Android Kernel eventpoll.c Use-After-Free Vulnerability (CVE-2021-1048) Analysis and Mitigation Guide
Executive & SecOps Summary
SECOPS ASSESSMENTAndroid kernel and upstream Linux kernel integrated into mobile devices and embedded systems
Local attackers can corrupt kernel memory and escalate privileges to Root without requiring any user interaction or additional execution privileges
Apply the Android November 2021 Security Bulletin patch or upgrade to the patched upstream Linux kernel version
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Device manufacturers and system administrators must apply the Android security patch level of 2021-11-01 or later. For custom kernel builds, merge the official upstream fix for fs/eventpoll.c.
# Example: Syncing and merging the patched Android common kernel
cd /path/to/android/kernel
git fetch aosp
git merge aosp/kernel/common/android-4.19-stable
# Rebuild the kernel
make ARCH=arm64 defconfig
make ARCH=arm64 -j$(nproc)2. Temporary Workaround & Hardening Configuration
If immediate patching is not feasible, restrict access to debugging interfaces and enforce strict SELinux policies to limit the attack surface of untrusted applications.
# Force SELinux to Enforcing mode
setenforce 1
# Restrict ptrace scope to prevent local process injection and debugging abuse
sysctl -w kernel.yama.ptrace_scope=33. Log Audit & IoC Verification Commands
Verify the current security patch level and inspect kernel logs for potential exploitation attempts or epoll-related crashes.
# Check the current Android security patch level
getprop ro.build.version.security_patch
# Audit kernel ring buffer for epoll-related Use-After-Free or crash signatures
dmesg | grep -Ei 'use-after-free|epoll|ep_loop_check'Block Kernel Privilege Escalation & Ransomware Payloads Before You Can Reboot
Protect unpatched Windows, Linux, and macOS workloads against in-memory exploitation and lateral movement with autonomous EDR.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2021-1048) is a Use-After-Free (UAF) flaw located in the ep_loop_check_proc function within fs/eventpoll.c of the Linux kernel. During the validation of epoll file descriptor loops and reference relationships, the reference counting mechanism fails to properly manage object lifecycles, leading to a scenario where the kernel references an already freed memory block. This results in kernel memory corruption.
2. Exploitation Mechanics & Attack Scenario
A local attacker with basic execution privileges can construct complex circular references (loops) by creating multiple epoll instances. During the execution of ep_loop_check_proc, the attacker triggers a race condition or abnormal file descriptor release sequence to free specific epitem or eventpoll structures prematurely. By leveraging heap spraying techniques, the attacker reclaims the freed memory space with controlled data. When the kernel subsequently dereferences the stale pointer, the attacker hijacks the control flow to execute arbitrary code with kernel privileges, requiring no user interaction.
3. Infrastructure & Business Impact
As a vulnerability listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, this flaw has been actively exploited in the wild. Successful exploitation allows malicious local applications or compromised shell accounts to bypass the Android sandbox and gain full kernel-level (Root) access. This compromises the entire device, leading to sensitive data exfiltration, system instability, and persistent backdoor installation, posing a severe threat to enterprise mobile fleets and user data security.
Official Vendor Advisories & References
In ep_loop_check_proc of eventpoll.c, there is a possible way to corrupt memory due to a use after free. This could lead to local escalation of privilege with no additional execution privileges needed. User interaction is not needed for exploitation.Product: AndroidVersions: Android kernelAndroid ID: A-204573007References: Upstream kernel
Frequently Asked SecOps Questions (CVE-2021-1048)
FAQQ1. How critical is CVE-2021-1048 and is it actively exploited in the wild?
CVE-2021-1048 is rated CVSS 7.8 (HIGH) with a 30-day EPSS exploitation probability of 1.1%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2021-1048?
Primary impacted vendors include google, linux across android, linux_kernel. Scope summary: All devices running vulnerable versions of the Android kernel (Android ID: A-204573007) and corresponding upstream Linux kernels
Q3. What temporary mitigations or workarounds exist for CVE-2021-1048 before patching?
[Mitigation] Apply the Android November 2021 Security Bulletin patch or upgrade to the patched upstream Linux kernel version Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
Generate an Internal Engineering Notice or CISO Report for CVE-2021-1048
Instantly format this advisory into an executive CISO summary or engineering patch ticket.
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