The extractFrom function in Internals/QuotedString.cpp in Arduino JSON before 4.5 allows remote attackers to cause a denial of service (crash) via a JSON string with a \ (backslash) followed by a terminator, as demonstrated by "\\\0", which triggers a buffer overflow and over-read.
https://github.com/bblanchon/ArduinoJson/commit/5e7b9ec688d79e7b16ec7064e1d37e8481a31e72
https://github.com/bblanchon/ArduinoJson/blob/master/CHANGELOG.md