CVE-2018-1000134 and the UnboundID LDAP SDK for Java

On Friday, March 16, 2018, CVE-2018-1000134 was published, describing a vulnerability in the UnboundID LDAP SDK for Java. The vulnerability has been fixed in LDAP SDK version 4.0.5, which is available for immediate download from the LDAP.com website, from the releases page of our GitHub repository, from the Files page of our SourceForge project, and from the Maven Central Repository.

This post will explain the issue in detail (see the release notes for information about other changes in LDAP SDK version 4.0.5). However, to quickly determine whether your application is vulnerable, you should check to see if all of the following conditions are true:

  • You are using the LDAP SDK in synchronous mode. Although this mode is recommended for applications that do not require asynchronous functionality, the LDAP SDK does not use this mode by default.
  • You use the LDAP SDK to perform simple bind operations for the purpose of authenticating users to a directory server. This is a very common use case for LDAP-enabled applications.
  • Your application does not attempt to verify whether the user actually provided a password. This is unfortunately all too common for LDAP-enabled applications.
  • The simple bind requests are sent to a directory server that does not follow the RFC 4513 section 5.1.2 recommendation to reject simple bind requests with a non-empty DN and an empty password. Although this recommendation is part of the revised LDAPv3 specification published in 2006, there are apparently some directory servers that still do not follow this recommendation by default.

If your application meets all of these criteria, then you should take action immediately to protect yourself. The simplest way to fix the vulnerability in your application is to update it to use the 4.0.5 release of the LDAP SDK. However, you should also ensure that your applications properly validate all user input, and it may also be a good idea to consider switching to a more modern directory server.

The Vulnerability in LDAPv3

The original LDAPv3 protocol specification was published as RFC 2251 in December 1997. LDAPv3 is a very impressive protocol in most regards, but perhaps the most glaring problem in the specification lies in the following paragraph in section 4.2.2:

If no authentication is to be performed, then the simple authentication option MUST be chosen, and the password be of zero length. (This is often done by LDAPv2 clients.) Typically the DN is also of zero length.

It’s that word “typically” in this last sentence that has been the source of a great many vulnerabilities in LDAP-enabled applications. Usually, when you want to perform an anonymous simple bind, you provide an empty string for both the DN and the password. However, according to the letter of the specification above, you don’t have to provide an empty DN. As long as the password is empty, the server will treat it as an anonymous simple bind.

In applications that use an LDAP simple bind to authenticate users, it’s a very common practice to provide two fields on the login form: one for the username (or email address or phone number or some other kind of identifier), and one for the password. The application first performs a search to see if they can map that username to exactly one user in the directory, and if so, then it performs a simple bind with the DN of that user’s entry and the provided password. As long as that the server returns a “success” response to the bind request, then the application considers the user authenticated and will grant them whatever access that user is supposed to have.

However, a problem can arise if the application just blindly takes whatever password was provided in the login form and plugs it into the simple bind request without actually checking to see whether the user provided any password at all. In such cases, if the user provided a valid username but an empty password, then the application will perform a simple bind request with a valid DN but no password. The directory server will interpret that as an anonymous simple bind and will return a success result, and the application will assume that the user is authenticated even though they didn’t actually provide any password at all.

This is such a big problem in LDAP-enabled applications that it was specifically addressed in the updated LDAPv3 specification published in June 2006. RFC 4513 section 5.1.2 states the following:

Unauthenticated Bind operations can have significant security issues (see Section 6.3.1). In particular, users intending to perform Name/Password authentication may inadvertently provide an empty password and thus cause poorly implemented clients to request Unauthenticated access. Clients SHOULD be implemented to require user selection of the Unauthenticated Authentication Mechanism by means other than user input of an empty password. Clients SHOULD disallow an empty password input to a Name/Password Authentication user interface. Additionally, Servers SHOULD by default fail Unauthenticated Bind requests with a resultCode of unwillingToPerform.

Further, section 6.3.1 of the same RFC states:

Operational experience shows that clients can (and frequently do) misuse the unauthenticated access mechanism of the simple Bind method (see Section 5.1.2). For example, a client program might make a decision to grant access to non-directory information on the basis of successfully completing a Bind operation. LDAP server implementations may return a success response to an unauthenticated Bind request. This may erroneously leave the client with the impression that the server has successfully authenticated the identity represented by the distinguished name when in reality, an anonymous authorization state has been established. Clients that use the results from a simple Bind operation to make authorization decisions should actively detect unauthenticated Bind requests (by verifying that the supplied password is not empty) and react appropriately.

In directory servers that follow the recommendation from RFC 4513 section 5.1.2, clients can perform an anonymous simple bind by providing an empty DN and an empty password, but an attempt to bind with a non-empty DN and an empty password will be rejected. This very good recommendation was made over ten years ago, and the code change needed to implement it is probably very simple. However, for some reason, there are directory server implementations out there that haven’t been updated to follow this recommendation, and therefore leave client applications open to this inadvertent vulnerability.

The Vulnerability in the UnboundID LDAP SDK for Java

Ever since its initial release, the UnboundID LDAP SDK for Java has attempted to protect against simple bind requests that include a non-empty DN with an empty password. The LDAPConnectionOptions class provides a setBindWithDNRequiresPassword(boolean) method that you can use to indicate whether the LDAP SDK will reject a simple bind request that has a non-empty DN with an empty password. If you don’t explicitly use this option, then the LDAP SDK will assume a default value of true. If you try to send a simple bind request that includes a non-empty DN and an empty password, then the LDAP SDK won’t actually send any request to the server but will instead throw an LDAPException with a result code of ResultCode.PARAM_ERROR and a message of “Simple bind operations are not allowed to contain a bind DN without a password.”

Or at least, that’s the intended behavior. And that is the behavior that you’ll get if you send the bind request in the asynchronous mode that the LDAP SDK uses by default. However, Stanis Shkel created GitHub issue #40 (“processSync in SimpleBindRequest allows empty password with set bindDN”), which points out that this check was skipped for connections operating in synchronous mode.

LDAP is an asynchronous protocol. With a few exceptions, it’s possible to have multiple operations in progress simultaneously over the same LDAP connection. To support that asynchronous capability, the LDAP SDK maintains an extra background thread that constantly read data from a connection and makes sure that any data sent from the server gets delivered to whichever thread is waiting for it. This is just fine most of the time, but it does come at the cost of increased resource consumption, and a small performance hit from handing off data from one thread to another. To minimize this impact for applications that don’t take advantage of the asynchronous capabilities that LDAP provides, we added a synchronous mode to the LDAP SDK way back in version 0.9.10 (released in July of 2009). In this mode, the same thread that sends a request to the server is the one that waits for and reads the response. This can provide better performance and lower resource consumption, but you have to explicitly enable it using the LDAPConnectionOptions.setUseSynchronousMode(boolean) method before establishing a connection.

In the course of implementing support for the synchronous mode for a simple bind request, we incorrectly put the check for synchronous mode before the check for an empty password. For a connection operating in synchronous mode, we branched off to another part of the code and skipped the check for an empty password. The fix for the problem was simple: move the check for an empty password above the check for synchronous mode, and it was committed about three and a half hours after the issue was reported, including a unit test to ensure that a simple bind request with a non-empty DN and an empty password is properly rejected when operating in synchronous mode (there was already a test to ensure the correct behavior in the default asynchronous mode).

Conditions Necessary for the Vulnerability

Although there was unquestionably a bug in the LDAP SDK that created the possibility for this bug, there are a number of factors that could have prevented an application from being susceptible to it. Only an application that meets all of the following conditions would have been vulnerable:

  • The application must have explicitly enabled the use of synchronous mode when creating an LDAP connection or connection pool. If the application was using the default asynchronous mode, it would not have been vulnerable.
  • The application must have created simple bind requests from untrusted and unverified user input. If the application did not create simple bind requests (for example, because it did not perform binds at all, or because it used SASL authentication instead of simple), then it would not have been vulnerable. Alternately, if the application validated the user input to ensure that it would not attempt to bind with an empty password, then it would not have been vulnerable.
  • The application must have sent the simple bind request to a server that does not follow the RFC 4513 recommendations. If the server is configured to reject simple bind requests that contain a non-empty DN with an empty password, then an application communicating with that server would not have been vulnerable.

While we strongly recommend updating to LDAP SDK version 4.0.5, which no longer has the bug described in CVE-2018-1000134, we also strongly recommend ensuring that applications properly validate all user input as additional mitigation against problems like this. And if you’re using a directory server that hasn’t been updated to apply a very simple update to avoid a problem that has been well known and clearly documented for well over a decade, then perhaps you should consider updating to a directory server that takes security and standards compliance more seriously.