OpenSSL Out-of-bounds Write Vulnerability - CVE-2026-7383

Issue summary: A signed integer overflow when sizing the destination buffer for Unicode output in ASN1_mbstring_ncopy() can lead to a heap buffer overflow. Impact summary: A heap buffer overflow may lead to a crash or possibly attacker controlled code execution or other undefined behaviour. In ASN1_mbstring_copy() and ASN1_mbstring_ncopy() the destination size for Unicode output is computed in a signed int: by left shift of the input character count for BMPSTRING (UTF-16) and UNIVERSALSTRING (UTF-32) and by summing per-character byte counts for UTF8STRING. The calculation overflows when the input reaches around 230 characters. In the worst case (UNIVERSALSTRING at 230 characters) the size wraps to zero OPENSSL_malloc(1) is called and the subsequent character copy writes several gigabytes past the one-byte allocation. X.509 certificate processing routes through ASN1_STRING_set_by_NID() whose DIRSTRING_TYPE mask excludes UNIVERSALSTRING and whose per-NID size limits cap the input length no network protocol or certificate-handling path in OpenSSL exercises the overflow. Triggering the bug requires an application that calls ASN1_mbstring_copy() or ASN1_mbstring_ncopy() directly or registers a custom string type via ASN1_STRING_TABLE_add() with attacker-controlled input on the order of half a gigabyte or more. For these reasons this issue was assigned Low severity. The FIPS modules in 4.0 3.6 3.5 3.4 and 3.0 are not affected by this issue as the affected code is outside the OpenSSL FIPS module boundary.