CVE-2021-25489

What it actually does

This vulnerability is an improper input validation flaw in the Samsung Exynos modem interface driver used on affected Android/Samsung devices prior to the October 2021 Samsung Mobile Security Release. A local attacker who has already obtained the required radio-level permission can pass crafted input containing format-string sequences to the modem interface. Because the driver does not safely validate or constrain that input before using it in a formatting context, the kernel can panic, causing a device crash or reboot. The disclosed impact is denial of service against the device kernel; the CVSS vector does not indicate confidentiality or integrity impact.

Why it matters to the business

Successful exploitation can make affected mobile devices crash or repeatedly reboot, disrupting user productivity, field operations, mobile communications, and any business processes that rely on those devices. The vulnerability is listed in CISA KEV with a remediation due date of 2023-07-20 and has been observed exploited in the wild, so organizations with legacy Samsung Exynos/Android devices should treat unpatched devices as higher priority even though the EPSS score is low and stable. Devices no longer receiving Samsung/Android security updates present the greatest operational risk because the practical mitigation may be replacement rather than patching.

How an attack unfolds

An attacker first needs local code execution on the device in a context that has the required radio permission, such as through a malicious application, a compromised privileged app, supply-chain abuse of a preinstalled app, or another prior privilege/escalation step. The attacker then interacts with the affected modem interface driver and submits malformed input containing format-string tokens. The vulnerable driver fails to validate the input and processes it unsafely in kernel context. This can trigger a kernel panic, crashing or rebooting the device. In real-world attack chains, this type of flaw is most useful as a disruption mechanism, an anti-forensics step, or a reliability component after the attacker has already achieved a privileged local foothold.

How to fix it

Update affected Samsung/Android devices to Samsung Mobile Security Release October 2021 Release 1 or later, or to a vendor firmware build that includes the corresponding fix for CVE-2021-25489. Where exact package-level fixed versions are not available, use the device vendor security patch level as the authoritative remediation marker and require an Android/Samsung security patch level of 2021-10-01 or later that includes the Samsung October 2021 fixes. Retire or replace affected devices that cannot receive vendor security updates. As compensating controls, restrict installation of untrusted applications, enforce mobile device management controls, prevent sideloading where possible, monitor for privileged app abuse, and limit access to radio/modem permissions to trusted signed system components only.

How to confirm the fix worked

Confirm each device reports a vendor/Android security patch level that includes Samsung SMR Oct-2021 Release 1 or later. Validate through MDM inventory, device settings, or ADB where authorized, such as checking the Android security patch level and installed firmware/build identifiers against Samsung advisories for the device model. For exposure assessment, identify Samsung Exynos-based devices that are below the October 2021 security release or are no longer supported. For mitigation validation, confirm that untrusted apps cannot be installed or sideloaded, that only approved system or carrier components hold privileged radio permissions, and that no unexpected privileged applications are present.

Typical attack path

1. Identify an affected Samsung Exynos/Android device running security patches earlier than Samsung SMR Oct-2021 Release 1.
2. Gain local execution on the device through a malicious app, compromised privileged component, physical access, or another exploit chain.
3. Obtain or abuse a context with the required radio permission needed to access the vulnerable modem interface.
4. Send crafted input containing format-string sequences to the modem interface driver.
5. Trigger unsafe format-string handling in kernel context.
6. Cause a kernel panic, device crash, or reboot, resulting in denial of service.

How to detect it

Detection is difficult because the described outcome is a kernel panic rather than a durable file or network artifact. Focus on signs of prior local exploitation: repeated unexpected reboots or kernel panics on affected Samsung Exynos devices, Android tombstone/last_kmsg/pstore entries referencing modem, radio, RIL, or kernel panic activity, and suspicious applications or privileged components holding radio-related permissions. Review MDM telemetry for devices with abnormal reboot frequency after installation or update of an app. Where ADB or forensic collection is authorized, inspect logcat buffers, kernel logs, pstore/console-ramoops, tombstones, and bugreports for modem interface failures shortly before a reboot. Also review package inventories for recently installed or sideloaded apps, apps signed outside the expected vendor/carrier trust chain, and apps requesting or holding privileged telephony/radio permissions. Threat intelligence currently indicates active exploitation in the wild, but no public attacker-specific network indicators are inherent to the vulnerability.

• Kernel panic
• console-ramoops
• /sys/fs/pstore/console-ramoops
• /sys/fs/pstore/console-ramoops-0
• /proc/last_kmsg
• /data/tombstones/
• modem interface
• radio permission
• RIL
• rild
• vendor.rild
• android.permission.MODIFY_PHONE_STATE
• android.permission.READ_PRIVILEGED_PHONE_STATE
• Fatal signal
• watchdog bite
• Subsystem restart
• SSR
• modem crash

What your security team should do

• Prioritize inventory of Samsung Exynos/Android devices and identify any below Samsung SMR Oct-2021 Release 1 or without current security patch support.
• Patch affected devices to the October 2021 Samsung security release or later; replace devices that cannot be patched.
• Because this is in CISA KEV and exploited in the wild, treat remaining vulnerable devices as exceptions requiring risk acceptance, isolation, or removal from sensitive workflows.
• Use MDM controls to block sideloading, restrict unknown sources, enforce application allowlisting, and monitor privileged app installations.
• Review apps with privileged or radio-related permissions and investigate any unexpected package with those capabilities.
• Collect and retain mobile crash, reboot, kernel panic, and Android log artifacts from high-risk device fleets to support exploitation review.
• Educate support teams that repeated unexplained crashes or reboots on legacy Samsung Exynos devices may warrant security triage, not only hardware troubleshooting.