CVE-2026-56787 is an off-by-one out-of-bounds read vulnerability in RTKLIB through version 2.4.3, specifically in the decode_ssr3 function at src/rtcm3.c line 1446. Remote attackers can exploit this by sending malicious SSR correction streams over NTRIP or serial connections with attacker-controlled signal mode fields, triggering a global buffer overflow. This can cause denial of service or crash RTKLIB-based rovers and CORS servers. The vulnerability has a CVSS 4.0 base score of 6.9, indicating medium severity. No official patch or remediation level has been provided by the vendor as of the published date.

Successful exploitation results in a global buffer overflow due to an off-by-one out-of-bounds read, which can cause denial of service or crash the affected RTKLIB rovers and CORS servers. There is no indication of privilege escalation or data confidentiality impact. The vulnerability is remotely exploitable without authentication.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is available, users should consider restricting or monitoring incoming SSR correction streams over NTRIP or serial connections from untrusted sources to reduce exposure.