Kernel stack disclosure via MPLS label over-read
Published Jun 18, 2026 · Updated Jun 18, 2026
Out-of-bounds read in OpenBSD src before commit 6a23123 allows remote attackers to disclose kernel stack data through crafted MPLS frames. In mpls_do_error, a label stack without a Bottom-of-Stack bit leaves nstk at 16, but copy paths use nstk+1 against the fixed 16-entry stack array and return the four-byte over-read in an ICMP/MPLS error. The path requires MPLS enabled on the receiving interface, 16 labels, an outer TTL of 1, and an IPv4 payload; each request discloses four adjacent kernel-stack bytes without crashing the host.
Summary
What happened
Out-of-bounds read in OpenBSD src before commit 6a23123 allows remote attackers to disclose kernel stack data through crafted MPLS frames. In mpls_do_error, a label stack without a Bottom-of-Stack bit leaves nstk at 16, but copy paths use nstk+1 against the fixed 16-entry stack array and return the four-byte over-read in an ICMP/MPLS error. The path requires MPLS enabled on the receiving interface, 16 labels, an outer TTL of 1, and an IPv4 payload; each request discloses four adjacent kernel-stack bytes without crashing the host.
The record
- CVE
- CVE-2026-56099
- Published
- Jun 18, 2026
- Updated
- Jun 18, 2026
- Vendor
- OpenBSD Project
- Product
- src
- Classifications
- CWE-125
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jun 18, 2026CVE publishedPublication date reported by the CVE source.
- Jun 18, 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?
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- Affected versionversion=0 <6a23123ec05f1eb29cfcaae0f3a468b2e1983cfd
What conditions does exploitation require?
What is affected?
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.
Weakness, pattern, technique
Public exploit references
- poc-008-mpls-stack-leak.pyproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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