Medium [CVE-2026-57076] YAML::Syck: Heap use-after-free via anchor name reuse

YAML::Syck versions before 1.47 for Perl allow a heap use-after-free via an anchor name reused as an anchors-table key in syck_hdlr_add_anchor. In the bundled libsyck an anchor name allocated by syck_strndup is stored both as node->anchor, freed when the node is freed, and as the key in the parser's anchors table.

Freeing the node frees the shared key, and a later anchor redefinition makes st_delete compare against the freed key, so st_strcmp reads freed heap memory. Anchors are a standard YAML feature and need no special flags, so this is reached on the default Load path.

Any caller that runs Load or LoadFile on an untrusted document that redefines an anchor reaches the read of freed memory. An attacker could exploit a heap use-after-free vulnerability by providing a specially crafted YAML document that reuses an anchor name as an anchors-table key.

This flaw causes the software to read freed heap memory, which may lead to information disclosure or denial of service. This Important heap use-after-free vulnerability in `perl-YAML-Syck` can be triggered by processing a specially crafted YAML document.

The flaw, exploitable through the default YAML loading mechanism, may lead to information disclosure or a denial of service in applications handling untrusted YAML input. Red Hat severity: Moderate — CVSS 6.3 (CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:N/A:H).

Weakness: CWE-825.