CVE-2026-75004

Executive Summary

CVE-2026-75004 is an improper neutralization vulnerability (CWE-77) identified in Roundcube Webmail instances utilizing the managesieve plugin. Prior to versions 1.6.18 and 1.7.3, improper rule name quoting in Sieve script generation allows an authenticated remote user to bypass the managesieve_disabled_actions security restriction. Successful exploitation enables attackers to execute Sieve script actions that were explicitly restricted by mail administrators, such as unauthorized email forwarding, automated responses, or filtering actions.

Technical Analysis

Roundcube Webmail integrates the managesieve plugin to allow end users to author and manage server-side email filtering rules via the ManageSieve protocol (RFC 5804). System administrators utilize the managesieve_disabled_actions configuration setting to restrict specific capabilities—such as automated email redirection, file-into operations, or auto-replies—thereby preventing data exfiltration, policy violations, or automated relay abuse.

The flaw exists within the rule name processing mechanism of the managesieve plugin. When a user defines a Sieve filtering rule, the rule name string is processed and formatted before being incorporated into the active Sieve script structure or parsed against administrative policy blocks. Due to improper quoting of the rule name parameter, an attacker can input crafted control characters or nested quoting structures within the rule name field.

This improper quoting causes a parser differential between the policy enforcement layer (which evaluates managesieve_disabled_actions) and the constructed Sieve script interpreter. Because the parameter quoting is mishandled, the policy evaluation logic fails to properly detect and strip restricted Sieve actions encapsulated within or immediately following the crafted rule name string. Consequently, the application constructs and submits a valid Sieve script to the underlying backend server (e.g., Dovecot Pigeonhole) that contains actions explicitly designated as disabled in the Roundcube configuration.

The vulnerability requires authenticated access as a valid webmail user and only impacts environments where the managesieve plugin is active and configured with action restrictions.

Exploitation Scenarios

No public proof-of-concept (PoC) code is currently available; the following attack chain is inferred from the underlying CWE-77 vulnerability class and published advisory metrics.

1. Reconnaissance and Target Identification

An attacker identifies a target organization operating a vulnerable deployment of Roundcube Webmail. Access to the web interface is validated, and the presence of the managesieve plugin is confirmed via web user interface components (such as the presence of the "Filters" management tab under user settings) or directory enumeration.

2. Low-Privilege Authentication

The attacker obtains valid low-privilege mail credentials (via phishing, password spraying, or credential compromise). The attacker logs into the Roundcube Webmail portal and navigates to the Sieve filter management interface powered by the managesieve plugin.

3. Payload Injection via Rule Name

The attacker creates or modifies a filter rule within the web interface. In the rule name field, the attacker submits a string containing crafted quoting syntax designed to bypass character escaping mechanisms. Embedded within or adjacent to this payload are Sieve script actions prohibited by the administrator in managesieve_disabled_actions (for instance, a redirect or vacation action command).

4. Policy Bypass and Sieve Script Activation

When the form is saved, the managesieve plugin processes the input. Due to the improper quoting, the policy validation mechanism fails to match the disabled action against the managesieve_disabled_actions blocklist. The plugin serializes the filter into a Sieve script and transmits it to the ManageSieve daemon. The server accepts and activates the script.

5. Post-Exploitation Execution

Incoming mail triggering the crafted Sieve rule executes the forbidden action. If the administrator had restricted external email forwarding to prevent data loss, the attacker's newly established redirect rule actively exfiltrates incoming messages to an off-system email address.

Impact Assessment

Business Impact

Exploitation of CVE-2026-75004 compromises organizational mail policy enforcement. Bypassing managesieve_disabled_actions allows malicious insiders or compromised user accounts to establish unauthorized message forwarding rules, leading to unintended data exfiltration or regulatory non-compliance (e.g., HIPAA, GDPR, PCI-DSS).

Severity & CVSS Breakdown

• CVSS v3.1 Base Score: 4.3 (Medium)
• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N
  • Attack Vector (AV:N): Network-accessible via HTTP/HTTPS web interface.
  • Attack Complexity (AC:L): Low complexity; requires standard user input manipulation.
  • Privileges Required (PR:L): Requires an authenticated webmail user account.
  • User Interaction (UI:N): No interaction from other users or administrators is required.
  • Scope (S:U): Impact is confined to the Roundcube/Sieve environment.
  • Confidentiality (C:N): No direct disclosure of system secrets or host files via the bug itself.
  • Integrity (I:L): Low integrity impact; allows unauthorized execution of restricted mail filtering rules.
  • Availability (A:N): No availability impact or denial of service condition.

Detection Guidance

Sigma Rule

title: Roundcube Webmail ManageSieve Rule Name Quoting Bypass
id: 8c3f91d2-0b1a-4d22-b2f7-08e1f5a89412
status: experimental
description: Detects potential exploitation attempts of CVE-2026-75004 in Roundcube Webmail web server logs, targeting improper rule name quoting in the managesieve plugin.
author: ValtersIT Security
date: 2026-03-30
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-75004
logsource:
    category: webserver
    service: http
detection:
    selection_endpoint:
        c-uri-path|contains:
            - '_task=settings'
            - '_action=plugin.managesieve'
    selection_payload:
        c-uri-query|contains:
            - 'quote'
            - '%22'
            - '%27'
            - '\'
            - '"'
    selection_sieve_actions:
        c-uri-query|contains:
            - 'redirect'
            - 'vacation'
            - 'fileinto'
            - 'reject'
            - 'discard'
    condition: selection_endpoint and (selection_payload and selection_sieve_actions)
falsepositives:
    - Legitimate administrative customization of Sieve rules with complex names containing single or double quotes.
level: medium

YARA Rule

rule Roundcube_ManageSieve_Quoting_Bypass_CVE_2026_75004 {
    meta:
        description = "Detects unpatched Roundcube ManageSieve plugin file handling rule name strings susceptible to CVE-2026-75004"
        author = "ValtersIT Threat Intelligence"
        date = "2026-03-30"
        cve = "CVE-2026-75004"
        severity = "Medium"

    strings:
        $plugin_identifier = "managesieve" ascii fullword
        $config_disabled = "managesieve_disabled_actions" ascii fullword
        $sieve_rule_parse = "rcube_sieve_engine" ascii
        $improper_quote_pattern = /rule_name\s*=\s*['"][^'"]*[\"'\\][^'"]*['"]/ ascii

    condition:
        uint32(0) == 0x66703f3c // PHP file signature ('<?ph')
        and $plugin_identifier
        and ($config_disabled or $sieve_rule_parse)
        and $improper_quote_pattern
}

Suricata IDS Rule

alert http $EXTERNAL_NET any -> $HOME_NET $HTTP_PORTS (
    msg:"ET WEB_SPECIFIC_APPS Roundcube Webmail managesieve improper rule name quoting attempt (CVE-2026-75004)";
    flow:established,to_server;
    http.uri; content:"_task=settings"; fast_pattern;
    http.uri; content:"_action=plugin.managesieve";
    http.request_body; content:"_rule_name=";
    http.request_body; pcre:"/_rule_name=[^&]*%22|%27|%5C|["'\\]/i";
    http.request_body; pcre:"/(redirect|vacation|fileinto|reject|discard)/i";
    classtype:web-application-attack;
    sid:20267500;
    rev:1;
    reference:cve,CVE-2026-75004;
)

Elastic Detection

sequence by host.name, user.name with maxspan=5m
  [network where event.type == "start" and process.name in ("apache2", "httpd", "php-fpm", "nginx") 
   and destination.port in (80, 443)]
  [http where http.request.method == "POST" 
   and http.request.uri.path contains "_action=plugin.managesieve"
   and http.request.body.content contains "_rule_name="
   and http.request.body.content regex ".*(_rule_name=.*[\"'\\]).*"
   and http.request.body.content regex ".*(redirect|vacation|fileinto|reject|discard).*"
   and http.response.status_code == 200]

Splunk SPL Query

index=webmail OR index=proxy sourcetype=access_combined
| search uri_path="*index.php*" uri_query="*_action=plugin.managesieve*"
| eval payload=urldecode(_raw)
| where match(payload, "_rule_name=.*[\"'\\]") AND match(payload, "(redirect|vacation|fileinto|reject|discard)")
| stats count min(_time) as first_seen max(_time) as last_seen by src_ip, user, http_user_agent, uri_query
| convert timeformat="%Y-%m-%d %H:%M:%S" ctime(first_seen) ctime(last_seen)
| table first_seen, last_seen, src_ip, user, uri_query, count

Wazuh OSSEC Rule

<group name="web,roundcube,">
  <rule id="100850" level="8">
    <if_sid>31100</if_sid>
    <url>_action=plugin.managesieve</url>
    <match>_rule_name=</match>
    <regex>_rule_name=.*%22|%27|%5C|["'\\]</regex>
    <description>Roundcube ManageSieve potential rule name quoting bypass attempt (CVE-2026-75004)</description>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence

Patch Confidence: 4/4 (CONFIRMED) — Official vendor releases and independent distribution security trackers (NVD, OSV, Debian) verify the availability of fixes for this vulnerability.

1. Immediate Mitigations

• Disable the Plugin: If ManageSieve script management is not strictly necessary for operational workflows, temporarily disable the managesieve plugin in Roundcube's config.inc.php file by removing 'managesieve' from the $config['plugins'] array.
• Server-Level Enforcement: If user Sieve scripts are required, enforce Sieve feature restrictions directly on the backend ManageSieve/MDA server (e.g., via Dovecot Pigeonhole settings using sieve_disallow_actions or restricted extensions) rather than relying exclusively on Roundcube application-level policy (managesieve_disabled_actions).

2. Patch and Upgrade Steps

Deploy the official software updates provided by the Roundcube project:

• If running the 1.6.x release branch, upgrade to Roundcube Webmail 1.6.18 or later.
• If running the 1.7.x release branch, upgrade to Roundcube Webmail 1.7.3 or later.

Upgrade Verification Process:

1. Back up existing Roundcube installation files and the backend database.
2. Unpack the updated Roundcube release archive over the existing installation directory or apply vendor package updates via system package managers (apt, dnf).
3. Execute the standard Roundcube update script (bin/update.sh) if database or configuration changes are required.
4. Verify that the managesieve plugin files are updated to match the target version.
5. Validate Sieve filter creation through the web UI using special character inputs to confirm proper quoting enforcement.

3. Long-Term Hardening

• Audit existing Sieve script files stored on the mail server to identify any active scripts that contain restricted actions (e.g., unauthorized auto-redirects).
• Enforce strict input validation boundaries across all custom or third-party webmail plugins.
• Integrate continuous vulnerability scanning for public-facing web applications.

References

1. NVD CVE Detail: https://nvd.nist.gov/vuln/detail/CVE-2026-75004
2. Roundcube Webmail Releases: https://roundcube.net/news/
3. OSV Vulnerability Database: https://osv.dev/vulnerability/CVE-2026-75004
4. Debian Security Tracker (Roundcube): https://security-tracker.debian.org/tracker/source-package/roundcube