Tuesday, January 6, 2015


Multiple LDAP APIs are Asking for Trouble

LDAP filter injection is a classic injection flaw that occurs when user-supplied values find their way into LDAP search filters ("queries") without proper encoding or input validation. The issue has been publicly described since at least 2002 [1] and I still find these flaws on a fairly regular basis. For those unfamiliar with it, these vulnerabilities most often show up in application login forms and can allow an attacker to extract the usernames that exist in the directory and often allow the extraction of attribute values from user objects. In older LDAP servers (or poorly configured ones), it is sometimes possible to extract user password hashes, since these are just an attribute on user objects. LDAP filter injection typically isn't as severe as SQL injection, but it can be serious, depending on how the application uses the filter and what sensitive attributes exist on objects.

Recently, I was describing to a client how to correct a filter injection in their code based on the LDAP API they were using and I was struck by how poorly constructed the API was. In this case, it was the "cfldap" tag provided by Adobe ColdFusion [2][3]. Throughout the documentation, I see no mention anywhere about the dangers of dynamically constructing search filters. In addition, there's apparently no function provided to escape special characters in user-supplied strings for use in filters or DN syntax. Of course any developer could read the fairly straight-forward RFC on the topic [4] and write an encoding function themselves to convert things like "(" to "\28". But as I mentioned in my previous post, one can't really expect developers to be experts in external technologies like LDAP, directory servers, and filter syntax. To add insult to injury, the documentation even provides code examples which are vulnerable to filter injection:

...
<form action="cfldap.cfm" method="POST"> 
    <input type="text" name="name"><br><br> 
    <input type="submit" value="Search"> 
</form> 
<!--- make the LDAP query ---> 
<!-- Note that some search text is required. 
    A search filter of cn=** would cause an error --> 
<cfif (isdefined("form.name") AND (form.name IS NOT ""))> 
    <cfldap 
        server="ldap.airius.com" 
        action="query" 
        name="results" 
        start="ou=People, o=Airius.com" 
        scope="onelevel" 
        filter="(&(cn=*#form.Name#*)(l=Santa Clara))" 
        attributes="cn,sn,ou,mail,telephonenumber" 
        sort="ou,sn" 
        maxrows=100 
        timeout=20000 
    > 
...

There you go, taking a value straight from the form submission and inserting it into a query. How can you expect the typical ColdFusion developer to protect their code from this kind of vulnerability when the API and documentation is effectively setting them up for failure? As one can imagine, ColdFusion isn't alone in this kind of API negligence:
  • PHP's LDAP API does provide an escaping function [5], but it isn't mentioned at all on the ldap_search page, and this page even provides a code example that is almost as vulnerable as ColdFusion's [6].
  • Under Java, the Apache Directory LDAP API does not appear to offer any escaping function and also doesn't mention anything about the risks of dynamically constructed filter expressions [7].
  • For .NET, I have yet to find a method that allows one to escape values prior to including them in search filters (outside of some provided in older C++ APIs). The DirectorySearcher class doesn't seem to contain any mention of the security risks of dynamic filter expressions [8]. Other MSDN pages [9][10] do discuss the escape syntax, but I haven't yet found any security warnings associated with it.
Ok, so if these API vendors aren't doing enough to dissuade developers from making search filter mistakes, what would things look like if they were doing it "right"? Well, we can start by taking inspiration from modern SQL query APIs. The data flow and syntax concerns are very similar, but a lot more attention has been paid to constructing SQL queries safely. The best solutions we have come up with include:
  • Query template APIs, A.K.A. "parameterized prepared statements". Here, programmers are expected to provide a query template along with a series of (potentially untrusted) dynamic values, each mapped to a particular element within the template. Encoding is automatically performed based on data type (or is unnecessary if the database server supports parsing the query templates).
  • Object-Relational Mapping (ORM) or more abstract APIs. Here, an abstract object representation is provided to developers which effectively eliminates the need to construct queries in most situations.
Indeed, these approaches have worked wonders for the safety of relational database access. Over the last several years I have noticed the frequency of SQL injection flaws drop dramatically for newer applications, since most are now leveraging APIs that use one of these approaches. So why don't we do the same for LDAP filter expressions? At very a minimum, each API should have a string escaping function available, along with documentation of the risks of injection. However, I'm not convinced that providing only an escaping mechanism is sufficient, as this approach hasn't been enough to protect SQL queries in the past.

For those keeping score, here's a summary of the limited set of LDAP APIs I've looked at so far and how well they fair in providing safe interfaces and documentation:


API Unsafe
Examples
Security
Risks
Documented
Encoding
Function
Filter
Templates
ORM-like
API
ColdFusion 10 YESNONONONO
PHP 5 YESNOYESNONO
Apache NONONONONO
.NET NONONONONO
python-ldap NOYESYESYESNO
Perl Net::LDAPNOYESNONONO

There's a lot of documentation to cover here, so it's certainly possible I missed something in one or more of these. If I misrepresented anything, post a comment below and I'll happily fix it.

References

  1. LDAP Injection: Are your web applications vulnerable? -- Sacha Faust, SPI Dynamics Inc.
    http://www.networkdls.com/articles/ldapinjection.pdf
  2. ColdFusion Documentation Wiki: cfldap
    https://wikidocs.adobe.com/wiki/display/coldfusionen/cfldap
  3. Adobe ColdFusion 10 - Querying an LDAP directory
    http://help.adobe.com/en_US/ColdFusion/10.0/Developing/WSc3ff6d0ea77859461172e0811cbec0eb56-7fe5.html
  4. RFC 4515: LDAP String Representation of Search Filters
    https://tools.ietf.org/search/rfc4515
  5. PHP: ldap_escape
    http://php.net/manual/en/function.ldap-escape.php
  6. PHP: ldap_search
    http://php.net/manual/en/function.ldap-search.php
  7. Apache Directory API: Searching (...)
    http://directory.apache.org/api/user-guide/2.3-searching.html
  8. MSDN: DirectorySearcher.Filter Property
    http://msdn.microsoft.com/en-us/library/system.directoryservices.directorysearcher.filter(v=vs.110).aspx
  9. MSDN: Search Filter Syntax
    http://msdn.microsoft.com/en-us/library/aa746475%28v=vs.85%29.aspx
  10. MSDN: Creating a Query Filter
    http://msdn.microsoft.com/en-us/library/ms675768%28v=vs.85%29.aspx

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