CVE-2026-6351

Executive Summary

CVE-2026-6351 is a high-severity CRLF injection vulnerability (CWE-93) affecting Openfind MailGates and MailAudit email security gateway products. The flaw allows unauthenticated remote attackers to inject arbitrary HTTP headers and carriage-return/line-feed sequences into server responses, which can be leveraged to read arbitrary system files from the underlying operating system. This vulnerability carries a CVSS score of 8.7 (High) with a network attack vector requiring no authentication or user interaction, making it trivially exploitable by any remote attacker able to reach the affected management interface. Organizations running vulnerable versions of MailGates or MailAudit (5.0 prior to 5.2.10.099, or 6.0 prior to 6.1.10.054) should treat this as an urgent patching priority given the confidentiality impact and the pre-authentication exploitation window.

Technical Analysis

CRLF injection (CWE-93) occurs when an application fails to sanitize user-controlled input before embedding it into an HTTP response header. The attacker supplies a payload containing ASCII carriage return (\r, 0x0D) and line feed (\n, 0x0A) characters. When these bytes are reflected into a response header without proper encoding or validation, the attacker gains the ability to terminate the legitimate header prematurely and inject arbitrary additional headers, or in some cases, manipulate the response body.

In the context of MailGates/MailAudit, the vulnerability exists in the web-based management interface. These products act as mail security gateways, sitting inline between the internet and the organization's mail infrastructure. The management console is typically bound to an administrative port but is frequently exposed to the network for remote administration.

The CRLF injection vector in this specific case allows unauthenticated attackers to exploit the flaw to read system files. The mechanism typically works as follows:

1. Header Injection to Response Splitting: By injecting \r\n sequences into a parameter that is reflected in a redirect (Location header) or other HTTP response headers, the attacker can cause the server to emit additional headers and separate HTTP responses.

2. Body Manipulation: A common CRLF-to-LFI (Local File Inclusion) chain involves injecting a \r\n\r\n sequence to terminate the headers prematurely and begin a body section. If the application then processes the injected content in a way that allows file path inclusion — for example, through a X-Accel-Redirect header (nginx) or a file:// URI handling mechanism — the attacker can read arbitrary files from the server's filesystem.

3. Unauthenticated Access: Because the injection occurs before authentication checks are applied (or in a code path that doesn't require authentication, such as a login redirect handler), the attacker does not need valid credentials.

The attack surface is the HTTP management interface listening on TCP ports 80, 443, or the configured admin port. MailGates and MailAudit are often deployed in DMZ segments, meaning the vulnerable interface may be directly reachable from the internet. The CVSS vector confirms this: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N — network-accessible, low attack complexity, no privileges required, no user interaction, and a high confidentiality impact with no integrity or availability impact. The attack is entirely read-oriented, indicating the file disclosure is the primary and likely the only impact.

Exploitation Scenarios

The following is a step-by-step attack chain an adversary would execute to exploit CVE-2026-6351. No public PoC has been confirmed, but the techniques described are standard for CRLF injection-to-file-read chains.

Step 1 — Reconnaissance and Target Identification

The attacker begins by discovering exposed MailGates/MailAudit instances. Using Shodan or similar search engines, the attacker queries for http.title:"MailGates" or http.title:"MailAudit" to find management interfaces. Alternatively, the attacker uses a port scan across target IP ranges looking for typical admin ports (e.g., 8080, 8443, or 443) and then fingerprints the login page to confirm the product and version.

Confirming the version is critical: the attacker checks if the target runs 5.0.x (vulnerable if < 5.2.10.099) or 6.0.x (vulnerable if < 6.1.10.054). Version information is often disclosed in the login page footer or in the HTML meta tags.

Step 2 — Crafting the CRLF Payload

The attacker identifies a parameter that is reflected into a response header. Common candidates in such appliances include:

• Redirect parameters (e.g., /login?redirect=<injected>)
• Error message parameters reflected in a Location header
• Any cgi-bin handler that echoes user input into headers

The attacker crafts a payload structured to achieve response splitting followed by a file read. A typical payload for file disclosure via CRLF injection in a redirect parameter is:

/login?redirect=/../../../../etc/passwd%0d%0aX-Accel-Redirect:%20file:///etc/passwd

URL-decoded, this injects a CRLF (%0d%0a) after the legitimate redirect path, then adds a new header X-Accel-Redirect: file:///etc/passwd. If the backend is an nginx reverse proxy or the application honors the X-Accel-Redirect header, the server responds with the contents of /etc/passwd instead of a redirect.

Step 3 — Triggering the Vulnerability

The attacker sends an HTTP request directly to the vulnerable endpoint using curl:

curl -i "https://target:8443/login?redirect=/../../../../etc/shadow%0d%0aX-Accel-Redirect:%20file:///etc/shadow"

If the injection succeeds, the attacker receives a response where the injected header is honored and the system file is returned as the response body. Even if /etc/shadow is permission-protected, the process running the web server may have sufficient privileges to read it, depending on the system's configuration.

Step 4 — Post-Exploitation and Impact

With arbitrary file read capabilities, the attacker harvests sensitive files such as:

• /etc/passwd and /etc/shadow — for credential cracking and lateral movement
• Application configuration files containing database credentials, API keys, and mail server passwords
• /var/log/mail or similar mail logs containing internal email metadata
• TLS private keys if stored on the local filesystem, enabling man-in-the-middle attacks against mail traffic
• Any other readable files on the system, including other mail queue data

The attacker does not need to maintain persistent access through this vector alone — the file read is a one-shot operation — but the harvested credentials and configuration data enable further penetration of the email infrastructure.

Impact Assessment

The CVSS score of 8.7 (High) reflects the severity when considering the confidentiality impact (C:H). The vector AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N indicates the following:

• Attack Vector (AV:N): Exploitable remotely over the network; no physical or local access required.
• Attack Complexity (AC:L): No special conditions or race conditions; the injection is straightforward.
• Privileges Required (PR:N): Pre-authentication exploitation; the attacker needs no credentials.
• User Interaction (UI:N): No victim interaction required; the target application is attacked directly.
• Scope (S:U): The impact is confined to the vulnerable component, not the entire infrastructure.
• Confidentiality Impact (C:H): High — arbitrary system file read exposes secrets, credentials, and mail data.
• Integrity Impact (I:N): None directly; the attacker cannot modify data through this specific CVE.
• Availability Impact (A:N): None directly; the service remains operational.

Affected Systems: All MailGates and MailAudit deployments running version 5.0 (unless patched to 5.2.10.099 or later) and version 6.0 (unless patched to 6.1.10.054 or later). These products are typically deployed as gateway appliances in front of Microsoft Exchange, IBM Domino, or Linux-based mail servers.

Blast Radius: The immediate blast radius is the mail gateway host itself. However, because mail gateways hold configuration data for the entire mail infrastructure — including credentials for downstream mail servers, SMTP relay settings, and directory service credentials (LDAP/AD) — the compromise of this host can provide a pivot point into the core identity infrastructure. If TLS private keys for the mail domain are stored on the appliance, the attacker could decrypt captured mail traffic if they also have network position.

Business Impact: Direct business impact includes exposure of internal email metadata and user credentials, potential regulatory violations (e.g., GDPR, HIPAA, PCI-DSS) if mail data is considered personal or sensitive, and reputational damage. There is no direct availability impact, so the service continues running, but the exposure of highly sensitive configuration files escalates the risk of a full network compromise.

Detection Guidance

Sigma Rule

title: Openfind MailGates/MailAudit CRLF Injection Attempt (CVE-2026-6351)
id: 3f4a9c1e-7b2d-4e8f-9a6c-1d2e3f4a5b6c
status: experimental
description: Detects HTTP requests containing CRLF injection payloads targeting the MailGates/MailAudit management interface.
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-6351
    - https://www.twcert.org.tw/
author: Valtersit Threat Intelligence
date: 2026/06/01
logsource:
    category: webserver
    product: apache
    definition: 'Requirements: web server access logs or reverse proxy logs for MailGates/MailAudit management endpoints.'
detection:
    selection_crlf_encoded:
        c-uri|contains:
            - '%0d%0a'
            - '%0D%0A'
            - '%0d'
            - '%0a'
    selection_header_injection:
        c-uri|contains|all:
            - 'X-Accel-Redirect'
            - 'file:'
            - '/etc/'
    selection_common_paths:
        cs-uri-stem|contains:
            - '/login?'
            - '/redirect?'
            - '/auth'
    condition: selection_crlf_encoded and (selection_header_injection or selection_common_paths)
    timeframe: 5m
level: high
falsepositives:
    - Legitimate redirect URLs containing encoded newlines are extremely rare and should be reviewed if they occur.
    - Other applications behind the same proxy may generate false positives if they use similar redirect patterns; tune by source IP or URI.
fields:
    - src_ip
    - cs-uri-stem
    - c-uri
    - cs-host
    - cs-user-agent
tags:
    - attack.t1190
    - cve.2026-6351
    - cwe.93

YARA Rule

rule Openfind_CRLF_Injection_CVE_2026_6351 {
    meta:
        author = "Valtersit Threat Intelligence"
        description = "Detects CRLF injection payloads targeting Openfind MailGates/MailAudit"
        cve = "CVE-2026-6351"
        cwe = "CWE-93"
        date = "2026-06-01"
        severity = "high"
    strings:
        $crlf_encoded = /%0[dD]%0[aA]/
        $xaccel = "X-Accel-Redirect" nocase
        $file_uri = "file:///etc/" nocase
        $path_traversal = /\.\.\/\.\.\/\.\.\//
        $login_uri = "/login?" nocase
        $redirect_uri = "/redirect?" nocase
        $shadow_ref = "/etc/shadow" nocase
        $passwd_ref = "/etc/passwd" nocase
    condition:
        uint16(0) == 0x4854 and // "HT" for HTTP log line
        $crlf_encoded and
        (
            ($xaccel and $file_uri) or
            ($path_traversal and ($passwd_ref or $shadow_ref)) or
            ($login_uri or $redirect_uri and $crlf_encoded)
        )
}

Suricata IDS Rule

alert http any any -> any any (msg:"ET EXPLOIT Openfind MailGates/MailAudit CRLF Injection Attempt (CVE-2026-6351)"; flow:established,to_server; content:"%0d%0a"; http_uri; content:"X-Accel-Redirect"; http_uri; content:"file:"; http_uri; content:"etc"; http_uri; content:"login"; http_uri; pcre:"/\.\.\/\.\.\/\.\.\//Ui"; reference:cve,2026-6351; classtype:attempted-admin; sid:2026063501; rev:1; metadata:created_at 2026_06_01, updated_at 2026_06_01;)

alert http any any -> any any (msg:"ET EXPLOIT Openfind MailGates/MailAudit Encoded CRLF in Redirect URI (CVE-2026-6351)"; flow:established,to_server; content:"%0d%0a"; http_uri; pcre:"/(\/login\?redirect=|\/redirect\?url=|\/auth\?next=)[^&\"]*%0[dD]%0[aA]/Ui"; reference:cve,2026-6351; classtype:attempted-admin; sid:2026063502; rev:1; metadata:created_at 2026_06_01, updated_at 2026_06_01;)

Elastic Detection

// Elastic EQL query for detecting CRLF injection attempts against MailGates/MailAudit
// Place in a detection rule with a filter for the affected host names or IPs

sequence by source.ip, destination.ip with maxspan=5m
  [ network where network.protocol == "http" and
    http.request.uri.path like "*/login*" or http.request.uri.path like "*/redirect*" ]
  [ http where http.request.uri contains "%0d%0a" or
    http.request.uri contains "%0D%0A" or
    http.request.uri matches ".*%0[dD]%0[aA].*" ]
  [ http where http.request.uri contains "file:///etc/" or
    http.request.uri contains "X-Accel-Redirect" ]

Splunk SPL Query

index=web sourcetype=access_combined
| where (uri LIKE "%login%" OR uri LIKE "%redirect%")
    AND (uri LIKE "%0d%0a%" OR uri LIKE "%0D%0A%")
| eval crlf_detected=if(uri LIKE "%0d%0a%" OR uri LIKE "%0D%0A%", "CRLF_PRESENT", "NONE")
| where crlf_detected="CRLF_PRESENT"
| eval file_read_attempt=if(uri LIKE "%file:///etc/%" OR uri LIKE "%/etc/passwd%" OR uri LIKE "%/etc/shadow%", "FILE_READ", "HEADER_ONLY")
| stats count by src_ip, uri, status, dest_host, file_read_attempt
| sort - count
| where count > 0

Wazuh OSSEC Rule

<group name="web, attack, crlf_injection,">
  <rule id="796142" level="12">
    <decoded>web-access</decoded>
    <match>%0d%0a|%0D%0A</match>
    <regex>(\/login\?redirect=|\/redirect\?url=|\/auth\?next=)</regex>
    <regex>file:\/\/\/etc\/|\.\.\/\.\.\/\.\.\/</regex>
    <description>Openfind MailGates/MailAudit CRLF injection attempt (CVE-2026-6351) - potential system file read</description>
    <group>attack.t1190,cve-2026-6351,cwe-93,</group>
    <mitre>
      <id>T1190</id>
    </mitre>
    <info type="cve">CVE-2026-6351</info>
    <info type="cwe">CWE-93</info>
    <info type="ossec">web-access</info>
  </rule>

  <rule id="796143" level="8">
    <if_sid>2026351</if_sid>
    <regex>%0d%0a</regex>
    <regex>^\S+ \S+ \S+ 200</regex>
    <description>CRLF injection likely successful - HTTP 200 response for injected request</description>
    <group>attack.t1190,cve-2026-6351,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 2/4 (CLAIMED — single source claims patch, not independently confirmed).

The CNA (TWNCERT — Taiwan National Computer Emergency Response Team, given the twcert CVE assignment) has issued a patch advisory. However, only a single source asserts the fix, and it has not been independently verified by third parties. Treat the patch information as actionable but verify the fix on your own test systems before deploying to production.

1) Immediate Mitigations (apply within hours):

• Restrict network access: Immediately place firewall rules or security group restrictions to limit access to the MailGates/MailAudit management interface (typically ports 80, 443, or configured admin ports) to only trusted administrative IP ranges. If the interface is internet-exposed, block external access entirely and use a VPN or jump host for administration.
• Disable or restrict the web management interface: If possible, disable the web UI and use CLI/console access exclusively until patching is complete.
• Enable logging: Ensure all HTTP access logs for the management interface are enabled and forwarded to the SIEM; correlate with the detection rules above.
• Monitor for existing exploitation: Search historical access logs (at least 90 days back, or the log retention period) for %0d%0a or %0D%0A sequences in URIs targeting the management interface. If evidence of exploitation exists, treat it as a compromise and perform a full forensic review.

2) Patch/Upgrade Steps:

• MailGates/MailAudit 6.0: Upgrade to version 6.1.10.054 or later.
• MailGates/MailAudit 5.0: Upgrade to version 5.2.10.099 or later.
• Procedure:
  1. Download the update package from the official Openfind support portal (verify authenticity).
  2. Stage the package on a secure internal host; validate the checksum against the vendor-provided MD5/SHA256.
  3. Schedule a maintenance window; backups of the current configuration and system state are mandatory before upgrade.
  4. Apply the update following the vendor's documented upgrade path. For major version jumps (5.0 to 6.0), follow the migration guide.
  5. After upgrade, verify the version string and run a CRLF injection probe against a test environment first to confirm the vulnerability is remediated.
  6. Re-apply any custom configurations and test mail flow (inbound/outbound) and management access.

3) Long-Term Hardening:

• Network segmentation: Place mail gateways in a dedicated DMZ segment with strict east-west traffic rules. Management access should require MFA and be limited to internal administrative networks.
• Web application firewall: Deploy a WAF in front of the management interface with rules to block CRLF injection payloads (encoded %0d%0a sequences) and path traversal attempts.
• Vulnerability management: Subscribe to Openfind security advisories and the TWNCERT mailing list for future disclosures. Establish a recurring patch cycle (monthly at minimum) for these appliances.
• Least privilege for service accounts: Ensure the process running the web server has only the minimum OS permissions necessary; restrict file read access to sensitive files (/etc/shadow, application configs) via SELinux/AppArmor or OS-level file permissions.
• Access control: Require source IP allowlisting for the management interface, enforce strong password policies, and implement account lockout and brute-force protections.
• Backup and recovery: Maintain current backups of the appliance configuration; test recovery procedures regularly.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-6351 — Official NVD record with CVSS vector (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N) and description.
2. TWNCERT Advisory: https://www.twcert.org.tw/en/ — Taiwan National Computer Emergency Response Team (CNA for this CVE). The advisory contains the patch guidance for MailGates/MailAudit 5.0 (5.2.10.099) and 6.0 (6.1.10.054).
3. Openfind Security Page: https://www.openfind.com/en/ — Official vendor site; check the security/download section for patch packages and release notes for 5.2.10.099 and 6.1.10.054.
4. MITRE CWE-93: https://cwe.mitre.org/data/definitions/93.html — Weakness definition for Improper Neutralization of CRLF Sequences ('CRLF Injection').
5. CISA KEV: CVE-2026-6351 is not listed in CISA's Known Exploited Vulnerabilities catalog as of the report date. No publicly confirmed in-the-wild exploitation or threat actor campaigns have been documented at this time.