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mcp-memgraph: harden read-only mode - #623
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…d list In read-only mode, queries now run in a read transaction, so Memgraph rejects anything that writes, including write procedures. Queries that can't run in a transaction are allowed only for SHOW and PROFILE of a read query. Reads that only mention a write keyword are no longer refused.
katarinasupe
marked this pull request as draft
October 28, 2026 14:47
A read transaction alone lets through procedures Memgraph flags as reads that write through their own transaction or connection (do.*, periodic.*), code and file procedures, and auth commands. Refuse those and write keywords outside strings and comments before the query runs, strip leading comments (some Memgraph versions crash on a comment before EXPLAIN/PROFILE), and allow DATA DIRECTORY LOCK STATUS. Tests now check that blocked queries leave the database unchanged. The README says read-only mode is best effort.
Match procedures that can write anywhere in the query and under their APOC names, unquote backticked names in the same pass as strings and comments, treat U+180E and zero-width characters as whitespace, and add CLEAR, model training and outbound-request procedures. Keywords after a dot or used as a map key no longer count, so map.merge and n.set are allowed. The README says the text check is not a parser and can be bypassed.
katarinasupe
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katarinasupe
marked this pull request as ready for review
October 28, 2026 15:21
antejavor
approved these changes
Oct 29, 2026
Block only the procedures that change something (run other queries, write files, load code, change a shared model, write to Elasticsearch), not whole modules, so xml_module.parse, tgn.get, json_util.load_from_url, llm.complete and Elasticsearch search run again. Require CALL before a procedure name and match GRANT, REVOKE, DENY and CLEAR only at the start of a query, so variables and labels with those names aren't refused.
Start the server as its own process with MCP_READ_ONLY=true, call run_cypher_query through an MCP client, and check that writes are refused without changing the database while reads still return rows.
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Not a good solution. |
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Read-only mode (the default) only blocked queries containing
CREATE,MERGE,DELETE,REMOVE,SETorDROP. Writes without those words ran: write procedures (schema.assert,refactor.*), admin commands (ANALYZE GRAPH,FREE MEMORY,STORAGE MODE), and procedures flagged as reads that write in their own transaction or touch files and code (do.*,periodic.*,mg.create_module_file+mg.load,csv_utils.*). Reads likeRETURN 'set'were refused.Now, in layers:
SHOW ...,DATA DIRECTORY LOCK STATUSandPROFILEof a read query run.EXPLAIN/PROFILEcrashes Memgraph 3.14).This is still a best-effort guard: the text check is not a Cypher parser, and a custom module flagged as read-only can do anything. The README says so. A sturdier version would check Memgraph's
EXPLAINplan instead of the query text. Known refusals:EXPLAINof a write query,SET SESSION TRACE.Tests compare database state before and after each case (25 blocked, 15 reads) against
memgraph-mage:3.14.0, plus 12 readsmainallowed that must pass the text check, and one test that talks to the server process over stdio; the full suite passes. Every read-flagged procedure and function in that image was checked: the only ones refused thatmainallowed change something (run other queries, write files, load code, change a shared model).