Enterprise and many consumer apps authenticate users or look up directory information against an LDAP service, often a corporate directory. If the backend builds an LDAP query by dropping user input straight into a search filter, it has the same problem as SQL injection in a different language: an attacker can inject LDAP filter syntax to change what the query matches, bypassing authentication or reaching data they should not. LDAP injection is less talked about than SQL injection, but the cause and the fix are the same idea. Here is what LDAP injection is, how it shows up in a mobile app's backend, and how to prevent it.
Short answer
LDAP injection is a vulnerability where untrusted input is placed into an LDAP search filter or query without being escaped, letting an attacker inject LDAP filter syntax that changes the query's meaning. Per OWASP, this can bypass authentication, return unauthorized directory entries, or otherwise manipulate the query, much like SQL injection but against an LDAP directory. It affects mobile apps whose backend authenticates users or looks up data against LDAP, often a corporate directory, using user-supplied values like a username or search term. The fix is to escape LDAP special characters in user input according to the directory's rules before building a filter, use safe directory APIs, validate input, and run the directory bind account with least privilege. Keep user input as a value in the filter, never as filter syntax.
What you should know
- LDAP injection manipulates a search filter: via unescaped untrusted input.
- It can bypass authentication: making a filter match when it should not.
- It can expose directory data: returning entries it should not.
- It affects apps that use LDAP: typically a corporate directory for auth or lookup.
- The fix is escaping and safe APIs: input as a value, not filter syntax.
What is LDAP injection?
It is injecting LDAP filter syntax through untrusted input used to build a directory query. LDAP search filters have their own syntax, with special characters and operators that combine conditions, for example matching an attribute to a value, or combining clauses. When an application builds a filter by inserting user input, a username to authenticate, a term to search, without escaping it, an attacker can include those special characters in their input so that what they supply is interpreted as part of the filter rather than as a plain value. That lets them change which entries the query matches. The canonical result is authentication bypass, where crafted input turns a lookup for a specific user into a filter that matches more broadly, or alters the logic so authentication succeeds when it should not. The root cause is identical to other injection flaws: untrusted input placed into a structured query without separating data from the query syntax, here the syntax being an LDAP filter.
How does it manifest?
In a few forms, mirroring other injection flaws. The table lists them.
| Form | What happens |
|---|---|
| Authentication bypass | A crafted filter matches a user when it should not |
| Unauthorized data access | The filter returns directory entries it should not |
| Filter logic manipulation | Injected operators change the query's conditions |
| Wildcard abuse | Injected wildcards broaden what the query matches |
| Blind injection | Inferring directory data from response differences |
The most impactful form is authentication bypass: if login builds an LDAP filter from a username and the attacker injects filter syntax, they can make the filter match an account or evaluate in a way that lets them in without valid credentials. Beyond auth, injecting into a directory search can return entries the user should not see, by broadening the filter or altering its conditions, which discloses directory data. Injected operators can change the logical structure of the filter, and injected wildcards can widen what it matches. As with SQL injection, blind variants exist, where an attacker infers information from how the directory's responses differ. Every form traces back to the same gap, user input treated as filter syntax instead of a value, so the directory query does something other than intended.
How do you prevent it?
Escape input for LDAP, use safe APIs, and limit the bind account. The core defense is to escape LDAP special characters in any user input before it is placed into a filter, according to the directory's escaping rules, so that characters which would otherwise be filter syntax are treated as literal parts of a value, which neutralizes the injection. Use your platform's safe directory APIs and query-building facilities rather than concatenating raw input into a filter string by hand, since they handle escaping correctly. Validate input against what it should be, for example constraining a username to an expected format, and allowlist where feasible, to reduce what can reach the filter. Run the directory bind account your backend uses with least privilege, so even a successful injection is limited in what it can read or do, and never use an over-privileged account for routine lookups. And enforce all of this on the server, since the attacker supplies the input by calling your API directly. The principle is the same as for SQL and other injection: separate data from query syntax, here by escaping user input for LDAP so it can only ever be a value in the filter, not part of its structure.
What to watch out for
The first trap is building an LDAP filter by concatenating a username or search term directly into it, which lets injected filter syntax bypass authentication or expose data; escape the input and use safe APIs. The second is an over-privileged directory bind account, which widens the impact of any injection; apply least privilege. The third is assuming LDAP is immune because it is not SQL, when the same injection pattern applies in LDAP syntax. LDAP injection is server-side, so a pre-submission scan such as PTKD.com (https://ptkd.com), which reads the binary against OWASP MASVS, surfaces the endpoints and parameters your app sends as the surface to protect, while the filter escaping and validation are yours to implement on the backend.
What to take away
- LDAP injection places untrusted input into an LDAP filter without escaping, letting an attacker inject filter syntax that can bypass authentication, expose directory entries, or manipulate the query.
- It affects mobile apps whose backend authenticates or looks up data against an LDAP directory using user-supplied values, the SQL-injection pattern in LDAP form.
- Prevent it by escaping LDAP special characters in input, using safe directory APIs instead of building filters by hand, validating input, and running the bind account with least privilege.
- LDAP injection is server-side; use a pre-submission scan such as PTKD.com to surface the parameters your app sends, then escape and validate them on the backend.
