CVE-2026-52967 Overview
CVE-2026-52967 is a Linux kernel vulnerability in the SMB client component, specifically within the symlink_data() function. The flaw affects 32-bit architectures and can result in either an infinite loop or an out-of-bounds read when processing crafted SMB error response data. The issue stems from improper validation of the ErrorDataLength field when computing the offset for the next error context entry. A malicious or malformed SMB server response containing specific length values such as 0xfffffff8 or 0xfffffff0 triggers the unsafe pointer arithmetic.
Critical Impact
A malicious SMB server can cause an SMB client running a 32-bit Linux kernel to enter an infinite loop or read memory outside the bounds of the response buffer.
Affected Products
- Linux kernel SMB client (fs/smb/client) on 32-bit architectures
- Stable kernel branches referenced by upstream fix commits
- Distributions shipping vulnerable kernel versions prior to the patch
Discovery Timeline
- 2026-06-24 - CVE-2026-52967 published to NVD
- 2026-06-24 - Last updated in NVD database
Technical Details for CVE-2026-52967
Vulnerability Analysis
The vulnerability resides in the symlink_data() routine of the Linux kernel SMB client. When parsing SMB2 error response symlink data, the code walks a chain of error context entries by adding p->ErrorDataLength to p->ErrorContextData to locate the next entry. On 32-bit systems, the len value is treated such that very large unsigned values wrap pointer arithmetic into the original or earlier memory regions.
Two conditions emerge from this flaw. When ErrorDataLength equals 0xfffffff8, the computed next pointer equals p, producing an infinite loop because the parser never advances. When ErrorDataLength equals 0xfffffff0, the next pointer resolves to (u8 *)p - 8, causing the parser to read memory before the original structure.
The infinite loop creates a denial-of-service condition by hanging the kernel thread servicing the SMB response. The out-of-bounds read can leak kernel memory contents into subsequent processing or panic the kernel depending on the surrounding allocation layout.
Root Cause
The root cause is missing validation of the ErrorDataLength field before pointer arithmetic. The code accepts attacker-controlled length values without bounding them against the remaining response buffer size, allowing integer wraparound to produce non-progressing or backward-pointing offsets [CWE-125, CWE-835].
Attack Vector
An attacker controlling or impersonating an SMB server returns a crafted SMB2 error response with a manipulated ErrorDataLength field. A Linux SMB client running a 32-bit kernel that mounts or queries a share on that server triggers the flaw when parsing the symlink error data. No authentication beyond what the mount operation requires is needed if the client initiates the connection to the malicious server.
The vulnerability is described in prose because no proof-of-concept code has been published. See the upstream commits for the precise patch logic, including Linux Kernel Commit 1cfa2d59f6 and Linux Kernel Commit cd4b9b662f.
Detection Methods for CVE-2026-52967
Indicators of Compromise
- Kernel threads stuck at 100% CPU within SMB client code paths such as symlink_data or smb2_parse_symlink_data
- dmesg warnings referencing KASAN out-of-bounds reads inside the fs/smb/client module
- Repeated soft lockup messages associated with SMB mount or stat operations against untrusted servers
Detection Strategies
- Inventory hosts running 32-bit Linux kernels and identify which mount SMB shares using the in-kernel cifs.ko client
- Compare installed kernel package versions against the fixed versions referenced in the upstream stable commits
- Audit SMB client connections to servers outside the trusted network perimeter
Monitoring Recommendations
- Alert on unexpected kernel soft lockups or hung task warnings on systems that mount remote SMB shares
- Track outbound SMB (TCP 445) connections from Linux endpoints to untrusted or external destinations
- Monitor for new SMB mounts initiated by non-administrative users on multi-user 32-bit systems
How to Mitigate CVE-2026-52967
Immediate Actions Required
- Apply the upstream stable kernel update that includes the symlink_data() bounds check fix
- Restrict Linux SMB clients to connect only to trusted, authenticated SMB servers
- Prioritize patching 32-bit Linux systems acting as SMB clients, as the flaw is specific to that architecture
Patch Information
The fix is available in multiple stable branches via the following upstream commits: Linux Kernel Commit 1b9331b16b, Linux Kernel Commit 1cfa2d59f6, Linux Kernel Commit 7d9a7f1f96, Linux Kernel Commit 97a05b0ae9, Linux Kernel Commit b41598bf54, and Linux Kernel Commit cd4b9b662f. Install the kernel package update from your distribution vendor that incorporates these commits and reboot the affected systems.
Workarounds
- Unmount SMB shares from untrusted servers until the kernel patch is applied
- Block outbound SMB traffic (TCP 445) to networks outside organizational control using host or perimeter firewalls
- Where feasible, migrate 32-bit systems to a 64-bit kernel build, which is not affected by the wraparound condition described
# Block outbound SMB to untrusted networks (example using nftables)
nft add rule inet filter output tcp dport 445 ip daddr != 10.0.0.0/8 drop
# Verify running kernel version against patched releases
uname -r
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
