CVE-2010-2023 in Exim
Summary
by MITRE
transports/appendfile.c in Exim before 4.72, when a world-writable sticky-bit mail directory is used, does not verify the st_nlink field of mailbox files, which allows local users to cause a denial of service or possibly gain privileges by creating a hard link to another user s file.
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Analysis
by VulDB Data Team • 09/14/2021
The vulnerability identified as CVE-2010-2023 resides within the Exim mail transfer agent software and represents a critical security flaw in the transports/appendfile.c component. This issue affects Exim versions prior to 4.72 and stems from improper validation of file system metadata when processing mail delivery. The vulnerability specifically manifests when Exim encounters world-writable mail directories that have the sticky bit set, creating a dangerous condition where the software fails to properly verify the st_nlink field of mailbox files. The st_nlink field in Unix file systems represents the number of hard links pointing to a file, and its improper verification creates a significant security gap that can be exploited by malicious local users.
The technical exploitation of this vulnerability occurs through a carefully crafted hard link attack that manipulates the st_nlink field to bypass normal file system checks. When Exim processes mail delivery to a world-writable sticky-bit directory, it relies on the st_nlink field to determine if a file is a legitimate mailbox or if it has been manipulated by an attacker. This failure in validation allows attackers to create hard links to other users' files, potentially causing Exim to incorrectly process file operations. The attack vector specifically targets the appendfile.c module which handles file appending operations, making it particularly dangerous as it can be leveraged to either cause a denial of service by corrupting file operations or potentially escalate privileges by manipulating file access patterns and ownership structures.
The operational impact of this vulnerability extends beyond simple denial of service scenarios to encompass potential privilege escalation capabilities that can severely compromise system security. Local users with access to the affected mail directories can exploit this flaw to either disrupt mail services by causing Exim to malfunction or to gain unauthorized access to other users' mail files. The vulnerability creates a persistent threat that can be maintained across system reboots, as the hard link manipulation persists until manually corrected. This issue particularly affects multi-user systems where Exim handles mail delivery to shared directories, making it a significant concern for server administrators managing multiple user accounts with varying privilege levels. The vulnerability's classification aligns with CWE-276, which addresses improper file permissions and access control issues, and can be mapped to ATT&CK technique T1068, which involves local privilege escalation through exploitation of system vulnerabilities.
Mitigation strategies for CVE-2010-2023 require immediate implementation of several protective measures to secure Exim installations. System administrators should upgrade to Exim version 4.72 or later, which includes the necessary patches to properly validate the st_nlink field during file operations. Additionally, the affected mail directories should be secured by removing the world-writable permissions or ensuring that the sticky bit is properly enforced. Regular monitoring of file system metadata and hard link creation should be implemented as part of security auditing procedures. The vulnerability also highlights the importance of proper file system permission management and access control enforcement, particularly in multi-user environments where shared resources are common. Organizations should conduct comprehensive security assessments to identify all instances of world-writable sticky-bit directories that might be vulnerable to similar exploitation patterns. The remediation process should include not only software updates but also policy enforcement to prevent future occurrences of similar permission misconfigurations that could create analogous security risks.