Описание
Handlebars.js has Denial of Service via Malformed Decorator Syntax in Template Compilation
Summary
When a Handlebars template contains decorator syntax referencing an unregistered decorator (e.g. {{*n}}), the compiled template calls lookupProperty(decorators, "n"), which returns undefined. The runtime then immediately invokes the result as a function, causing an unhandled TypeError: ... is not a function that crashes the Node.js process. Any application that compiles user-supplied templates without wrapping the call in a try/catch is vulnerable to a single-request Denial of Service.
Description
In lib/handlebars/compiler/javascript-compiler.js, the code generated for a decorator invocation looks like:
When "n" is not a registered decorator, lookupProperty(decorators, "n") returns undefined. The expression immediately attempts to call undefined as a function, producing:
Because the error is thrown inside the compiled template function and is not caught by the runtime, it propagates up as an unhandled exception and — when not caught by the application — crashes the Node.js process.
This inconsistency is notable: references to unregistered helpers produce a clean "Missing helper: ..." error, while references to unregistered decorators cause a hard crash.
Attack scenario: An attacker submits {{*n}} as template content to any endpoint that calls Handlebars.compile(userInput)(). Each request crashes the server process; with process managers that auto-restart (PM2, systemd), repeated submissions create a persistent DoS.
Proof of Concept
Expected crash output:
Workarounds
- Wrap compilation and rendering in
try/catch:try { const result = Handlebars.compile(userInput)(context); res.send(result); } catch (err) { res.status(400).send('Invalid template'); } - Validate template input before passing it to
compile(). Reject templates containing decorator syntax ({{*...}}) if decorators are not used in your application. - Use the pre-compilation workflow: compile templates at build time and serve only pre-compiled templates; do not call
compile()at request time.
Ссылки
Пакеты
handlebars
>= 4.0.0, <= 4.7.8
4.7.9
Связанные уязвимости
Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, when a Handlebars template contains decorator syntax referencing an unregistered decorator (e.g. `{{*n}}`), the compiled template calls `lookupProperty(decorators, "n")`, which returns `undefined`. The runtime then immediately invokes the result as a function, causing an unhandled `TypeError: ... is not a function` that crashes the Node.js process. Any application that compiles user-supplied templates without wrapping the call in a `try/catch` is vulnerable to a single-request Denial of Service. Version 4.7.9 fixes the issue. Some workarounds are available. Wrap compilation and rendering in `try/catch`. Validate template input before passing it to `compile()`; reject templates containing decorator syntax (`{{*...}}`) if decorators are not used in your application. Use the pre-compilation workflow; compile templates at build time and serve only pre-compiled templates; do ...
Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, when a Handlebars template contains decorator syntax referencing an unregistered decorator (e.g. `{{*n}}`), the compiled template calls `lookupProperty(decorators, "n")`, which returns `undefined`. The runtime then immediately invokes the result as a function, causing an unhandled `TypeError: ... is not a function` that crashes the Node.js process. Any application that compiles user-supplied templates without wrapping the call in a `try/catch` is vulnerable to a single-request Denial of Service. Version 4.7.9 fixes the issue. Some workarounds are available. Wrap compilation and rendering in `try/catch`. Validate template input before passing it to `compile()`; reject templates containing decorator syntax (`{{*...}}`) if decorators are not used in your application. Use the pre-compilation workflow; compile templates at build time and serve only pre-compiled templates; do ...
Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, when a Handlebars template contains decorator syntax referencing an unregistered decorator (e.g. `{{*n}}`), the compiled template calls `lookupProperty(decorators, "n")`, which returns `undefined`. The runtime then immediately invokes the result as a function, causing an unhandled `TypeError: ... is not a function` that crashes the Node.js process. Any application that compiles user-supplied templates without wrapping the call in a `try/catch` is vulnerable to a single-request Denial of Service. Version 4.7.9 fixes the issue. Some workarounds are available. Wrap compilation and rendering in `try/catch`. Validate template input before passing it to `compile()`; reject templates containing decorator syntax (`{{*...}}`) if decorators are not used in your application. Use the pre-compilation workflow; compile templates at build time and serve only pre-compiled templates; do not
Handlebars.js has Denial of Service via Malformed Decorator Syntax in Template Compilation
Handlebars provides the power necessary to let users build semantic te ...