CVE-2025-39682: Linux Kernel Improper Check for Unusual or Exceptional Conditions Vulnerability
Linux Kernel contains an improper check for unusual or exceptional conditions vulnerability in the TLS receive path which allows a zero-length record retrieved from the rx_list to bypass the intended recvmsg() record-type handling, potentially causing subsequent TLS records to be processed using incorrect zero-copy and queuing assumptions. The impacted product(s) could be end-of-life (EoL) and/or end-of-service (EoS). Users are advised to discontinue use and/or transition to a supported version.
Overview
The Linux kernel's in-kernel TLS receive path contains a logic flaw in how it handles zero-length records retrieved from the rx_list. When such a record appears as the first entry processed by a recvmsg() call, the kernel fails to check the record type before proceeding with zero-copy decryption, causing subsequent TLS records to be processed under incorrect memory assumptions. The result can be kernel memory corruption with potential for arbitrary code execution, privilege escalation, or denial of service. Kernel versions from 6.0 through the 6.1, 6.6, and 6.12 stable lines are affected, as are Siemens SIMATIC CN 4100 firmware versions before 5.0.
Vulnerability details
The flaw sits in net/tls/tls_sw.c, specifically in tls_sw_recvmsg(). The kernel's TLS software path enforces a rule: each recvmsg() call must process either contiguous DATA records or exactly one non-DATA record. When record types change mid-call and a record has already been decrypted, the kernel queues the pending socket buffer to rx_list for the next call to pick up. Zero-copy decryption is only permitted for DATA records, and after a zero-copy operation there is no socket buffer to queue. The missed edge case is a zero-length record sitting at the head of rx_list: the type-check logic does not fire, allowing the zero-copy path to proceed incorrectly when the following record is a different type, violating the kernel's own memory safety invariants.
A remote peer controls the TLS records delivered over an established connection. By sending a zero-length TLS record followed by a record of a different type, an attacker can trigger the incorrect zero-copy path in the kernel receive loop. Exploitation requires that kernel TLS (kTLS) be active on the target socket and that the attacker be the TLS peer. If those conditions are met, the resulting memory corruption could yield arbitrary code execution or privilege escalation in kernel context. The precondition of an established TLS connection limits opportunistic exploitation but does not require any special privilege on the attacker's side.
Severity and impact
Exploitation status
This vulnerability is confirmed as actively exploited in the wild — it is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog.
Detection and monitoring
- Monitor for unexpected kernel panics, oops messages, or BUG() traces in syslog or the kernel ring buffer (dmesg) referencing tls_sw_recvmsg or net/tls/tls_sw.c, which would indicate the corrupted state being reached.
- Audit systems for active kTLS socket usage (SO_TLS_TX / SO_TLS_RX socket options set) exposed to untrusted or external peers; a kTLS socket accepting connections from outside a defined peer set is the prerequisite surface for this flaw.
Remediation
Additional hardening
- Update the Linux kernel to 6.1.149 or later on the 6.1.x line, 6.6.103 or later on the 6.6.x line, or 6.12.44 or later on the 6.12.x line; see References for additional stable-branch boundaries. Siemens SIMATIC CN 4100 firmware should be updated to V5.0 or later.
- Disable kernel TLS offload (kTLS) on sockets that accept connections from untrusted or external peers where application-layer TLS termination is a viable alternative, removing the vulnerable code path entirely.
- Restrict network access to services using kTLS through host-based firewall rules or network segmentation, limiting which peers can establish TLS connections and reducing the pool of potential attackers who meet the precondition.
- Apply CISA forensic triage guidance referenced in the vendor advisory to any system where kernel memory corruption indicators are observed before patching, as exploitation may leave the kernel in an inconsistent state.
Key dates
References
- https://git.kernel.org/stable/c/2902c3ebcca52ca845c03182000e8d71d3a5196f
- https://git.kernel.org/stable/c/29c0ce3c8cdb6dc5d61139c937f34cb888a6f42e
- https://git.kernel.org/stable/c/3439c15ae91a517cf3c650ea15a8987699416ad9
- https://git.kernel.org/stable/c/62708b9452f8eb77513115b17c4f8d1a22ebf843
- https://git.kernel.org/stable/c/c09dd3773b5950e9cfb6c9b9a5f6e36d06c62677
- https://www.cisa.gov/news-events/directives/bod-26-04-prioritizing-security-updates-based-risk
- https://www.cisa.gov/news-events/directives/bod-26-04-implementation-guidance-prioritizing-security-updates-based-risk
- https://nvd.nist.gov/vuln/detail/CVE-2025-39682
Frequently asked questions
Does CVE-2025-39682 affect my FedRAMP authorization?
If Linux Kernel runs inside your authorization boundary, yes. CVE-2025-39682 appears on CISA's Known Exploited Vulnerabilities (KEV) catalog with a remediation deadline of September 21, 2026. For a FedRAMP-authorized service, an unpatched KEV within the boundary is an assessor finding. You must either remediate it or formally document a mitigation before your assessor and sponsoring agency raise it during review.
How does Knox help me handle CVE-2025-39682?
Knox does not patch your software. Remediating Linux Kernel is your responsibility under the FedRAMP shared-responsibility model. What Knox provides is the pre-authorized, single-tenant boundary to remediate within, along with continuous compliance monitoring and audit-artifact coverage that document the fix for your next assessment. The work of applying the patch is yours; managing your compliance posture while you do it is not something you handle alone.
How does Knox's monitoring help with vulnerabilities like this?
Knox's automated continuous monitoring platform watches your environment for newly disclosed vulnerabilities and compliance issues, including CVE-2025-39682. Exposure surfaces during ongoing monitoring rather than only when an assessor flags it at review time, giving you the opportunity to act before a finding becomes a formal problem.
How do I get FedRAMP authorized with Knox?
Knox runs a FedRAMP-as-a-Service platform. It gives SaaS vendors a pre-authorized cloud boundary on AWS, Azure, and GCP. Your application inherits 60-80% of the required security controls. You reach FedRAMP authorization in about 90 days for roughly 90% less than the traditional $3.5M path. Book a meeting and Knox will map your path to authorization.
What happens if CVE-2025-39682 isn't remediated by September 21, 2026?
If CVE-2025-39682 is not remediated by September 21, 2026, it becomes a Plan of Action and Milestones (POA&M) item. A growing POA&M list is what turns a routine continuous-monitoring review into a difficult agency conversation. Closing the finding before the deadline keeps your authorization clean and the agency relationship intact.
Ready to achieve FedRAMP authorization in 90 days or less?
Schedule a meeting to discuss scope, parse readiness, and map your company’s accelerated path to FedRAMP authorization.
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