CVE-2021-25489

What is the Samsung Modem Interface Driver?

Samsung Galaxy devices include a modem interface driver that manages communication between the Android application processor and the baseband modem processor (handling cellular radio communications). This kernel-mode driver processes commands, configuration data, and status messages exchanged between Android and the baseband firmware. Because the modem interface processes string data for logging, debugging, or status reporting, improper handling of format specifiers in those strings — a format string vulnerability — can cause the kernel to interpret attacker-controlled data as printf-style format specifiers. In the kernel context, an uncontrolled format string causes a kernel panic (system crash) at minimum, and in some cases can be leveraged for information disclosure or code execution.

Overview

CVE-2021-25489 is an improper input validation vulnerability (CWE-20) in the Samsung Galaxy modem interface driver that results in a format string bug causing a kernel panic. When the modem interface driver processes input containing format string specifiers (%n, %s, %x, etc.) without sanitization, it passes attacker-controlled strings directly to a kernel format string function, causing uncontrolled kernel memory access and a system crash (availability impact). Patched in Samsung's October 2021 Security Bulletin. CISA added it to KEV in June 2023 alongside multiple other Samsung kernel driver CVEs (CVE-2021-25394/25395/25371/25372), indicating Samsung-specific driver vulnerabilities were being actively exploited in targeted attacks.

Affected Versions

Technical Details

• Root cause: Improper input validation (CWE-20) leading to a format string bug in the modem interface kernel driver — input received from the modem or processed by the driver interface is passed as the format string argument to a kernel logging or printf-family function without sanitization; attacker-controlled format specifiers in the input cause the kernel to interpret stack or heap memory as format arguments
• Format string in kernel context: A kernel format string vulnerability with %n specifier can write to arbitrary kernel memory addresses; in practice, uncontrolled kernel format strings reliably cause kernel panics (%s dereferencing invalid pointers) even when arbitrary write is not achieved; the CVSS A:L (Low availability) reflects the kernel panic DoS outcome
• Modem interface attack surface: The modem interface driver processes data from the baseband modem firmware, which may be indirectly controllable by an attacker who has compromised the baseband (via separate baseband vulnerabilities) or by an attacker who can inject data into the modem communication channel; the AV:L/PR:L suggests exploitation requires a local attacker with some kernel or modem access
• June 2023 KEV context: CISA's addition of CVE-2021-25489 in June 2023 (simultaneously with CVE-2021-25394/25395/25371/25372) indicates it was discovered as part of the same Samsung Galaxy exploitation investigation — likely confirming that the modem interface vulnerability was used alongside kernel UAF and DSP vulnerabilities in a comprehensive Samsung exploitation toolkit
• Availability impact in chain context: While the standalone impact is Low (kernel panic / DoS), format string vulnerabilities in kernel drivers can be chained with information leaks or used to map kernel memory layout; the primary in-chain use may be as a persistence disruption tool or to trigger specific kernel states that support other exploitation steps

Discovery

Patched in Samsung's October 2021 Security Bulletin. CISA's June 2023 KEV addition alongside multiple other Samsung-specific kernel driver vulnerabilities confirms the modem interface vulnerability was part of the broader Samsung Galaxy exploitation toolkit documented in targeted surveillance investigations.

Exploitation Context

Samsung modem interface driver vulnerabilities have unique significance because the modem processor operates as a separate, highly-privileged component with access to all cellular communications. Format string bugs in the modem interface driver can be triggered by actors who control data flowing through the modem communication channel — including sophisticated actors with the ability to target specific devices via cellular network positioning. While CVE-2021-25489's primary observed impact is kernel panic, modem-path vulnerabilities are of interest to advanced persistent threat actors targeting mobile device surveillance, where disrupting device function or mapping kernel memory via the modem interface supports broader exploitation objectives.

Remediation

1. Apply Samsung October 2021 Security Bulletin updates — patches the format string vulnerability in the modem interface driver
2. Verify security patch level is 2021-10-01 or later: Settings → About Phone → Android Security Update
3. Enable automatic Samsung security updates
4. For enterprise MDM: enforce minimum October 2021 Samsung security patch level for managed Samsung Galaxy devices
5. Samsung Knox Real-time Kernel Protection (RKP) provides kernel integrity monitoring that can detect kernel panic conditions caused by format string exploitation attempts
6. Replace Samsung Galaxy devices that no longer receive security updates — end-of-life devices remain permanently vulnerable to modem interface and other kernel driver vulnerabilities