CVE-2025-1390: libcap PAM Privilege Escalation Flaw

CVE-2025-1390 Overview

A vulnerability exists in the PAM module pam_cap.so of libcap that allows local privilege escalation through improper parsing of configuration entries in /etc/security/capability.conf. The module supports group names starting with "@", but during parsing, configurations not starting with "@" are incorrectly recognized as group names. This parsing flaw allows attackers to craft specific usernames that result in unintended users being granted inherited capability sets.

Critical Impact

Attackers with local access can exploit this configuration parsing flaw to escalate privileges by constructing usernames that match intended group name entries, resulting in unauthorized capability inheritance.

Affected Products

• libcap pam_cap.so PAM module
• Systems using /etc/security/capability.conf for user inherited privileges

Discovery Timeline

• 2025-02-18 - CVE CVE-2025-1390 published to NVD
• 2025-02-18 - Last updated in NVD database

Technical Details for CVE-2025-1390

Vulnerability Analysis

This vulnerability stems from improper access control (CWE-284) in the pam_cap.so PAM module's configuration parser. The PAM module is designed to manage Linux capabilities that can be inherited by user sessions, configured through /etc/security/capability.conf. The configuration format uses "@" as a prefix to denote group names, distinguishing them from individual usernames.

The flaw occurs because the parsing logic incorrectly identifies entries without the "@" prefix as group names under certain conditions. When an attacker creates a username that matches an entry intended to grant capabilities to a group, the system may erroneously grant those elevated capabilities to the attacker's session.

Linux capabilities provide fine-grained control over privileged operations, and improper assignment can allow users to perform actions normally restricted to root, such as binding to privileged ports, bypassing file permission checks, or manipulating process credentials.

Root Cause

The root cause is a logic error in the configuration parsing code of pam_cap.so. The parser fails to properly enforce the "@" prefix requirement for group name identification, leading to a condition where plain configuration entries are misinterpreted. This allows username strings to match against what should be group-only capability assignments.

Attack Vector

The attack requires local access to the system and the ability to create or control a user account. An attacker can exploit this vulnerability through the following mechanism:

1. The attacker identifies capability configurations in /etc/security/capability.conf that grant elevated privileges to groups
2. The attacker creates a username (or uses an existing account) that matches the group name entry without the "@" prefix
3. When the user authenticates, pam_cap.so incorrectly parses the configuration and grants the user the inherited capabilities intended for group members
4. The attacker's session now has elevated capabilities, enabling privilege escalation

The vulnerability is exploited locally (AV:L) with low attack complexity (AC:L) and requires low privileges (PR:L) to execute. No user interaction is needed. The primary impact is on integrity, as the attacker gains unauthorized capabilities.

Detection Methods for CVE-2025-1390

Indicators of Compromise

• Unusual user accounts with names matching group entries in /etc/security/capability.conf
• Processes running with unexpected Linux capabilities for non-privileged users
• Authentication logs showing successful logins for suspicious usernames that match group names
• Capability auditing showing unexpected capability grants during PAM authentication

Detection Strategies

• Monitor /etc/security/capability.conf for configuration entries that could be exploited (non-group entries granting high-value capabilities)
• Implement file integrity monitoring on /etc/security/capability.conf to detect unauthorized modifications
• Use getcap and capability auditing tools to enumerate processes with unexpected inherited capabilities
• Review user creation logs for accounts with names that suspiciously match group names defined in capability configurations

Monitoring Recommendations

• Enable PAM debugging to log detailed authentication decisions including capability assignments
• Configure auditd rules to track changes to /etc/security/capability.conf and user/group management operations
• Deploy endpoint detection solutions to monitor for privilege escalation patterns following user authentication
• Regularly audit user account names against capability configuration entries to identify potential exploitation vectors

How to Mitigate CVE-2025-1390

Immediate Actions Required

• Review /etc/security/capability.conf to identify entries that could be exploited through this parsing flaw
• Audit existing user accounts for names that match any capability configuration entries
• Ensure all group-based capability assignments use the "@" prefix correctly
• Consider temporarily disabling pam_cap.so if inherited capabilities are not business-critical until a patch is applied

Patch Information

Refer to the OpenAnolis Bug Report #18804 for the latest patch status and updates from the libcap maintainers. System administrators should monitor their distribution's security advisories for backported fixes.

Workarounds

• Remove or comment out capability configuration entries in /etc/security/capability.conf that do not explicitly use the "@" prefix for group designations
• Implement strict user naming policies that prevent creation of accounts matching group names
• Use alternative capability management mechanisms that are not affected by this parsing vulnerability
• Restrict access to user creation functions to prevent attackers from creating malicious usernames

Administrators should audit their configuration file to ensure proper formatting. Group entries should always begin with the "@" symbol. Review and restrict any capability assignments to ensure they follow the documented format and cannot be exploited through username manipulation.