Medium [CVE-2026-57075] YAML::Syck: YAML::Syck: Information disclosure via out-of-bounds read in base64 decoder

YAML::Syck versions before 1.47 for Perl allow an out-of-bounds read via a signed-char lookup-table index in syck_base64dec. The base64 decoder in the bundled libsyck indexes the 256-entry static table b64_xtable with a signed char, so any!!binary byte >= 0x80 sign-extends to a negative index and reads before the table.

The decoder receives the raw bytes of any!!binary node, a standard YAML type not gated by $LoadBlessed or $LoadCode, so it is reached on the default Load path. Any caller that runs Load or LoadFile on an untrusted document containing a!!binary scalar with a high-bit byte triggers the read, and the value read can surface in the decoded result.

An out-of-bounds read vulnerability exists in the base64 decoder, specifically in the `syck_base64dec` function. This occurs because the decoder uses a signed character to index a lookup table, allowing specially crafted `!!binary` YAML nodes with high-bit bytes to cause a read beyond the intended memory region.

A remote attacker could exploit this by providing a malicious YAML document, potentially leading to information disclosure. This Moderate impact flaw in `perl-YAML-Syck` can lead to information disclosure.

Exploitation requires an application to parse a specially crafted YAML file, making the impact dependent on the application's handling of untrusted input.