An integer overflow occurs in simdjson's document-builder API within the escape_and_append function, causing the calculated buffer size for very large input strings to wrap around on systems with 32‑bit size_t. This underallocation can lead to out‑of‑bounds reads inside SIMD routines, potentially exposing sensitive data, corrupting memory, or generating malformed JSON output. The flaw is exploitable when an attacker supplies an extremely large JSON string that the application parses.

The vulnerability affects all releases of the simdjson library version 4.6.3 and earlier when installed on 32‑bit platforms or built with a 32‑bit size_t type. The fix was introduced in release 4.6.4. Systems using older versions of simdjson, especially those that process externally supplied JSON, are at risk.

The CVSS score of 6.9 indicates medium severity, and the vulnerability is not listed in CISA's KEV catalog. EPSS data is unavailable, so exploitation likelihood cannot be quantified from the current dataset. The likely attack vector is an application that builds JSON documents from untrusted input on a 32‑bit environment. An attacker who can supply such input could trigger out‑of‑bounds reads, leading to information disclosure or memory corruption, depending on the target process's privileges.