CVE-2026-35538 Overview
CVE-2026-35538 is an IMAP injection vulnerability affecting Roundcube Webmail before versions 1.5.14 and 1.6.14. The vulnerability exists due to unsanitized IMAP SEARCH command arguments, which could allow an attacker to perform IMAP injection attacks or bypass Cross-Site Request Forgery (CSRF) protections during mail search operations.
Critical Impact
Authenticated attackers may exploit improper input sanitization in the mail search functionality to inject malicious IMAP commands or bypass CSRF protections, potentially compromising email integrity.
Affected Products
- Roundcube Webmail versions prior to 1.5.14
- Roundcube Webmail versions prior to 1.6.14
- Roundcube Webmail versions prior to 1.7-rc5
Discovery Timeline
- April 3, 2026 - CVE-2026-35538 published to NVD
- April 7, 2026 - Last updated in NVD database
Technical Details for CVE-2026-35538
Vulnerability Analysis
This vulnerability falls under CWE-88 (Improper Neutralization of Argument Delimiters in a Command), commonly known as Argument Injection. The flaw stems from inadequate sanitization of user-supplied input when constructing IMAP SEARCH commands within Roundcube Webmail's mail search functionality.
IMAP (Internet Message Access Protocol) uses a text-based command structure where commands and arguments are delimited by specific characters. When user input is incorporated into these commands without proper validation or escaping, attackers can inject additional IMAP commands or manipulate the intended command structure.
The vulnerability requires authenticated access to exploit, as the attacker must be logged into the Roundcube webmail interface to perform mail search operations. The attack is network-based but requires high complexity to execute successfully, limiting the immediate risk.
Root Cause
The root cause of CVE-2026-35538 is improper neutralization of special characters and argument delimiters in user-supplied search parameters before they are passed to the IMAP SEARCH command. The affected code paths failed to adequately sanitize or escape metacharacters that have special meaning in the IMAP protocol, allowing attackers to break out of the intended search context.
Attack Vector
The attack vector is network-based and requires low privileges (authenticated user access). An attacker with valid credentials to a Roundcube Webmail instance can craft malicious search queries containing IMAP metacharacters or command sequences. When processed by the vulnerable search functionality, these specially crafted inputs could:
- Inject arbitrary IMAP commands that the mail server would execute
- Bypass CSRF protection mechanisms designed to prevent unauthorized actions
- Potentially manipulate mail search results or access unintended mailbox data
The vulnerability requires high attack complexity, as successful exploitation depends on specific timing and configuration factors. However, no user interaction is required beyond the attacker's own authenticated session.
Detection Methods for CVE-2026-35538
Indicators of Compromise
- Unusual or malformed search queries in Roundcube application logs containing IMAP metacharacters such as \, *, %, or parentheses
- Unexpected IMAP command sequences in mail server logs that don't match typical user search patterns
- Anomalous search activity patterns from authenticated users, particularly requests containing encoded special characters
Detection Strategies
- Monitor Roundcube webmail access logs for search requests containing suspicious patterns or IMAP command syntax
- Implement Web Application Firewall (WAF) rules to detect and block search parameters containing IMAP injection payloads
- Review mail server IMAP logs for unexpected or malformed SEARCH commands that may indicate injection attempts
Monitoring Recommendations
- Enable verbose logging on both Roundcube Webmail and the underlying IMAP server to capture detailed search activity
- Set up alerting for failed or malformed IMAP commands that could indicate exploitation attempts
- Monitor for increased error rates in mail search functionality that may suggest injection testing
How to Mitigate CVE-2026-35538
Immediate Actions Required
- Upgrade Roundcube Webmail to version 1.5.14, 1.6.14, or 1.7-rc5 immediately
- Review application logs for any evidence of exploitation attempts before patching
- Temporarily restrict access to the webmail interface if immediate patching is not possible
Patch Information
Roundcube has released security updates addressing this vulnerability. The fixes are available in the following versions:
The patches implement proper sanitization of IMAP SEARCH command arguments to prevent injection attacks. Multiple commits address this issue:
For additional details, see the Roundcube Security Updates Announcement.
Workarounds
- Implement a reverse proxy or WAF with rules to sanitize search input parameters before they reach Roundcube
- Restrict network access to the Roundcube webmail interface to trusted IP ranges until patching is complete
- Disable or limit the mail search functionality at the application configuration level if operationally feasible
# Example: Restrict Roundcube access via Apache configuration
<Location "/roundcube">
Require ip 10.0.0.0/8 192.168.0.0/16
# Limit access to internal networks until patch is applied
</Location>
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
