A critical robustness bug in the Linux kernel’s SMB/CIFS client—tracked as CVE-2024-0565—creates an integer-underflow condition in the function receive_encrypted_standard that can lead to out‑of‑bounds memory reads, denial-of-service, and in some vendor assessments the potential for remote code execution; operators who run CIFS/SMB client code (kernel-side mounts of SMB shares) should treat this as a high-priority patch-and-mitigation item and verify their environments immediately.
The defect lives in the kernel’s SMB client implementation: the vulnerable code path is in fs/smb/client/smb2ops.c inside the function named receive_encrypted_standard. When the client processes an encrypted “standard” SMB2 response it trusts a server-controlled header field and computes a length that is later passed to memcpy. Under specially crafted inputs that compute to an unexpectedly small or negative value, an integer underflow can cause memcpy to operate on a large unsigned length, producing an out‑of‑bounds read and corrupting kernel memory. The immediate operational impact is a crash or kernel panic (denial of service), and multiple security trackers and vendors have documented that a more sophisticated exploit could, under particular conditions, be used to escalate that corruption into arbitrary code execution in kernel context. The National Vulnerability Database (NVD) and multiple Linux distribution advisories record the bug as an out‑of‑bounds read triggered by integer underflow in receive_encrypted_standard.
This is not an isolated CIFS/SMB client bug: the kernel’s SMB/CIFS client and server subsystems have been the focus of several robustness fixes in recent years, and defenders should treat CIFS/SMB client code as a high-value attack surface for memory-safety issues. Prior kernel mailing‑list discussions and community writeups have tracked related SMB client fixes and CVEs, underscoring that maintaining up-to-date kerpackages is the right first-line defense.
Because vendor CVSS vectors differ (adjacent vs network, complexity high vs low, user interaction required vs not), treat the higher‑impact assessments as the conservative posture for your environment. If your systems are accessible to potentially untrusted SMB servers or your users can be induced to connect to attacker-controlled shares, assume an elevated risk level until patched.
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background / Overview
The defect lives in the kernel’s SMB client implementation: the vulnerable code path is in fs/smb/client/smb2ops.c inside the function named receive_encrypted_standard. When the client processes an encrypted “standard” SMB2 response it trusts a server-controlled header field and computes a length that is later passed to memcpy. Under specially crafted inputs that compute to an unexpectedly small or negative value, an integer underflow can cause memcpy to operate on a large unsigned length, producing an out‑of‑bounds read and corrupting kernel memory. The immediate operational impact is a crash or kernel panic (denial of service), and multiple security trackers and vendors have documented that a more sophisticated exploit could, under particular conditions, be used to escalate that corruption into arbitrary code execution in kernel context. The National Vulnerability Database (NVD) and multiple Linux distribution advisories record the bug as an out‑of‑bounds read triggered by integer underflow in receive_encrypted_standard.This is not an isolated CIFS/SMB client bug: the kernel’s SMB/CIFS client and server subsystems have been the focus of several robustness fixes in recent years, and defenders should treat CIFS/SMB client code as a high-value attack surface for memory-safety issues. Prior kernel mailing‑list discussions and community writeups have tracked related SMB client fixes and CVEs, underscoring that maintaining up-to-date kerpackages is the right first-line defense.
What exactly went wrong — technical root cause
The vulnerable code path
- The vulnerability happens when processing encrypted standard SMB2 responses; the code path reads and parses SMB headers and chained packet metadata that is controlled by the server.
- A header field named NextCommand (or equivalent length/offset fields used to walk chained SMB messages) is not validated correctly before the code computes a length and calls memcpy to copy encrypted payload fragments into kernel buffers.
- The arithmetic used to compute the memcpy size can underflow (signed arithmetic wrapped or unsigned wraparound), producing a huge positive unsigned length passed to memcpy or causing a read of memory outside intended bounds.
Consequences of the integer underflow
Integer underflow in a kernel memcpy-length calculation has two clear, predictable consequences:- Immediate instability: the kernel reads or writes outside its intended buffer and rapidly hits invalid memory, producing oops/panic/crash and a reliable denial-of-service. This is the simplest and most widely documented impact for CVE-2024-0565 and is what most vendors emphasize in their advisories.
- Potential for exploit escalation: if an attacker can precisely control layout and the copied data, out‑of‑bounds reads/writes in kernel memory can sometimes be leveraged into arbitrary code execution in kernel context. Several advisories and third‑party trackers note that, while the immediate, easily-achieved impact is DoS, a more skillful exploit (and particular kernel memory layout conditions) could allow remote code execution. Vendor and ZDI writeups explicitly document the theoretical RCE potential. Administrators should therefore not treat the problem as a mere crash bug.
Who and what is affected
Kernel versions and distributions
- The upstream kernel code affected is the in‑tree SMB/CIFS client implementation. Public trackers place the introduction point and patch timeline within the 6.x kernel era; several disclosures say kernels prior to 6.7-rc6 are affected and that upstream has landed fixes. Distribution advisories show a broad footprint: multiple major distributions released errata and patches (Red Hat, SUSE, Amazon Linux, and others).
- Red Hat and its downstream advisories list affected Enterprise Linux versions and provide fixes in kernel errata. Amazon Linux, SUSE and other downstreams followed with their own patched kernels and advisories. Tenable, Rapid7 and other vulnerability databases aggregate those vendor advisories and point to vendor-specific errata for remediation.
Exposure model — who can reach this code path?
- Attack vector classification for this CVE is typically Adjacent Network or Network depending on the vendor assessment: the SMB client acts when a host mounts or accesses an SMB share from a remote server. An attacker exploiting this path generally must be able to send the kernel an SMB response the client will process — which may be accomplished by a malicious SMB server on the local network, a compromised file server, or via user action that causes the client to connect to an attacker-controlled server (for example, opening a link or file served from a hostile SMB server). Vendor scoring varies, but most agree the attacker can be remote and that user interaction or network adjacency is often required.
- In practice, the highest risk scenarios are:
- Desktop or laptop users who mount or access SMB shares automatically (e.g., through file explorers or network discovery).
- Servers or appliances that mount remote SMB shares (for backups, storage, or inter‑host file access).
- Virtual appliances or containers bridging networks that allow an untrusted SMB server to be reached.
Exploitability, severity, and public discussion
- CVSS calculations differ across CNAs and trackers — but consensus places the bug in the High severity range. Typical CVSS v3.1 base scores reported by vendors range from about 6.8 to 7.5 depending on vector assumptions (attack complexity, privileges required, whether user interaction is needed). NVD documents the out‑of‑bounds read and lists a high severity assessment in its enriched record.
- ZDI’s advisory describes scenarios where remote code execution is feasible and assigns a higher severity vector for exploitability if the attacker can get a user to open a malicious file or visit a malicious resource; ZDI’s write-up highlights the RCE potential beyond the immediate DoS. This is consistent with public exploitability analysis by other commercial trackers.
- At the time of initial public disclosure some trackers reported the existence of proof‑of‑concept code and experimental exploits; other sources described the vulnerability as no-remote-exploit or requiring user interaction. Because this range of statements impacts how defenders prioritize mitigation, treat claims about public exploit code cautiously: confirm directly from vendor errata and your own environment’s telemetry before assuming PoCs are trivially usable against your fleet. Several databases that aggregate PoC reports list at least one public PoC; however, PoC quality and real-world applicability vary. Flagged as caution: the presence of a PoC does not necessarily mean a reliable, broadly applicable RCE exploit exists for all kernel versions or distributions.
Patching and mitigation guidance (practical steps for administrators)
If you maintain Linux hosts that mount or access SMB/CIFS shares, take the following actions now.1. Inventory first (required)
- Identify systems that load the cifs module or have SMB client mounts (look for mount points using cifs or smbfs and kernel modules named cifs).
- Prioritize systems that automatically mount network shares at boot, have scheduled cron jobs accessing remote SMB stores, or act as clients for backup/replication to remote SMB servers.
2. Apply vendor patches immediately
- Install the kernel updates supplied by your distribution vendor: Red Hat, SUSE, Amazon Linux and others released errata to address the issue. Where possible, apply the distribution kernel update and reboot according to vendor guidance. Patching is the definitive fix. Vendor advisories and errata are the authoritative source for which package versions close CVE-2024-0565.
3. Short-term mitigations if you cannot patch immediately
- Unload or blacklist the cifs kernel module to prevent its use until you can patch. This will remove SMB client functionality from the kernel, which may be acceptable for systems that do not require SMB mounts. Several vendor advisories explicitly recommend blacklisting cifs as a temporary mitigation if patching is impossible in the short window. Confirm the operational impact before blacklisting.
- Limit SMB client access: firewall off L2/L3 connectivity to untrusted SMB servers and limit network adjacency; prevent users from mounting arbitrary SMB servers unless approved. Where possible, block outgoing SMB ports (commonly TCP 445) from client hosts that do not need to reach SMB servers.
- User education and access controls: instruct users not to connect to unknown network shares or open files from untrusted SMB servers. Because some exploit paths require user action (open file or navigate to an SMB link), reducing risky user behavior lowers near-term exposure.
4. Verify remediation
- After patching, reboot if the vendor requires it and confirm the kernel version contains the fix (check the distribution’s security advisory and package changelog).
- Use kernel oops logs and crashdump tooling to ensure no residual crashes are occurring. If cifs is blacklisted as a temporary fix, verify the module is not loaded on reboot.
Detection and hunting — what to look for
Defenders should tune detection to identify both attempted exploitation and post‑compromise indicators.- Monitor kernel oops/panic and dmesg outputs for SMB/CIFS-related stack traces or messages referencing receive_encrypted_standard, smb2ops, or unexpected calls to memcpy in fs/smb code paths. Kernel crash messages and repeated oops are reliable indicators of attempted exploitation causing a DoS.
- Network telemetry: identify unusual SMB responses from origin servers, especially from servers that were not previously contacted or that show nonstandard SMB header fields or unusual chaining behavior. Capture packet traces for forensic analysis if you see crashes correlated with SMB traffic.
- Host activity: monitor for unexpected module loads (cifs), mounts to new SMB servers, or user actions that triggered new network mounts; combine this with endpoint logs showing crashes or restarts.
- Vulnerability scanners and CMDB: ensure your asset management includes kernel package versions; flag unpatched kernels for remediation.
Risk assessment — who should be most worried
- High priority: servers and appliances that mount remote SMB shares (backup servers, NAS clients, virtualization hosts with SMB-backed storage). These systems are high-value and often long‑lived; an unpatched kernel on such a host leaves critical services vulnerable to persistent DoS and, in worst cases, kernel compromise.
- Medium priority: desktops and laptops that auto-discover or mount SMB shares (e.g., enterprise file shares). These hosts can be weaponized via malicious SMB servers on local networks or by user actions that initiate connections to attacker-controlled servers.
- Lower priority: systems that never load the cifs module, do not mount SMB shares, and are otherwise isolated from SMB servers.
Why this matters: operational and security implications
- Kernel crashes on production servers cause service interruptions and can cascade across clusters, scheduled backups, or storage systems that rely on SMB mounts. Outages triggered by remote SMB servers or by compromised internal servers are operationally costly.
- Kernel code execution would give an attacker maximum privilege — root/kernel mode — undermining confidentiality, integrity, and availability of affected hosts. While exploitation to RCE may be harder than causing a crash, the possibility alone changes remediation from “important” to “urgent” for high-value hosts.
- The vulnerability emphasizes a persistent problem in long-lived kernel subsystems: protocol parsers that accept externally-supplied network data remain high-risk. SMB is a complex, multi-version protocol and handling chained/encrypted responses increases the complexity and attack surface.
Practical recommendations — an operational checklist
- Inventory all systems that use SMB/CIFS client functionality and mark those that must remain operational during a patch window.
- Apply vendor-supplied kernel patches today where available; follow distribution restart guidance.
- If immediate patching is impossible, blacklist the cifs kernel module on non-critical systems after validating operational impact.
- Harden network controls: block outbound SMB (TCP 445) from endpoints that do not need it, and restrict SMB traffic to known, trusted servers inside protected network segments.
- Update detection: add firewall rules, EDR rules, and SIEM alerts for SMB anomalies, kernel oops messages mentioning smb2ops/receive_encrypted_standard, and repeated crashes tied to SMB traffic.
- Test patch rollouts in staging and then proceed with production, ensuring systems are rebooted where required and verifying fixes via reported kernel versions and vendor changelogs.
- Track vendor advisories for follow-up fixes and backports — some vendors issued multiple errata and backports for different enterprise kernel branches.
Cross‑reference validation and caveats
This article synthesizes the official and community reporting for CVE-2024-0565: the NVD description naming the out‑of‑bounds read in receive_encrypted_standard is the canonical technical summary, and multiple distribution advisories (Red Hat, SUSE, Amazon Linux) provide vendor-corrected kernels and errata entries. Independent trackers and vendors (Tenable, Rapid7, ZDI) add context on exploitability and RCE potential. Where public PoCs exist, their practical applicability varies by kernel version and distribution-specific mitigations; defenders should not assume a single PoC will succeed across all environments. Confirmed vendor errata and patched package versions are the authoritative source for remediation.Because vendor CVSS vectors differ (adjacent vs network, complexity high vs low, user interaction required vs not), treat the higher‑impact assessments as the conservative posture for your environment. If your systems are accessible to potentially untrusted SMB servers or your users can be induced to connect to attacker-controlled shares, assume an elevated risk level until patched.
Long-term lessons and hardening for SMB/CIFS
- Minimize kernel protocol parsing exposure: when possible, prefer userland SMB clients that isolate parsing from kernel memory (this is not always feasible for performance reasons, but separation reduces kernel attack surface).
- Network segmentation and zero-trust: restrict which hosts can serve SMB, require authenticated and encrypted SMB channels, and consider blocking SMB traffic across trust boundaries where not needed.
- Shift-right on testing: add fuzzing and sanitizers in kernel development workflows for protocol parsers and consider enabling memory-safety tooling where practical.
- Faster patch pipelines: ensure kernel updates for critical services (storage clients, backup servers) are assessed and staged quickly; these subsystems often sit on long-lived hosts that don’t get frequent kernel refreshes.
Conclusion
CVE-2024-0565 is a high‑impact kernel vulnerability rooted in an integer underflow inside receive_encrypted_standard in the CIFS/SMB client code. The predictable result — kernel crashes and denial-of-service — is already serious for any production host that mounts SMB shares. Multiple vendor analyses and zero‑day researchers note the theoretical possibility of turning this flaw into kernel‑level code execution under certain conditions, elevating the business risk. Organizations should immediately inventory affected systems, apply distribution-supplied kernel updates, and use short-term mitigations (module blacklisting, network controls) when immediate patching is not possible. Finally, add SMB/CIFS client parsing to your kernel risk model and detection playbooks: this class of protocol-parsing weaknesses will continue to produce high-impact vulnerabilities until we reduce kernel exposure or significantly improve memory safety around network parsers.Source: MSRC Security Update Guide - Microsoft Security Response Center
