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.
Attack Parameters
Attack Vector
Network
Can be exploited remotely
Attack Complexity
Low
Easy to exploit
Attack Requirements
None
No additional conditions
Privileges Required
Low
Basic privileges needed
User Interaction
None
No user interaction needed
Impact Assessment
Confidentiality
None
No data leak
Integrity
None
No data modification
Availability
High
Complete denial of service
CVSS Vector v4.0
Weakness Type (CWE)
References 5
https://github.com/svarshavchik/courier-libs/commit/b5b5581aabea3efadf5e2944947…
disclosure@vulncheck.com
https://github.com/svarshavchik/courier/releases/tag/courier%2F2.0.2%2F20260627…
disclosure@vulncheck.com
https://github.com/svarshavchik/courier/releases/tag/courier-imap%2F6.0.1%2F202…
disclosure@vulncheck.com
https://packages.debian.org/sid/courier-imap
disclosure@vulncheck.com
https://www.vulncheck.com/advisories/courier-imap-mail-server-stack-overflow-do…
disclosure@vulncheck.com