CVE-2025-21905: Linux Kernel Buffer Overflow Vulnerability

CVE-2025-21905 Overview

CVE-2025-21905 is an out-of-bounds read vulnerability [CWE-125] in the Linux kernel's iwlwifi Intel wireless driver. The flaw resides in the firmware (FW) file parsing logic, where the driver prints a string extracted from a Type-Length-Value (TLV) structure without verifying NUL-termination. When the TLV lacks a terminating null byte, the print routine can read past the end of the TLV buffer and, if the affected TLV is the last one in the file, beyond the end of the entire firmware file buffer.

Critical Impact
A local user with the ability to load a crafted firmware file can trigger kernel memory disclosure or a kernel crash, impacting confidentiality and availability of the affected Linux system.

Affected Products

Linux kernel stable branches containing the vulnerable iwlwifi firmware parsing code
Linux kernel 6.14-rc1 through 6.14-rc5 release candidates
Debian LTS distributions shipping the affected kernel versions

Discovery Timeline

2025-04-01 - CVE-2025-21905 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-21905

Vulnerability Analysis

The vulnerability exists in the iwlwifi driver, which handles Intel Wi-Fi hardware in the Linux kernel. During firmware file parsing, the driver reads TLV structures embedded within the FW binary and prints string data extracted from them. The code path assumes the string within the TLV is NUL-terminated, but the firmware file format provides no such guarantee.

When the print routine encounters a non-terminated string, it continues reading memory beyond the TLV payload searching for a NUL byte. If the malformed TLV is the last record in the firmware file, the read can extend past the end of the file buffer into adjacent kernel memory. This constitutes an out-of-bounds read that can leak kernel memory contents into logs or trigger a fault.

The upstream fix limits the print format specifier to the known size of the TLV buffer, preventing the string routine from reading beyond the bounded region.

Root Cause

The root cause is missing input validation on data supplied by a firmware file. The iwlwifi driver invoked a string-printing function against a byte range without enforcing a maximum length derived from the TLV header. Because the TLV specifies a length field, the driver should have bounded the string read to that length rather than trusting the presence of a NUL terminator.

Attack Vector

Exploitation requires local access with privileges sufficient to influence firmware loading — typically low-privileged access to load or replace files in the firmware search path, or the ability to supply a crafted firmware image to the wireless subsystem. Remote exploitation over the network is not applicable. Successful exploitation can result in kernel memory disclosure through kernel logs or a denial-of-service condition through invalid memory access.

See the upstream commits referenced in the Kernel Git Commit Notice for the exact source-level change.

Detection Methods for CVE-2025-21905

Indicators of Compromise

Unexpected kernel log entries from the iwlwifi driver containing malformed or unusually long strings during firmware load
Kernel oops or panic messages referencing iwlwifi firmware parsing functions
Presence of modified or non-vendor firmware files in /lib/firmware/iwlwifi-*

Detection Strategies

Monitor dmesg and journalctl -k output for anomalous iwlwifi firmware load messages, especially garbled string output that suggests memory over-read
Track running kernel versions across the fleet and flag hosts on unpatched 6.14-rc series or older stable branches lacking the fix commits
Audit filesystem integrity of firmware directories to detect unauthorized modification of iwlwifi firmware blobs

Monitoring Recommendations

Forward kernel logs to a centralized logging platform and alert on iwlwifi parse failures or KASAN reports involving TLV processing
Establish baseline hashes for vendor-supplied firmware files and continuously validate them against known-good values
Correlate firmware file modification events with user session and privilege context to identify suspicious tampering

How to Mitigate CVE-2025-21905

Immediate Actions Required

Apply the stable kernel updates that include the upstream fix commits, then reboot affected systems
Restrict write access to /lib/firmware/ and related directories to root only, ensuring no world-writable permissions
For Debian systems, install the DSA/DLA updates referenced in the Debian LTS Security Notice

Patch Information

The Linux kernel maintainers resolved the issue by bounding the print format to the TLV buffer size. Fix commits are available across multiple stable branches, including 88ed69f924638c7503644e1f8eed1e976f3ffa7a, 47616b82f2d42ea2060334746fed9a2988d845c9, 59cdda202829d1d6a095d233386870a59aff986f, and f265e6031d0bc4fc40c4619cb42466722b46eaa9. See the Kernel Git Commit Notice for the reference patch. Debian users should apply the packaged updates from the Debian LTS Security Notice.

Workarounds

Blacklist the iwlwifi module on systems that do not require Intel Wi-Fi functionality by adding blacklist iwlwifi to /etc/modprobe.d/blacklist.conf
Enforce strict filesystem permissions on /lib/firmware/ to prevent non-root modification of firmware files
Deploy kernel lockdown mode where feasible to restrict loading of unsigned or modified firmware images

bash

# Configuration example: verify kernel version and firmware permissions
uname -r
ls -l /lib/firmware/iwlwifi-*.ucode
chmod 644 /lib/firmware/iwlwifi-*.ucode
chown root:root /lib/firmware/iwlwifi-*.ucode

# Optional: disable the iwlwifi module if not required
echo 'blacklist iwlwifi' | sudo tee /etc/modprobe.d/disable-iwlwifi.conf
sudo update-initramfs -u