High [CVE-2026-13713] YAML::Syck: Denial of Service via crafted YAML document

YAML::Syck versions before 1.47 for Perl allow a use-after-free and double-free via an anchor node freed while still on the parser value stack. In the bundled libsyck, when an anchor name is redefined or removed, syck_hdlr_add_anchor and syck_hdlr_remove_anchor free the node stored under that name with syck_free_node.

That node can still be live on the parser's value stack, so syck_hdlr_add_node reaches it again and frees it a second time. On a normal build the 48-byte node chunk is freed twice and the interpreter aborts.

Anchors need no special flags, so this is reached on the default Load path, and a 7-byte document that redefines an anchor triggers it. Any caller that runs Load or LoadFile on an untrusted document that redefines an anchor mid-parse crashes the interpreter, a denial of service.

This vulnerability allows a remote attacker to cause a denial of service (DoS) by providing a specially crafted YAML document. This Important vulnerability in `perl-YAML-Syck` can lead to a denial of service when processing untrusted YAML documents.

The flaw, a use-after-free and double-free, causes the interpreter to crash if a specially crafted YAML document redefines an anchor during parsing. This poses a risk to Red Hat systems that handle external YAML input without sufficient validation.

Red Hat severity: Important — CVSS 7.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).