Executive brief
Facebook Hermes is a JavaScript engine used in React Native applications. An out-of-bounds memory access vulnerability in the lazy compilation of generator functions could allow an attacker to execute arbitrary code if the application evaluates untrusted JavaScript. However, most React Native apps are unaffected since they do not typically permit such evaluation.
Technical details
An out-of-bounds read and write vulnerability exists in Facebook Hermes prior to commit 091835377369c8fd5917d9b87acffa721ad2a168. When a generator function is large enough to trigger lazy compilation, the engine incorrectly loses information about whether the function is an inner generator. This causes the function to be generated as a regular function instead, leading to memory access violations. The vulnerability is exploitable only if untrusted JavaScript evaluation is permitted. The root cause is improper restoration of function context during lazy compilation. The fix involves correctly tracking the isGeneratorInnerFunction flag during serialization and deserialization.
Affected products
- Facebook Hermes prior to 0.5.2
Timeline
- 2020-09-09: disclosed
- 2020-09-08: patched: Fix committed to Hermes repository