CVE-2026-67194 in Courier IMAP
Summary
by MITRE • 07/29/2026
Courier IMAP before 6.0.1 and Courier Mail Server before 2.0.2 allow authenticated IMAP users to crash the imapd process via deeply nested parenthesized SEARCH queries. The SEARCH command parser (alloc_search_key in searchinfo.C) recursively descends on nested parenthesized groups through a mutual recursion chain with alloc_search_andlist() and alloc_search_notkey(), with no depth limit. Courier IMAP has no overall command line length limit, making exploitation trivial. A single IMAP command with ~2500 nested parentheses overflows the 8MB default stack, causing SIGSEGV.
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Analysis
by VulDB Data Team • 07/29/2026
This vulnerability represents a critical stack-based buffer overflow in Courier IMAP and Mail Server implementations that stems from inadequate input validation during SEARCH command processing. The flaw exists in the searchinfo.C source file where the alloc_search_key function performs recursive descent through nested parenthesized groups without implementing any depth limiting mechanisms. This recursive traversal creates a mutual recursion chain between alloc_search_andlist() and alloc_search_notkey() functions, allowing attackers to craft malicious SEARCH queries that exploit this unbounded recursion pattern. The vulnerability is particularly dangerous because Courier IMAP lacks overall command line length restrictions, making it trivial for authenticated users to construct and execute exploitative payloads.
The technical implementation of this vulnerability leverages the default 8MB stack limit on most Unix-like systems to trigger a segmentation fault through stack overflow conditions. An attacker needs only to create a single IMAP SEARCH command containing approximately 2500 nested parentheses to successfully crash the imapd process. This recursive descent mechanism operates without any depth counter or maximum nesting limit enforcement, allowing the call stack to grow uncontrollably until it exceeds system limits. The absence of input sanitization for search query structures means that legitimate authenticated users can exploit this weakness to cause denial-of-service conditions, effectively disrupting email services for all legitimate users.
The operational impact of this vulnerability extends beyond simple service disruption as it provides attackers with a reliable method to crash the IMAP daemon and potentially gain insights into system behavior through controlled crashes. This type of vulnerability aligns with CWE-674 (Uncontrolled Recursion) and represents a classic example of how insufficient input validation can lead to resource exhaustion attacks. From an adversarial perspective, this vulnerability enables attackers to perform persistent denial-of-service operations against email infrastructure without requiring special privileges beyond authentication access. The implementation also creates opportunities for information leakage through crash analysis, as the stack overflow behavior may reveal memory layout details or system configuration information.
Organizations using affected Courier IMAP and Mail Server versions should implement immediate mitigations including updating to patched releases that introduce depth limiting mechanisms in search query parsing. System administrators should consider implementing command line length restrictions and monitoring for unusual SEARCH command patterns that might indicate exploitation attempts. Network-level protections such as rate limiting and connection filtering can help reduce the impact of automated exploitation attempts. The vulnerability demonstrates the importance of input validation and resource management in server-side applications, reinforcing best practices from the OWASP Top Ten and MITRE ATT&CK framework categories related to resource exhaustion and input validation flaws. Organizations should also implement proper logging and monitoring of IMAP SEARCH commands to detect potential exploitation attempts.