CVE-2026-58839: MountRegistry.cpp Buffer Overflow Vulnerability

CVE-2026-58839 Overview

CVE-2026-58839 is an out-of-bounds read vulnerability in the forEachLine function of MountRegistry.cpp within Android. The flaw stems from a buffer overflow condition that allows a local attacker to read memory outside intended bounds. Successful exploitation results in local privilege escalation without requiring additional execution privileges or user interaction. The issue is tracked under CWE-120: Buffer Copy without Checking Size of Input and was disclosed as part of the Android Security Bulletin for September 2026.

Critical Impact
A local attacker with low privileges can escalate to higher privileges on affected Android devices without user interaction.

Affected Products

Android platform components referenced in the Android Security Bulletin September 2026
Devices incorporating the vulnerable MountRegistry.cpp implementation
Systems that have not yet applied the September 2026 Android security patch level

Discovery Timeline

2026-09-08 - CVE-2026-58839 published to NVD
2026-09-08 - Last updated in NVD database

Technical Details for CVE-2026-58839

Vulnerability Analysis

The vulnerability resides in the forEachLine function of MountRegistry.cpp, a component involved in parsing mount registry entries. The function iterates over lines in a buffer but fails to correctly validate boundary conditions during the read operation. This creates an out-of-bounds read primitive that leaks adjacent memory contents to the calling context.

Because the affected code path executes in a context with elevated privileges relative to the caller, the leaked data can be leveraged to escalate privileges locally. The attacker does not need to trigger user interaction, making the flaw suitable for chaining with other local exploits.

Root Cause

The root cause is improper size validation during buffer traversal, classified as [CWE-120]. The forEachLine routine advances through buffer contents without enforcing the declared buffer length as an upper bound. When input data lacks expected terminators or contains malformed line boundaries, the read continues past the allocated region.

Attack Vector

Exploitation requires local access to the affected device with low-privilege user context. An attacker delivers crafted input to a component that invokes forEachLine on attacker-influenced data. The out-of-bounds read returns memory contents from adjacent allocations, which the attacker uses to defeat address space layout randomization or extract sensitive kernel or system data to complete a privilege escalation chain.

No public proof-of-concept exploit is available at this time. Technical details are documented in the Android Security Bulletin September 2026.

Detection Methods for CVE-2026-58839

Indicators of Compromise

Unexpected process crashes or SIGSEGV signals originating from components that invoke mount registry parsing routines
Anomalous privilege transitions from unprivileged application UIDs to system or root contexts
Unusual read access patterns targeting mount configuration files on the device

Detection Strategies

Monitor Android security patch levels across managed device fleets to identify devices missing the September 2026 update
Review application behavior for repeated invocations of mount-related system interfaces from non-system applications
Correlate local privilege escalation attempts with recently installed or updated third-party applications

Monitoring Recommendations

Enroll devices in mobile device management (MDM) solutions that report Android security patch level in near real time
Track process crash telemetry for repeated faults in components handling mount registry data
Alert on installation of applications from unverified sources that request elevated system capabilities

How to Mitigate CVE-2026-58839

Immediate Actions Required

Apply the September 2026 Android security patch or later on all managed devices
Verify the reported Android security patch level reads 2026-09-01 or later after update
Restrict installation of applications from unknown sources on managed device fleets
Prioritize patching for devices used by high-value users or in sensitive environments

Patch Information

Google addressed CVE-2026-58839 in the September 2026 Android security patch. Device manufacturers integrate the fix into vendor builds and distribute updates through their standard channels. Refer to the Android Security Bulletin September 2026 for the complete list of affected components and patch identifiers.

Workarounds

No official workaround has been published; applying the vendor patch is the recommended remediation
Limit local access to devices pending patch deployment by enforcing screen lock and physical security controls
Reduce the attack surface by uninstalling untrusted third-party applications until updates are available

bash

# Verify Android security patch level on a device via adb
adb shell getprop ro.build.version.security_patch
# Expected output for patched devices: 2026-09-01 or later