Security checklist

This document aims as summarizing threats, security best practices as well as recommendations.

Threats

Operating an email server exposes you to the following threats:

  • Spammers might attempt to use your servers to send their spam messages on their behalf. We speak of open relay. In addition to the resources consumed being an open relay will affect the trust other mail installations have in you, and thus will cause legitimate traffic to be rejected.

  • Emails mostly consist of private data, which shall only be accessed by their legitimate user. Failure to do so might result in information disclosure.

  • Email forgery. An attacker might craft an email on the behalf of legitimate users.

  • Email protocols allow user to authenticate and thus can be used as oracles to guess user passwords.

  • Spam. Non legitimate traffic can be a real burden to your users.

  • Phishing: Crafted emails that tricks the user into doing unintended actions.

  • Viruses: An attacker sends an attachment that contains an exploit that could run if a user opens it.

  • Denial of service: A small request may result in a very large response and require considerable work on the server…​

  • Denial of service: A malicious JMAP client may use the JMAP push subscription to attempt to flood a third party server with requests, creating a denial-of-service attack and masking the attacker’s true identity.

  • Dictionary Harvest Attacks: An attacker can rely on SMTP command reply code to know if a user exists or not. This can be used to obtain the list of local users and later use those address as targets for other attacks.

Best practices

The following sections ranks best practices.

Best practices: Must

  • 1. Configure James in order not to be an open relay. This should be the case with the default configuration.

Be sure in smtpserver.xml to activate the following options: verifyIdentity.

We then recommend to manually test your installation in order to ensure that:

  • Unauthenticated SMTP users cannot send mails to external email addresses (they are not relayed)

  • Unauthenticated SMTP users can send mails to internal email addresses

  • Unauthenticated SMTP users cannot use local addresses in their mail from, and send emails both locally and to distant targets.

  • 2. Avoid STARTTLS usage and favor SSL. Upgrade from a non encrypted channel into an encrypted channel is an opportunity for additional vulnerabilities. This is easily prevented by requiring SSL connection upfront. Read more…​

Please note that STARTTLS is still beneficial in the context of email relaying, which happens on SMTP port 25 unencrypted, and enable opportunistic encryption upgrades that would not overwise be possible. We recommend keeping STARTTLS activated for SMTP port 25.

  • 3. Use SSL for remote delivery whenever you are using a gateway relaying SMTP server.

  • 4. Rely on an external identity service, dedicated to user credential storage. James supports LDAP. If you are forced to store users in James be sure to choose PBKDF2 as a hashing algorithm. Also, delays on authentication failures are supported via the verifyFailureDelay property. Note that IMAP / SMTP connections are closed after 3 authentication failures.

  • 5. Ensure that WebAdmin is not exposed unencrypted to the outer world. Doing so trivially exposes yourself. You can either disable it, activate JWT security, or restrict it to listen only on localhost.

  • 6. Set up HTTPS for http based protocols, namely JMAP and WebAdmin. We recommend the use of a reverse proxy like Nginx.

  • 7. Set up SPF and DKIM for your outgoing emails to be trusted.

  • 8. Prevent access to JMX. This can be achieved through a strict firewalling policy (blocking port 9999 is not enough) or disabling JMX. JMX is needed to use the existing CLI application but webadmin do offer similar features. Set the jmx.remote.x.mlet.allow.getMBeansFromURL to false to disable JMX remote code execution feature.

  • 9. If JMAP is enabled, be sure that JMAP PUSH cannot be used for server side request forgery. This can be configured using the push.prevent.server.side.request.forgery=true property, forbidding push to private addresses.

Best practice: Should

  • 1. Avoid advertising login/authenticate capabilities in clear channels. This might prevent some clients to attempt login on clear channels, and can be configured for both SMTP and IMAP using auth.plainAuthEnabled=false.

  • 2. Verify SPF and DKIM for your incoming emails.

  • 3. Set up reasonable storage quota for your users.

  • 4. We recommend setting up anti-spam and anti-virus solutions. James comes with some Rspamd and SpamAssassin integration, and some ClamAV tooling exists. Rspamd supports anti-phishing modules. Filtering with third party systems upstream is also possible.

  • 5. In order to limit your attack surface, disable protocols you or your users do not use. This includes the JMAP protocol, POP3, ManagedSieve, etc…​ Be conservative on what you expose.

  • 6. If operating behind a load-balancer, set up the PROXY protocol for TCP based protocols (IMAP and SMTP proxyRequired option)

Best practice: Could

Known vulnerabilities

Several vulnerabilities have had been reported for previous releases of Apache James server.

Be sure not to run those! We highly recommend running the latest release, which we put great effort in not to use outdated dependencies.

Reporting vulnerabilities

We follow the standard procedures within the ASF regarding vulnerability handling

CVE-2024-21742: Mime4J DOM header injection

Apache JAMES MIME4J prior to version 0.8.10 allow attackers able to specify the value of a header field to craft other header fields.

Severity: Moderate

Mitigation: Release 0.8.10 rejects the use of LF inside a header field thus preventing the issue.

Upgrading to Apache James MIME4J 0.8.10 is thus advised.

CVE-2023-51747: SMTP smuggling in Apache James

Apache James distribution prior to release 3.7.5 and release 3.8.1 is subject to SMTP smuggling, when used in combination of antother vulnerable server and can result in SPF bypass, leading to email forgery.

Severity: High

Mitigation: Release 3.7.5 and 3.8.1 interpret strictly the CRLF delimiter and thus prevent the issue.

Upgrading to Apache James 3.7.5 or 3.8.1 is thus advised.

CVE-2023-51518: Privilege escalation via JMX pre-authentication deserialisation

Apache James distribution prior to release 3.7.5 and 3.8.1 allow privilege escalation via JMX pre-authentication deserialisation. An attacker would need to identify a deserialization glitch before triggering an exploit.

Severity: Moderate

Mitigation:We recommend turning off JMX whenever possible.

Release 3.7.5 and 3.8.1 disable deserialization on unauthencited channels.

Upgrading to Apache James 3.7.5 on 3.8.1 is thus advised.

CVE-2023-26269: Privilege escalation through unauthenticated JMX

Apache James distribution prior to release 3.7.4 allows privilege escalation through the use of JMX.

Severity: Moderate

Mitigation: We recommend turning on authentication on. If the CLI is unused we recommend turning JMX off.

Release 3.7.4 set up implicitly JMX authentication for Guice based products and addresses the underlying JMX exploits.

Upgrading to Apache James 3.7.4 is thus advised.

CVE-2022-45935: Temporary File Information Disclosure in Apache JAMES

Apache James distribution prior to release 3.7.3 is vulnerable to a temporary File Information Disclosure.

Severity: Moderate

Mitigation: We recommend to upgrade to Apache James 3.7.3 or higher, which fixes this vulnerability.

CVE-2021-44228: STARTTLS command injection in Apache JAMES

Apache James distribution prior to release 3.7.1 is vulnerable to a buffering attack relying on the use of the STARTTLS command.

Fix of CVE-2021-38542, which solved similar problem from Apache James 3.6.1, is subject to a parser differential and do not take into account concurrent requests.

Severity: Moderate

Mitigation: We recommend to upgrade to Apache James 3.7.1 or higher, which fixes this vulnerability.

CVE-2021-38542: Apache James vulnerable to STARTTLS command injection (IMAP and POP3)

Apache James prior to release 3.6.1 is vulnerable to a buffering attack relying on the use of the STARTTLS command. This can result in Man-in -the-middle command injection attacks, leading potentially to leakage of sensible information.

Severity: Moderate

This issue is being tracked as JAMES-1862

Mitigation: We recommend upgrading to Apache James 3.6.1, which fixes this vulnerability.

Furthermore, we recommend, if possible to dis-activate STARTTLS and rely solely on explicit TLS for mail protocols, including SMTP, IMAP and POP3.

Read more about STARTTLS security here.

CVE-2021-40110: Apache James IMAP vulnerable to a ReDoS

Using Jazzer fuzzer, we identified that an IMAP user can craft IMAP LIST commands to orchestrate a Denial Of Service using a vulnerable Regular expression. This affected Apache James prior to 3.6.1

Severity: Moderate

This issue is being tracked as JAMES-3635

Mitigation: We recommend upgrading to Apache James 3.6.1, which enforce the use of RE2J regular expression engine to execute regex in linear time without back-tracking.

CVE-2021-40111: Apache James IMAP parsing Denial Of Service

While fuzzing with Jazzer the IMAP parsing stack we discover that crafted APPEND and STATUS IMAP command could be used to trigger infinite loops resulting in expensive CPU computations and OutOfMemory exceptions. This can be used for a Denial Of Service attack. The IMAP user needs to be authenticated to exploit this vulnerability. This affected Apache James prior to version 3.6.1.

Severity: Moderate

This issue is being tracked as JAMES-3634

Mitigation: We recommend upgrading to Apache James 3.6.1, which fixes this vulnerability.

CVE-2021-40525: Apache James: Sieve file storage vulnerable to path traversal attacks

Apache James ManagedSieve implementation alongside with the file storage for sieve scripts is vulnerable to path traversal, allowing reading and writing any file.

Severity: Moderate

This issue is being tracked as JAMES-3646

Mitigation:This vulnerability had been patched in Apache James 3.6.1 and higher. We recommend the upgrade.

This could also be mitigated by ensuring manageSieve is disabled, which is the case by default.

Distributed and Cassandra based products are also not impacted.

CVE-2017-12628 Privilege escalation using JMX

The Apache James Server prior version 3.0.1 is vulnerable to Java deserialization issues. One can use this for privilege escalation. This issue can be mitigated by:

  • Upgrading to James 3.0.1 onward

  • Using a recent JRE (Exploit could not be reproduced on OpenJdk 8 u141)

  • Exposing JMX socket only to localhost (default behaviour)

  • Possibly running James in a container

  • Disabling JMX all-together (Guice only)

Read more here.