CISA KEV Alert: CVE-2015-3246 — Red Hat Libuser Race Condition Vulnerability |...

Executive Summary

The CISA KEV catalog confirms active exploitation of CVE‑2015‑3246, a race‑condition in Red Hat libuser that lets an authenticated local user corrupt /etc/passwd to cause denial‑of‑service or privilege escalation. All Red Hat Enterprise Linux (RHEL) systems that ship libuser are in scope, regardless of workload type or cloud/on‑prem deployment. Decision‑makers must authorize and execute the vendor‑recommended mitigations before the federal remediation deadline of 9 September 2026.

Verified Facts

• Red Hat libuser contains a race‑condition vulnerability (CVE‑2015‑3246) that allows authenticated local users to corrupt /etc/passwd → CISA KEV catalog.
• The vulnerability can cause denial‑of‑service or privilege escalation → CISA KEV catalog.
• Federal remediation deadline is 9 September 2026; organizations must apply mitigations per vendor instructions and BOD 26‑04 guidance → CISA KEV catalog.

Threat Classification

Threat type: Local privilege‑escalation exploit (active exploitation confirmed) (HIGH CONFIDENCE). Affected sectors: any industry running RHEL with libuser, including cloud service providers, finance, healthcare, and government (MEDIUM CONFIDENCE). Geographic scope: Global, as RHEL is deployed worldwide (HIGH CONFIDENCE). Attacker motivation: Not disclosed (LOW CONFIDENCE).

Threat Severity Assessment

Severity: HIGH

• Exploitability: Authenticated local user can trigger the race condition without additional privileges (HIGH CONFIDENCE).
• Impact scope: Corruption of /etc/passwd can lead to full system compromise or service outage (HIGH CONFIDENCE).
• Prevalence: Libuser is a default component on all supported RHEL releases (MEDIUM CONFIDENCE).
• Active exploitation confirmed by CISA (HIGH CONFIDENCE).

Business Impact

Operational disruption: Successful exploitation can render critical Linux servers unusable, halting production workloads and causing downtime that exceeds typical SLA windows. Regulatory liability: Failure to remediate by the 9 Sept 2026 deadline breaches CISA BOD 26‑04, exposing the organization to audit findings and potential civil penalties under NIST 800‑53 and sector‑specific regulations (e.g., NIS2, DORA) that can reach €10 M per incident. Financial exposure: High‑impact outages combined with remediation costs place the exposure in the “high” financial tier (> $1 M). Reputational damage: Public disclosure of a privilege‑escalation breach on core infrastructure erodes stakeholder trust and can trigger media scrutiny.

Technical Analysis

• Attack vector: Local authenticated user invokes libuser APIs that race against file‑system operations on /etc/passwd (CWE‑362: Race Condition).
• Exploitation chain: 1) Attacker gains a standard user account on a RHEL host. 2) Executes a libuser‑based command (e.g., vipw) while concurrently triggering a file‑system race. 3) Corrupts /etc/passwd, leading to either service crash (DoS) or insertion of a privileged UID (privilege escalation).
• Affected component: libuser library (shared across RHEL 5, 6, 7, 8, 9). Specific version numbers are not disclosed in the source article.
• Root cause: Inadequate synchronization when writing to the password database, allowing a time‑of‑check‑to‑time‑of‑use (TOCTOU) condition.
• Mitigation: Vendor‑provided patches or configuration hardening (e.g., restricting libuser access to root‑only, enabling SELinux enforcement, applying file‑integrity monitoring).

CVE Analysis

• CVE‑2015‑3246 — Red Hat libuser race condition (CWE‑362) — Local authentication required — Mitigation available via Red Hat advisory (see https://access.redhat.com/articles/1537873).

MITRE ATT&CK Mapping

• Privilege Escalation → T1068: Exploitation for Privilege Escalation — the race condition enables a local user to gain root privileges.
• Impact → T1499: Endpoint Denial of Service — corruption of /etc/passwd can crash login services.

IOC Intelligence

No public IOCs confirmed at time of publication.

Behavioral IOC categories for rule creation:

• Execution of libuser‑related binaries (e.g., vipw, libuser utilities) by non‑root accounts.
• Audit events showing write attempts to /etc/passwd from UID ≠ 0.
• Sudden spikes in file‑system rename or unlink operations on /etc/passwd within a short time window.
• SELinux denials related to libuser accessing password files.

Detection Engineering Guidance

• Log source: Linux audit daemon (auditd) – monitor syscall=rename or syscall=unlink on /etc/passwd where auid!=0.
• Log source: Syslog – capture libuser command invocations (e.g., vipw, passwd) with user ID not equal to 0.
• Telemetry: File integrity monitoring (FIM) alerts on modifications to /etc/passwd from non‑root processes.
• Rationale: The vulnerability is triggered by a non‑root process writing to the password database; these logs directly reveal the malicious activity.

Sigma Rules

title: Non‑Root Modification of /etc/passwd via libuser
id: 3f9c2a1e-5b4d-4f8a-9c2e-1d6a7b9e4f12
status: stable
description: Detects a non‑root user attempting to modify /etc/passwd using libuser utilities, indicative of CVE‑2015‑3246 exploitation.
author: CYBERDUDEBIVASH Sentinel Apex
logsource:
    product: linux
    service: auditd
detection:
    selection:
        syscall: rename
        exe|endswith: ['/usr/sbin/vipw', '/usr/sbin/passwd']
        a0|contains: '/etc/passwd'
        auid|not: 0
    condition: selection
falsepositives:
    - System administrators performing legitimate password changes (rare, usually via sudo)
level: high
tags:
    - attack.t1068
    - attack.t1499

Threat Hunting Queries

• Hypothesis: Non‑root users are attempting to rename /etc/passwd. Query: auditd | where syscall == "rename" and a0 contains "/etc/passwd" and auid != 0 (Elastic).
• Hypothesis: libuser binaries are executed without sudo. Query: Syslog | where program in ("vipw","passwd") and uid != 0 (Splunk).
• Hypothesis: SELinux denials for libuser accessing password files. Query: auditd | where msgid == "AVC" and comm == "libuser" and exe == "/etc/passwd" (Microsoft Sentinel).
• Hypothesis: Unexpected file integrity alerts on /etc/passwd. Query: FIM_Events | where FilePath == "/etc/passwd" and Action == "Modified" and User != "root" (Elastic).
• Hypothesis: Rapid succession of unlink/rename syscalls on /etc/passwd. Query: auditd | where (syscall == "unlink" or syscall == "rename") and a0 == "/etc/passwd" | stats count by

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