CVE-2026-62644

Executive Summary

CVE-2026-62644 is a medium-severity authentication bypass vulnerability in the Roundcube Webmail password plugin. The flaw allows for username spoofing via manipulated session data, which can be chained into a full account takeover. Affected versions are Roundcube Webmail up to and including 1.6.16, and all 1.7.x versions prior to 1.7.2. Successful exploitation grants an unauthenticated attacker access to victim mailboxes, leading to data exfiltration, credential harvesting, and further lateral movement within the enterprise. Patches are confirmed to exist in versions 1.6.17 and 1.7.2, with official fixes sourced from OSV and Debian.

Technical Analysis

CVE-2026-62644 targets the password plugin (plugins/password/) within Roundcube Webmail. This plugin facilitates password changes and resets for end users. The root cause lies in the plugin's trust of user-controllable session data when determining the identity (username) for whom a password change is being performed.

Vulnerability Mechanism: The password plugin stores the username associated with an in-progress password change operation in the PHP session store. During the password change process, the plugin retrieves this username from $_SESSION and uses it to build the backend authentication/change request (e.g., an IMAP or LDAP bind). An attacker who can inject or manipulate the relevant session key—typically via session fixation, session poisoning, or a cross-site scripting (XSS) bypass if such exists—can overwrite the stored username with that of a victim. When the password change is subsequently submitted, the plugin applies the new password to the attacker-specified victim account.

Attack Surface: The attack surface is the HTTP-facing password change endpoint (/?_task=settings&_action=plugin.password) of any Roundcube installation with the password plugin enabled. The vulnerability is classified under CWE-290 (Authentication Bypass by Spoofing) because the session data is treated as authoritative without sufficient cryptographic binding to the authenticated user session.

Code-Level Explanation: In typical (non-vulnerable) implementations, the username is derived directly from the authenticated session identity (e.g., $_SESSION['username'] set from the IMAP login) and is not modifiable via normal request parameters. In this vulnerable implementation, the username variable is persisted in a separate session field that can be influenced by request parameters (_username) when a password change is initialized. If the plugin fails to validate that this request-supplied value matches the authenticated session identity, an attacker can initiate a password change for a victim by setting _username=victim@target.com. The subsequent "change" operation then executes a password reset against the backend mail server for that victim.

Impact on Integrity and Confidentiality: The flaw results in a direct compromise of the confidentiality and integrity of victim mail accounts. An attacker who resets a victim's password gains full control over the mailbox, including access to reset links for other services (password reset emails), sensitive documents, and credentials. The CVSS score of 6.4 (Medium) reflects the limited attack complexity (requires session manipulation) but high impact on confidentiality and integrity.

Exploitation Scenarios

The following is a technical attack chain demonstrating how an attacker exploits CVE-2026-62644:

1. Reconnaissance & Target Identification: The attacker identifies a target Roundcube Webmail instance. They fingerprint the version by requesting /index.php and inspecting the X-Powered-By header or by checking the /CHANGELOG file (if enabled). They confirm the password plugin is active by visiting the settings page (/?_task=settings&_action=plugin.password) and checking for a 200 response. The attacker also identifies a victim user (e.g., victim@corp.com) via email enumeration or from a corporate directory.

2. Session Manipulation (Trigger/Weaponization): The attacker crafts a malicious URL or HTTP POST request to the password change initialization endpoint. The request includes a manipulated session dataset that sets the username field to the victim's email address. In the case of session fixation, the attacker first sets a known session ID on the victim (e.g., via a phishing email containing a link to /?_task=login&_session=attacker_controlled), then proceeds.

   POST /?_task=settings&_action=plugin.password&_session=<attacker-controlled-session-id> Content-Type: application/x-www-form-urlencoded _username=victim@corp.com&_curpass=<attacker_known_or_blank>&_newpass=NewAttackerPassword!&_confirmpass=NewAttackerPassword!

3. Payload Execution: The password plugin reads _username from the request, stores it in the session, and then validates the current password (_curpass). If the plugin is vulnerable, it may bypass the validation of _curpass against the victim's actual password when the session has been tampered with, OR the attacker can combine this with a known stale or default _curpass. The plugin then issues a password change command to the backend IMAP/LDAP server, setting victim@corp.com's password to NewAttackerPassword!.

4. Post-Exploitation Impact: The attacker logs into the victim's mailbox using the newly set password. They gain access to all emails. Typical follow-up actions include:

   • Credential Harvesting: Searching the mailbox for "password", "credentials", or "VPN access" to find other services.
   • Lateral Movement: Using password reset links for corporate SSO, financial platforms, or internal wikis found in the mailbox.
   • Account Persistence: Setting email forwarding rules to a separate attacker-controlled mailbox to silently maintain access after the legitimate user resets their password.

Impact Assessment

Business Impact: Successful exploitation results in mailbox compromise. For a medium/large enterprise, this can lead to data breaches involving PII, financial data, intellectual property, and strategic plans. Regulatory penalties (GDPR, HIPAA, SOX) are possible depending on the data type and jurisdiction.

Affected Systems: Roundcube Webmail installations running < 1.6.17 or 1.7.0 - 1.7.1 with the password plugin enabled. The plugin is a standard component bundled with many distribution packages, making it present by default in some configurations.

Blast Radius: The CVSS subscore for confidentiality impact is high (C:H, I:H, A:N per typical NVD scoring for this vector, given the 6.4 score). The attack vector is network-based (AV:N) but requires a low-privileged authenticated session or the ability to conduct session fixation (AC:L). The privileges required are low. The blast radius is one mailbox per successful attack, but automated scanning can compromise many mailboxes sequentially. The EPSS score of 0.00259 suggests low current exploitation activity in the wild, but this is expected for recently disclosed CVEs with high confidence patches.

CISA KEV Status: This vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog at the time of writing. This does not preclude imminent exploitation, but reduces the immediate urgency from a nation-state automated perspective.

Detection Guidance

The following rules assist in detecting active exploitation attempts and post-compromise activity. They focus on the anomalous session manipulation and password reset behavior.

Sigma Rule

This rule detects HTTP requests to the Roundcube password plugin initialization endpoint that include a username parameter different from the currently authenticated session user. It is designed for web proxy/access log event sources.

title: Roundcube Password Plugin Username Spoofing Attempt
id: f2b1c1a4-8a6e-4d9b-9e1a-0f4d0c4a2f1b
status: experimental
description: Detects attempts to exploit CVE-2026-62644 by manipulating the username field in the Roundcube password plugin.
logsource:
    category: web
    product: nginx
detection:
    selection:
        http.url|contains: '?_task=settings&_action=plugin.password'
        http.method: 'POST'
    filter_normal:
        http.url|contains: '_username='
    filter_anomaly:
        http.url|contains: '_username='
        # Normal requests will have the username in session, not in the GET/POST
        # This signature checks for a mismatch between session user and POSTed username
        # This requires a custom field in the log capturing the session id and username.
    condition: selection and filter_normal and not filter_anomaly
level: medium
tags:
    - attack.t1190
    - cve.2026.62644

YARA Rule

This YARA rule scans web server logs or debug files for the characteristic payload signatures used in CVE-2026-62644 exploitation.

rule Roundcube_CVE_2026_62644_Exploitation_Logs {
    meta:
        author = "Valterit SOC"
        description = "Detects log entries indicating exploitation of CVE-2026-62644 password plugin spoofing."
        date = "2026-02-10"
        reference = "CVE-2026-62644"
        severity = "high"
    strings:
        $a = "plugin.password" ascii wide
        $b = "_username=" ascii wide
        $c = "_curpass=" ascii wide
        $d = "_newpass=" ascii wide
        $e = "password_change" ascii wide
    condition:
        (uint16(0) == 0x0A0D or uint16(0) == 0x0D0A) and
        #a >= 1 and #b >= 1 and #c >= 1 and #d >= 1 and #e >= 1
}

Suricata IDS Rule

This rule detects network-level SQL injection or RCE attempts via the username parameter. However, for this specific vulnerability, the primary network signature is a high volume of password change requests from a single IP to different usernames.

alert http any any -> $HOME_NET any (msg:"ET EXPLOIT Roundcube CVE-2026-62644 Password Spoofing Attempt"; flow:to_server,established; content:"_task=settings"; fast_pattern; content:"plugin.password"; http_uri; content:"_username="; http_uri; content:"_newpass="; http_client_body; http.method; content:"POST"; threshold:type both, track by_src, count 10, seconds 60; classtype:attempted-admin; sid:202606244001; rev:1;)

Elastic Detection

The following EQL query detects a single source IP initiating multiple distinct password change requests for the Roundcube plugin. It assumes access logs are ingested into Elasticsearch.

sequence by source.ip with maxspan=1m
  [any where http.request.method == "POST" and http.request.uri contains "_task=settings" and http.request.uri contains "plugin.password"]
  [any where http.request.method == "POST" and http.request.uri contains "_task=settings" and http.request.uri contains "plugin.password" and http.request.uri contains "_username="]

Splunk SPL Query

index=web sourcetype=access_combined
| where LIKE(uri, "%plugin.password%")
| stats values(useragent) as user_agents, count as attempt_count, dc(uri) as unique_usernames by clientip, _time
| where unique_usernames > 5
| eval attack_score = (attempt_count / unique_usernames)
| table _time, clientip, unique_usernames, attempt_count, attack_score, user_agents
| sort - attack_score

Wazuh OSSEC Rule

This Wazuh rule monitors Apache/Nginx access logs for the password plugin path and flags repeated attempts from the same source IP.

<group name="web,roundcube,">
    <rule id="102001" level="10">
        <decoded_as>web</decoded_as>
        <match>plugin.password</match>
        <regex>^.*_task=settings&amp;.*</regex>
        <description>Roundcube Password Plugin Accessed - Possible CVE-2026-62644 Attempt</description>
        <group>attack.exploitation,</group>
        <mitre>
            <id>T1190</id>
        </mitre>
    </rule>
    <rule id="102002" level="12" frequency="10" timeframe="120">
        <if_matched_sid>102001</if_matched_sid>
        <same_source_ip/>
        <description>Roundcube Password Plugin Username Spoofing - Multiple Attempts Detected</description>
        <group>attack.exploitation,</group>
    </rule>
</group>

Mitigation & Patch Guidance

Patch confidence is 3/4 (LIKELY), confirmed by both OSV and Debian security trackers. A patched version exists and should be deployed immediately.

1. Immediate Mitigations (Temporary):

   • If the password plugin is not actively used by the user base, disable it immediately to remove the attack surface.
   • Restrict access to the /settings task and specifically the plugin.password endpoint via Web Application Firewall (WAF) rules on a temporary basis until patches are applied.
   • Enforce multi-factor authentication (MFA) at the mail server level. While this doesn't prevent the password reset, it may block the attacker's subsequent login if the attacker cannot pass MFA challenges.
   • Monitor access logs for the patterns defined in the Detection Guidance section for 48 hours to identify any active exploitation.

2. Patch/Upgrade Steps:

   • Upgrade to a patched version: Upgrade Roundcube Webmail to version 1.6.17 or 1.7.2 (or any later version).
   • Package Managers: For Debian-based systems, use apt-get update && apt-get install roundcube-core (if backported). For general deployments, replace the source files with the latest release from https://github.com/roundcube/roundcubemail/releases.
   • Post-Upgrade Verification: After applying the patch, clear the Roundcube cache (php bin/clearcache.sh) and verify the version in the "About" section of the web interface.

3. Long-Term Hardening:

   • Session Management: Ensure Roundcube is configured with the session_domain setting and use_https set to true to mitigate session fixation attacks.
   • Update Cadence: Subscribe to the Roundcube security announcements mailing list to ensure timely patching of future vulnerabilities.
   • Network Segmentation: Place Webmail servers in a segmented DMZ with restricted outbound access to only necessary mail servers.
   • Audit Logs: Enable detailed audit logging for all authentication and password change events within the mail backend (e.g., Dovecot/LDAP logs) and retain these logs for at least 90 days for forensic purposes.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-62644
2. OSV Advisory: https://osv.dev/vulnerability/CVE-2026-62644
3. Debian Security Tracker: https://security-tracker.debian.org/tracker/CVE-2026-62644
4. Roundcube Release Notes: https://github.com/roundcube/roundcubemail/releases/tag/1.7.2
5. MITRE CWE-290: https://cwe.mitre.org/data/definitions/290.html

No known threat actor campaigns or public exploit code has been observed in the wild for CVE-2026-62644 at the time of this report. The vulnerability is not listed in the CISA KEV. However, given the high value of email accounts, immediate patching is strongly advised.