CVE-2015-3246: Red Hat Libuser Race Condition Vulnerability
Red Hat libuser contains a race condition vulnerability that allows authenticated local users to corrupt the /etc/passwd file to cause a denial of service or privilege escalation.
Overview
Libuser, the account-management library installed by default on Red Hat-derived Linux distributions, modifies /etc/passwd in place rather than writing to a temporary file and atomically renaming it. A local attacker with a shell account can exploit this behavior through the setuid-root userhelper binary to inject an arbitrary entry into /etc/passwd, including a root-equivalent account, achieving full local privilege escalation. Red Hat Enterprise Linux 6 and 7 are the primary affected platforms; libuser versions before 0.56.13-8 and 0.60 before 0.60-7 are vulnerable.
Vulnerability details
Libuser's generic_mod() function opens /etc/passwd directly with O_RDWR, writes the modified user record in place, and calls ftruncate() to trim any excess. Traditional utilities avoid this pattern by writing to a temporary file and using rename() for an atomic replacement. Because libuser holds the file open and partially written during the operation, any interruption leaves /etc/passwd in an inconsistent state. This in-place write pattern is the core of CWE-367: a window exists between successive write operations during which the file's contents are neither the old state nor the new one.
An attacker with a local account invokes the setuid-root userhelper binary repeatedly, each time supplying a crafted GECOS field sized to position a target character at a specific byte offset in /etc/passwd. Before each exec, the attacker sets RLIMIT_FSIZE to a value that causes the kernel to deliver SIGXFSZ and abort the write() at exactly the desired byte boundary, preventing ftruncate() from cleaning up the partial write. Repeating this process character by character injects an arbitrary line, such as a uid=0 account with no password, yielding local privilege escalation to root.
Severity and impact
Exploitation status
This vulnerability is confirmed as actively exploited in the wild — it is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog.
Detection and monitoring
- Monitor auditd for repeated rapid executions of userhelper by the same UID in a short window, particularly where each invocation carries a different-length GECOS argument. Normal userhelper use is infrequent; dozens of sequential forks from one user is anomalous.
- Check /etc/passwd integrity with a file-integrity monitor (e.g., auditd path watches or AIDE). Any modification to /etc/passwd outside a scheduled package update or administrator session warrants immediate review, especially additions of entries with UID 0.
- Audit RLIMIT_FSIZE changes via auditd setrlimit syscall rules scoped to processes that subsequently exec userhelper. Deliberately lowering the file-size limit immediately before invoking a setuid binary has no legitimate administrative use case.
Remediation
Additional hardening
- Update libuser to 0.56.13-8 or later on Red Hat Enterprise Linux 6, and to 0.60-7 or later on Red Hat Enterprise Linux 7, using the packages provided in RHSA-2015:1482 and RHSA-2015:1483 respectively; see References for advisory details.
- Restrict local shell access to the minimum set of users who require it. Because exploitation requires an existing /etc/passwd entry and interactive shell access, reducing the number of local accounts directly limits the attacker population.
- Apply or verify SELinux enforcing mode on affected hosts. A targeted SELinux policy can constrain userhelper's ability to write to /etc/passwd in ways that the advisory notes may affect exploit reliability.
- Audit systems for the presence of userhelper (from the usermode package) and remove it where account-management functionality is not required, eliminating the setuid-root attack surface this exploit depends on.
Key dates
References
- http://rhn.redhat.com/errata/RHSA-2015-1482.html
- http://rhn.redhat.com/errata/RHSA-2015-1483.html
- https://access.redhat.com/articles/1537873
- https://www.cisa.gov/news-events/directives/bod-26-04-prioritizing-security-updates-based-risk
- https://www.cisa.gov/news-events/directives/bod-26-04-implementation-guidance-prioritizing-security-updates-based-risk
- https://nvd.nist.gov/vuln/detail/CVE-2015-3246
- http://lists.fedoraproject.org/pipermail/package-announce/2015-August/163044.html
- http://lists.fedoraproject.org/pipermail/package-announce/2015-July/162947.html
Frequently asked questions
Does CVE-2015-3246 affect my FedRAMP authorization?
If Red Hat Libuser runs inside your authorization boundary, yes, CVE-2015-3246 affects your FedRAMP authorization. The vulnerability appears in CISA's Known Exploited Vulnerabilities (KEV) catalog with a remediation deadline of September 9, 2026. An unpatched KEV within your boundary is an assessor finding. You must either remediate it before that date or formally document a mitigation, or your sponsoring agency will raise it during review.
How does Knox help me handle CVE-2015-3246?
Knox does not patch Red Hat Libuser on your behalf. Under the FedRAMP shared-responsibility model, remediating CVE-2015-3246 is your obligation. What Knox provides is the pre-authorized, single-tenant boundary in which you carry out that remediation, plus continuous compliance monitoring and audit-artifact coverage to document the fix for your next assessment. The work of applying the patch is yours; managing your compliance posture while you do it is not something you handle alone.
How does Knox's monitoring help with vulnerabilities like this?
Knox's automated continuous monitoring platform watches your environment for newly disclosed vulnerabilities and compliance issues on an ongoing basis. When something like CVE-2015-3246 surfaces, exposure is identified during continuous monitoring rather than surfacing for the first time when an assessor reviews your boundary. That earlier signal gives you time to act before a finding becomes a formal problem.
How do I get FedRAMP authorized with Knox?
Knox runs a FedRAMP-as-a-Service platform. It gives SaaS vendors a pre-authorized cloud boundary on AWS, Azure, and GCP. Your application inherits 60-80% of the required security controls. You reach FedRAMP authorization in about 90 days for roughly 90% less than the traditional $3.5M path. Book a meeting and Knox will map your path to authorization.
What happens if CVE-2015-3246 isn't remediated by September 9, 2026?
If CVE-2015-3246 is not remediated by September 9, 2026, it becomes a Plan of Action and Milestones (POA&M) item. A growing POA&M list is what turns a routine continuous-monitoring review into a difficult conversation with your sponsoring agency. Meeting the deadline keeps your authorization in good standing and the agency relationship intact.
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