CVE-2026-21111: Samsung One UI Buffer Overflow Vulnerability

CVE-2026-21111 Overview

CVE-2026-21111 is an out-of-bounds write vulnerability in libsthmbc.so, a native library shipped in Samsung mobile devices running One UI. Local attackers with low privileges can write beyond allocated memory bounds by interacting with the vulnerable library. Samsung addressed the flaw in One UI 8.5. The issue is tracked under CWE-787: Out-of-bounds Write and requires local access on the affected device.

Critical Impact
Local attackers can corrupt adjacent memory in a privileged process, potentially leading to code execution, privilege escalation, or application compromise on affected Samsung devices.

Affected Products

Samsung Mobile Devices running One UI prior to version 8.5
Samsung libsthmbc.so native library
Samsung Android-based mobile firmware bundling the affected component

Discovery Timeline

2026-09-09 - CVE-2026-21111 published to NVD
2026-09-10 - Last updated in NVD database

Technical Details for CVE-2026-21111

Vulnerability Analysis

The vulnerability resides in libsthmbc.so, a shared library present on Samsung mobile devices. An out-of-bounds write occurs when the library processes attacker-controlled input without correctly validating buffer boundaries. This allows a local process to write data past the end of an allocated buffer.

Out-of-bounds writes in native Android libraries frequently corrupt heap metadata, adjacent object fields, or function pointers. When the vulnerable code runs within a privileged system service, the memory corruption can escalate into code execution in that service's security context. The vulnerability requires local access and low privileges, which typically maps to a malicious or compromised application on the device.

Root Cause

The root cause is missing or insufficient bounds checking in libsthmbc.so prior to One UI 8.5. The library writes data to a buffer without confirming that the destination offset or length remains within the allocated region. This condition falls under [CWE-787: Out-of-bounds Write], one of the most impactful memory safety defects in native code.

Attack Vector

Exploitation requires local access to the device and the ability to invoke functionality exposed by libsthmbc.so. An attacker delivers a malicious application through sideloading or a third-party store, then triggers the vulnerable code path with crafted input. Successful exploitation corrupts memory in the process consuming the library, undermining application isolation on the device. No verified public proof-of-concept exists for this issue.

See the Samsung Mobile Security Update for vendor-supplied technical details.

Detection Methods for CVE-2026-21111

Indicators of Compromise

Repeated crashes, tombstones, or SIGSEGV/SIGABRT signals in processes that load libsthmbc.so.
Newly installed or sideloaded applications that invoke Samsung-specific system libraries outside expected usage patterns.
Unexpected privilege transitions or service restarts on affected One UI versions prior to 8.5.

Detection Strategies

Inventory devices in the fleet and flag any Samsung device running One UI versions earlier than 8.5.
Review Mobile Threat Defense (MTD) telemetry for anomalous native crashes referencing libsthmbc.so in the stack trace.
Correlate application install events with subsequent native process instability to surface potential exploitation attempts.

Monitoring Recommendations

Forward Android tombstone and logcat data from managed devices to a centralized analytics pipeline for retrospective hunting.
Alert on installation of applications requesting privileged Samsung APIs from unmanaged sources.
Track One UI patch level compliance across the fleet and generate exceptions for devices below 8.5.

How to Mitigate CVE-2026-21111

Immediate Actions Required

Update affected Samsung devices to One UI 8.5 or later through the official system update channel.
Restrict sideloading and enforce installation only from vetted enterprise or official application stores using Mobile Device Management (MDM) policy.
Audit installed applications on managed devices and remove untrusted or unnecessary apps that could serve as a local delivery vector.

Patch Information

Samsung addressed CVE-2026-21111 in One UI 8.5. Details are published in the September 2026 Samsung Mobile Security Update. Administrators should confirm devices report a security patch level consistent with that bulletin.

Workarounds

No vendor-supplied workaround is documented; apply the One UI 8.5 update as the primary remediation.
Enforce MDM policies that block installation of applications from unknown sources on affected devices until patching completes.
Segment high-risk users onto devices already updated to One UI 8.5 while remediation proceeds across the fleet.

bash

# Verify Samsung One UI version on a managed device via ADB
adb shell getprop ro.build.version.oneui
adb shell getprop ro.build.version.release
adb shell getprop ro.build.version.security_patch