linux Kernel Organization
Product
Linux Kernel
Classifications
CWE-825
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 19, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 19, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=4cb47a8644cc9eb8ec81190a50e79e6530d0297f <50750d86a2e5266aba0c295483b3397843198b11
  2. Affected versionversion=4cb47a8644cc9eb8ec81190a50e79e6530d0297f <6dff77899b9e9fe5d854abda3a98ad04e7229ef7
  3. Affected versionversion=4cb47a8644cc9eb8ec81190a50e79e6530d0297f <7254aef4d1a7e18e887af9010e2f2dc34806789b
  4. Affected versionversion=4cb47a8644cc9eb8ec81190a50e79e6530d0297f <76cd9398a0470257ab765bdf5f358a2af2e17934
  5. Affected versionversion=4cb47a8644cc9eb8ec81190a50e79e6530d0297f <95b6d772bfe788331d9742d73eaa12e113b2adc4
  6. Affected versionversion=4cb47a8644cc9eb8ec81190a50e79e6530d0297f <b4bc94353050b1fa7b702bd4c6600710dd926cff
  7. Affected versionversion=4cb47a8644cc9eb8ec81190a50e79e6530d0297f <bf8b3f34c37c162357138e7c0942723b8b94fed1
  8. Affected versionversion=4cb47a8644cc9eb8ec81190a50e79e6530d0297f <f3f204541f280a6ecb04503a0d6794d93990ca43
  9. Affected versionversion=5.9

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Linux Kernel Organization · Linux Kernelversion=4cb47a8644cc9eb8ec81190a50e79e6530d0297f <50750d86a2e5266aba0c295483b3397843198b11; version=4cb47a8644cc9eb8ec81190a50e79e6530d0297f <6dff77899b9e9fe5d854abda3a98ad04e7229ef7; version=4cb47a8644cc9eb8ec81190a50e79e6530d0297f <7254aef4d1a7e18e887af9010e2f2dc34806789b; version=4cb47a8644cc9eb8ec81190a50e79e6530d0297f <76cd9398a0470257ab765bdf5f358a2af2e17934; version=4cb47a8644cc9eb8ec81190a50e79e6530d0297f <95b6d772bfe788331d9742d73eaa12e113b2adc4; version=4cb47a8644cc9eb8ec81190a50e79e6530d0297f <b4bc94353050b1fa7b702bd4c6600710dd926cff; version=4cb47a8644cc9eb8ec81190a50e79e6530d0297f <bf8b3f34c37c162357138e7c0942723b8b94fed1; version=4cb47a8644cc9eb8ec81190a50e79e6530d0297f <f3f204541f280a6ecb04503a0d6794d93990ca43; version=5.9

Published CVSS scores

7.0Amazon LinuxCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
7.0Red HatCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
9.8Linux Kernel CNACVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-825Expired Pointer Dereference

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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