CVE-2026-67194 Courier IMAP < 6.0.1 Mail Server < 2.0.2 Stack Overflow DoS via Nested SEARCH Queries
Authenticated users can crash the Courier IMAP server by sending deeply nested SEARCH queries, causing a stack overflow and service disruption.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-67194?
CVE-2026-67194 is a high-severity denial of service vulnerability in svarshavchik Courier IMAP, published on 29 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
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.
Weakness: CWE-674
Which versions are affected by CVE-2026-67194?
How do I fix CVE-2026-67194?
Upgrade to a fixed version.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-67194 being exploited?
There is no known exploitation of CVE-2026-67194 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-67194, GHSA-mcmr-g7gg-p242, UBUNTU-CVE-2026-67194.