CVE-2010-2024 in Exim
Summary
by MITRE
transports/appendfile.c in Exim before 4.72, when MBX locking is enabled, allows local users to change permissions of arbitrary files or create arbitrary files, and cause a denial of service or possibly gain privileges, via a symlink attack on a lockfile in /tmp/.
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Analysis
by VulDB Data Team • 09/14/2021
The vulnerability identified as CVE-2010-2024 resides within the Exim mail transfer agent software and represents a significant local privilege escalation flaw that affects versions prior to 4.72. This issue specifically manifests when the MBX locking mechanism is enabled, creating a dangerous race condition scenario that can be exploited by local attackers to manipulate file permissions and potentially execute arbitrary code with elevated privileges. The vulnerability stems from improper handling of lockfiles in the /tmp/ directory, which serves as a critical attack surface for malicious actors seeking to compromise system integrity.
The technical implementation of this vulnerability involves a symlink attack against lockfiles that Exim creates during message processing operations. When MBX locking is enabled, Exim attempts to create lockfiles in the /tmp/ directory to prevent concurrent access to mailboxes. However, the software does not properly validate the existence and ownership of these lockfiles before attempting to modify them, creating a window where a malicious user can establish a symbolic link pointing to a target file of their choosing. This race condition allows attackers to manipulate permissions of arbitrary files, create new files in locations where they should not have write access, or cause the system to behave unpredictably during the file creation process.
The operational impact of this vulnerability extends beyond simple privilege escalation to encompass potential denial of service conditions and arbitrary code execution capabilities. Local attackers can exploit this flaw to modify critical system files, inject malicious code into the mail processing pipeline, or disrupt normal mail delivery operations. The vulnerability's classification as a local privilege escalation issue means that an attacker with minimal system access can potentially elevate their privileges to root level, making it particularly dangerous in multi-user environments where different users may have varying levels of access. The attack vector is particularly concerning because it leverages the legitimate /tmp/ directory which is typically world-writable, making it a natural target for such exploitation techniques.
This vulnerability aligns with CWE-367, which describes the Time-of-Check to Time-of-Use (TOCTOU) race condition flaw, and demonstrates how improper file access validation can lead to serious security implications. The attack pattern follows established methodologies described in the ATT&CK framework under privilege escalation techniques, specifically targeting local system resources to gain unauthorized access. Organizations running Exim versions prior to 4.72 should prioritize immediate remediation, as the vulnerability can be exploited without any special privileges beyond basic system access. The fix implemented in Exim 4.72 involves proper validation of lockfile ownership and permissions before any file operations are performed, eliminating the race condition that enabled the symlink attack. System administrators should also implement monitoring for unusual file creation patterns in the /tmp/ directory and ensure that all mail server installations are updated to the patched versions to prevent exploitation of this critical vulnerability.