CVE-2012-5671 in Exim
Summary
by MITRE
Heap-based buffer overflow in the dkim_exim_query_dns_txt function in dkim.c in Exim 4.70 through 4.80, when DKIM support is enabled and acl_smtp_connect and acl_smtp_rcpt are not set to "warn control = dkim_disable_verify," allows remote attackers to execute arbitrary code via an email from a malicious DNS server.
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Analysis
by VulDB Data Team • 04/18/2021
The CVE-2012-5671 vulnerability represents a critical heap-based buffer overflow affecting Exim email servers running versions 4.70 through 4.80 with DKIM support enabled. This flaw exists within the dkim_exim_query_dns_txt function in the dkim.c source file, creating a significant security risk that can be exploited by remote attackers through malicious DNS server responses. The vulnerability specifically targets the handling of DNS TXT record queries used in DKIM signature verification processes, making it particularly dangerous in email infrastructure environments where DKIM validation is commonly implemented.
The technical implementation of this vulnerability stems from inadequate input validation and buffer management within the DNS TXT record processing logic. When Exim receives email with DKIM signatures and performs verification through DNS queries, the dkim_exim_query_dns_txt function fails to properly bounds-check the data received from DNS servers. This allows an attacker controlling a malicious DNS server to craft specially formatted TXT records that exceed the allocated buffer space, causing a heap overflow condition. The overflow occurs in the heap memory region where DNS query results are stored, potentially allowing attackers to overwrite adjacent memory locations and execute arbitrary code with the privileges of the Exim process.
The operational impact of this vulnerability extends beyond simple code execution, as it can enable complete system compromise when exploited successfully. Attackers can leverage this vulnerability to gain unauthorized access to email servers, potentially leading to data breaches, spam relay abuse, and further network infiltration. The vulnerability is particularly concerning because it requires minimal user interaction beyond sending malicious email, and the attack can be conducted entirely through DNS manipulation without requiring direct network access to the target server. The specific configuration requirements mentioned in the CVE description indicate that the vulnerability is mitigated when certain ACL controls are properly configured, but many installations may not have these protections enabled by default.
Mitigation strategies for CVE-2012-5671 should prioritize immediate patching of affected Exim installations to versions that address the buffer overflow issue. Organizations should also implement DNS security measures including DNSSEC validation and monitoring for suspicious DNS query patterns. Network segmentation and access controls can help limit the potential impact if exploitation occurs. The vulnerability aligns with CWE-121 heap-based buffer overflow classifications and represents a technique commonly seen in the ATT&CK framework under initial access and execution phases, specifically targeting email server infrastructure. Security teams should conduct thorough audits of their email server configurations to ensure proper ACL settings are implemented and regularly test their defenses against similar buffer overflow attack vectors that may exist in other components of their email infrastructure.