There exists an out of bounds read/write in LibJXL versions prior to commit 9cc451b91b74ba470fd72bd48c121e9f33d24c99. The JPEG decoder used by the JPEG XL encoder when doing JPEG recompression (i.e. if using JxlEncoderAddJPEGFrame on untrusted input) does not properly check bounds in the presence of incomplete codes. This could lead to an out-of-bounds write. In jpegli which is released as part of the same project, the same vulnerability is present. However, the relevant buffer is part of a bigger structure, and the code makes no assumptions on the values that could be overwritten. The issue could however cause jpegli to read uninitialised memory, or addresses of functions.
The product reads data past the end, or before the beginning, of the intended buffer.