CVE-2026-75615: ASN1SCC: Out-of-bounds read via unvalidated character index in ASN.1 string decoders

ASN1SCC is an open source ASN.1 compiler for embedded systems. Prior to 4.7.0.1, the ACN runtime function AcnDecStringCharIndexprivate in asn1crt/asn1crtencodingacn.c and the generated uPER decoder InternalItemstringwithalphadecode in StgC/uperc.stg do not validate a decoded character index before reading allowedCharSet. For string alphabets whose size is not a power of two, a crafted message can select an unused encoded index, read a small bounded window of adjacent static program constants, and return the byte as a successfully decoded character. The full 128-character IA5String alphabet is not affected. This issue is fixed in version 4.7.0.1.