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| 1 | +/** |
| 2 | + * Provides classes modeling security-relevant aspects of the `mcp` and `fastmcp` PyPI packages, |
| 3 | + * which implement Model Context Protocol (MCP) servers. |
| 4 | + * |
| 5 | + * See |
| 6 | + * - https://pypi.org/project/mcp/ |
| 7 | + * - https://pypi.org/project/fastmcp/ |
| 8 | + */ |
| 9 | + |
| 10 | +private import python |
| 11 | +private import semmle.python.dataflow.new.DataFlow |
| 12 | +private import semmle.python.dataflow.new.internal.DataFlowDispatch |
| 13 | +private import semmle.python.Concepts |
| 14 | +private import semmle.python.ApiGraphs |
| 15 | + |
| 16 | +/** |
| 17 | + * INTERNAL: Do not use. |
| 18 | + * |
| 19 | + * Provides models for the `mcp` and `fastmcp` PyPI packages. |
| 20 | + * |
| 21 | + * The parameters of tool, resource and prompt handlers are chosen by the MCP client, that is by a |
| 22 | + * language model or by anyone who can reach the server. The low-level server of `mcp`, the HTTP |
| 23 | + * headers and the bearer tokens are modeled as data extensions (`mcp.model.yml`, `fastmcp.model.yml`). |
| 24 | + */ |
| 25 | +module Mcp { |
| 26 | + /** Gets a reference to a server class whose instances register tool, resource and prompt handlers. */ |
| 27 | + private API::Node serverClass() { |
| 28 | + exists(API::Node mcpServer | mcpServer = API::moduleImport("mcp").getMember("server") | |
| 29 | + // `FastMCP` in 1.x of `mcp`, renamed to `MCPServer` in 2.x |
| 30 | + result = mcpServer.getMember("fastmcp").getMember("FastMCP") |
| 31 | + or |
| 32 | + result = mcpServer.getMember("fastmcp").getMember("server").getMember("FastMCP") |
| 33 | + or |
| 34 | + result = mcpServer.getMember("MCPServer") |
| 35 | + or |
| 36 | + result = mcpServer.getMember("mcpserver").getMember("MCPServer") |
| 37 | + or |
| 38 | + result = mcpServer.getMember("mcpserver").getMember("server").getMember("MCPServer") |
| 39 | + ) |
| 40 | + or |
| 41 | + exists(API::Node fastmcp | fastmcp = API::moduleImport("fastmcp") | |
| 42 | + result = fastmcp.getMember("FastMCP") |
| 43 | + or |
| 44 | + result = fastmcp.getMember("server").getMember("FastMCP") |
| 45 | + or |
| 46 | + result = fastmcp.getMember("server").getMember("server").getMember("FastMCP") |
| 47 | + ) |
| 48 | + } |
| 49 | + |
| 50 | + /** Gets a reference to a tool or prompt class of `fastmcp` that has a `from_function` method. */ |
| 51 | + private API::Node functionComponentClass() { |
| 52 | + exists(API::Node tools | tools = API::moduleImport("fastmcp").getMember("tools") | |
| 53 | + result = tools.getMember(["Tool", "FunctionTool"]) |
| 54 | + or |
| 55 | + // 2.x defines both classes in `fastmcp.tools.tool`, later versions split them up |
| 56 | + result = tools.getMember("tool").getMember(["Tool", "FunctionTool"]) |
| 57 | + or |
| 58 | + result = tools.getMember("base").getMember("Tool") |
| 59 | + or |
| 60 | + result = tools.getMember("function_tool").getMember("FunctionTool") |
| 61 | + ) |
| 62 | + or |
| 63 | + exists(API::Node prompts | prompts = API::moduleImport("fastmcp").getMember("prompts") | |
| 64 | + result = prompts.getMember(["Prompt", "FunctionPrompt"]) |
| 65 | + or |
| 66 | + result = prompts.getMember("prompt").getMember(["Prompt", "FunctionPrompt"]) |
| 67 | + or |
| 68 | + result = prompts.getMember("base").getMember("Prompt") |
| 69 | + or |
| 70 | + result = prompts.getMember("function_prompt").getMember("FunctionPrompt") |
| 71 | + ) |
| 72 | + } |
| 73 | + |
| 74 | + /** |
| 75 | + * Gets a reference to a standalone decorator of `fastmcp` (3.x and later) that marks a function |
| 76 | + * as a tool or prompt. |
| 77 | + */ |
| 78 | + private API::Node standaloneDecorator() { |
| 79 | + exists(API::Node fastmcp | fastmcp = API::moduleImport("fastmcp") | |
| 80 | + result = fastmcp.getMember("tools").getMember("tool") |
| 81 | + or |
| 82 | + result = fastmcp.getMember("tools").getMember("function_tool").getMember("tool") |
| 83 | + or |
| 84 | + result = fastmcp.getMember("prompts").getMember("prompt") |
| 85 | + or |
| 86 | + result = fastmcp.getMember("prompts").getMember("function_prompt").getMember("prompt") |
| 87 | + ) |
| 88 | + } |
| 89 | + |
| 90 | + /** Gets an API node for a function that is registered as a tool, resource or prompt handler. */ |
| 91 | + private API::Node handlerNode() { |
| 92 | + exists(API::Node server | server = serverClass().getAnInstance() | |
| 93 | + // `@server.tool()`, `@server.resource(uri)`, `@server.prompt()` |
| 94 | + result = server.getMember(["tool", "resource", "prompt"]).getReturn().getParameter(0) |
| 95 | + or |
| 96 | + // `@server.tool` and `@server.prompt` without parentheses (`fastmcp`) |
| 97 | + result = server.getMember(["tool", "prompt"]).getParameter(0) |
| 98 | + or |
| 99 | + result = server.getMember(["tool", "prompt"]).getKeywordParameter("name_or_fn") |
| 100 | + or |
| 101 | + result = server.getMember(["add_tool", "add_prompt"]).getParameter(0) |
| 102 | + or |
| 103 | + result = server.getMember("add_tool").getKeywordParameter(["fn", "tool"]) |
| 104 | + or |
| 105 | + result = server.getMember("add_prompt").getKeywordParameter("prompt") |
| 106 | + ) |
| 107 | + or |
| 108 | + exists(API::Node fromFunction | |
| 109 | + fromFunction = functionComponentClass().getMember("from_function") |
| 110 | + | |
| 111 | + result = fromFunction.getParameter(0) |
| 112 | + or |
| 113 | + result = fromFunction.getKeywordParameter("fn") |
| 114 | + ) |
| 115 | + or |
| 116 | + exists(API::Node decorator | decorator = standaloneDecorator() | |
| 117 | + result = decorator.getParameter(0) |
| 118 | + or |
| 119 | + result = decorator.getKeywordParameter("name_or_fn") |
| 120 | + or |
| 121 | + result = decorator.getReturn().getParameter(0) |
| 122 | + ) |
| 123 | + } |
| 124 | + |
| 125 | + /** Gets a reference to the `Context` class whose instances the SDKs inject into handlers. */ |
| 126 | + private API::Node contextClass() { |
| 127 | + exists(API::Node mcpServer | mcpServer = API::moduleImport("mcp").getMember("server") | |
| 128 | + result = mcpServer.getMember("fastmcp").getMember("Context") |
| 129 | + or |
| 130 | + result = mcpServer.getMember("fastmcp").getMember("server").getMember("Context") |
| 131 | + or |
| 132 | + result = mcpServer.getMember("mcpserver").getMember("Context") |
| 133 | + or |
| 134 | + result = mcpServer.getMember("mcpserver").getMember("context").getMember("Context") |
| 135 | + ) |
| 136 | + or |
| 137 | + exists(API::Node fastmcp | fastmcp = API::moduleImport("fastmcp") | |
| 138 | + result = fastmcp.getMember("Context") |
| 139 | + or |
| 140 | + result = fastmcp.getMember("server").getMember("context").getMember("Context") |
| 141 | + ) |
| 142 | + } |
| 143 | + |
| 144 | + /** |
| 145 | + * Holds if the annotation `e` refers to the injected `Context` class, possibly as |
| 146 | + * `Context[...]`, `Context | None`, `Optional[Context]`, `Union[Context, ...]`, |
| 147 | + * `Annotated[Context, ...]` or as a string such as `"Context"`. |
| 148 | + */ |
| 149 | + private predicate isContextAnnotation(Expr e) { |
| 150 | + e = contextClass().getAValueReachableFromSource().asExpr() |
| 151 | + or |
| 152 | + isContextAnnotation(e.(Subscript).getObject()) |
| 153 | + or |
| 154 | + isContextAnnotation(e.(Subscript).getIndex()) |
| 155 | + or |
| 156 | + isContextAnnotation(e.(Tuple).getAnElt()) |
| 157 | + or |
| 158 | + exists(BinaryExpr union | union = e and union.getOp() instanceof BitOr | |
| 159 | + isContextAnnotation(union.getLeft()) or isContextAnnotation(union.getRight()) |
| 160 | + ) |
| 161 | + or |
| 162 | + // The SDKs resolve string annotations, so `"Context"` counts in a module that imports it. |
| 163 | + e.(StringLiteral).getText().regexpMatch(".*\\bContext\\b.*") and |
| 164 | + contextClass().getAValueReachableFromSource().asExpr().getEnclosingModule() = |
| 165 | + e.getEnclosingModule() |
| 166 | + } |
| 167 | + |
| 168 | + /** Holds if `p` is a parameter of `handler` whose value is chosen by the MCP client. */ |
| 169 | + private predicate isClientParameter(Function handler, Parameter p) { |
| 170 | + handler = handlerNode().getAValueReachingSink().asExpr().(CallableExpr).getInnerScope() and |
| 171 | + ( |
| 172 | + p = handler.getAnArg() |
| 173 | + or |
| 174 | + p = handler.getAKeywordOnlyArg() |
| 175 | + or |
| 176 | + p = handler.getVararg() |
| 177 | + or |
| 178 | + p = handler.getKwarg() |
| 179 | + ) and |
| 180 | + not ( |
| 181 | + handler.isMethod() and |
| 182 | + not isStaticmethod(handler) and |
| 183 | + p = handler.getArg(0) |
| 184 | + ) and |
| 185 | + not isContextAnnotation(p.getAnnotation()) |
| 186 | + } |
| 187 | + |
| 188 | + /** A parameter of an MCP tool, resource or prompt handler, seen as a source of the `ai` threat model. */ |
| 189 | + private class HandlerParameter extends ThreatModelSource::Range, DataFlow::ParameterNode { |
| 190 | + HandlerParameter() { isClientParameter(_, this.getParameter()) } |
| 191 | + |
| 192 | + override string getThreatModel() { result = "ai" } |
| 193 | + |
| 194 | + override string getSourceType() { result = "MCP handler parameter" } |
| 195 | + } |
| 196 | +} |
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