CVE-2025-39849: Linux Kernel Buffer Overflow Vulnerability

CVE-2025-39849 Overview

CVE-2025-39849 is an out-of-bounds write vulnerability [CWE-787] in the Linux kernel's cfg80211 wireless configuration subsystem. The flaw resides in the __cfg80211_connect_result() function within the Station Management Entity (SME) code path. When the kernel processes wireless connection results, it copies the Service Set Identifier (SSID) into a fixed-size buffer bounded by IEEE80211_MAX_SSID_LEN (32 bytes). Missing bounds checking on ssid->datalen allows values larger than 32 to trigger memory corruption. The vulnerability affects multiple stable Linux kernel branches and Debian Linux distributions.

Critical Impact
Memory corruption in the kernel wireless stack can lead to local privilege escalation, kernel panic, or arbitrary kernel-mode code execution.

Affected Products

Linux Kernel (multiple stable branches, including 6.17-rc1 through 6.17-rc4)
Debian Linux 11.0
Siemens industrial products referencing this kernel (see Siemens Security Advisory SSA-032379)

Discovery Timeline

2025-09-19 - CVE-2025-39849 published to the National Vulnerability Database (NVD)
2026-07-30 - Last updated in NVD database

Technical Details for CVE-2025-39849

Vulnerability Analysis

The vulnerability exists in the cfg80211 subsystem, which provides the configuration Application Programming Interface (API) for full-MAC and soft-MAC wireless drivers in the Linux kernel. The affected function, __cfg80211_connect_result(), is invoked after a wireless connection attempt completes. It stores the SSID reported by the connection result into an internal structure whose SSID field is fixed at 32 bytes, matching the IEEE 802.11 standard maximum.

Before the fix, the function did not validate ssid->datalen against IEEE80211_MAX_SSID_LEN. A malformed connection result carrying an SSID longer than 32 bytes causes an out-of-bounds write into adjacent kernel memory. The corrected code caps the length at 32 bytes before the copy, preventing the overflow.

Root Cause

The root cause is a missing length check on attacker-influenced input flowing into a fixed-size kernel buffer. The datalen field originates from parsed Information Elements (IEs) associated with the connect result. Because the source data was trusted without validation, the copy operation extended past the destination buffer boundary and overwrote adjacent kernel structures.

Attack Vector

Exploitation requires local access with low privileges. An attacker with the ability to influence wireless connection results, either by loading a malicious driver, injecting crafted management frames within radio range on a vulnerable configuration, or manipulating a user-space wireless component that feeds cfg80211, can trigger the out-of-bounds write. Successful memory corruption in kernel context can be leveraged to escalate privileges to root, crash the system, or execute arbitrary code in ring 0.

No public proof-of-concept exploit is available at the time of publication, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2025-39849

Indicators of Compromise

Unexpected kernel panics or oops messages referencing __cfg80211_connect_result, cfg80211_sme, or nearby wireless stack symbols
KASAN (Kernel Address Sanitizer) reports flagging out-of-bounds writes in the cfg80211 subsystem
Wireless connection events containing SSID lengths greater than 32 bytes in dmesg or audit logs

Detection Strategies

Inventory running kernel versions across Linux hosts and compare against the fixed commits listed in the kernel.org stable tree
Monitor kernel crash telemetry and kdump artifacts for stack traces involving cfg80211 SME functions
Correlate wireless driver events with subsequent privilege escalation indicators such as unexpected setuid execution or new root sessions

Monitoring Recommendations

Enable auditd rules on wireless interface changes (iw, wpa_supplicant, NetworkManager) and kernel module loads for wireless drivers
Ship kernel logs and EDR telemetry to a centralized data lake for anomaly detection on kernel crashes and process behavior anomalies following wireless events
Alert on new or unsigned kernel modules loading on production endpoints, especially in wireless code paths

How to Mitigate CVE-2025-39849

Immediate Actions Required

Apply the upstream Linux kernel patches from the stable tree and reboot affected systems
Update Debian systems using the fixes shipped through the Debian LTS Announcement
For Siemens deployments, follow remediation guidance in Siemens Security Advisory SSA-032379
Restrict local access on systems that cannot be patched immediately, since exploitation requires local privileges

Patch Information

The fix adds bounds checking so that ssid->datalen is capped at IEEE80211_MAX_SSID_LEN (32 bytes) before the SSID is copied. The change is distributed across multiple stable branches in commits 31229145, 5cb7cab7, 62b635dc, 8e751d46, and e472f59d.

Workarounds

Disable or unload wireless drivers on servers and appliances that do not require Wi-Fi functionality (rmmod cfg80211 after removing dependent modules)
Blacklist wireless kernel modules using /etc/modprobe.d/ to prevent automatic loading
Enforce least-privilege on local accounts and restrict physical or console access to reduce local attack surface

bash

# Configuration example: prevent cfg80211 and dependent wireless modules from loading
echo 'install cfg80211 /bin/true' | sudo tee /etc/modprobe.d/disable-wireless.conf
echo 'install mac80211 /bin/true' | sudo tee -a /etc/modprobe.d/disable-wireless.conf
sudo update-initramfs -u
# Verify current kernel version against patched stable releases
uname -r