External risk intelligence

YAML-Syck Out-of-Bounds Read in Base64 Decoder

CVE advisorySeverity: CRITICAL (CVSS 9.1)

CVE-2026-57075

The vulnerability exists in a Perl library used for parsing YAML data. While it can be triggered by processing untrusted data, the exposure depends on whether an application accepts and parses external YAML input. There is no inherent network service or public-facing role defined for this library, making internet reachability dependent on specific, varied implementation patterns.

Out-of-bounds Read

Halo Surface Signal: 3 out of 5 — possibly public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A vulnerability has been identified in the YAML::Syck library for Perl, which could allow for unauthorized reading of memory when processing untrusted YAML data. This impacts applications that use this library to parse YAML files, potentially exposing sensitive information or leading to system instability. The primary concern at this stage is to confirm if our systems utilize this specific library and are exposed to untrusted YAML inputs.

Memory reading flaw in Perl YAML library.
Matters if untrusted YAML is processed.
Confirm relevance and exposure to this library.

Attack Path

How an attacker could exploit the issue

An attacker can reach this vulnerability by sending specially crafted YAML data containing a `!!binary` tag with specific byte values to an application that parses this data. The vulnerability lies within the `syck_base64dec` function, which processes these binary data. This function can be reached through the default `Load` or `LoadFile` path without any special permissions or user interaction. When triggered, this can lead to an out-of-bounds read, potentially exposing data.

No special access required.
Processing untrusted YAML documents.
Out-of-bounds read, potential data exposure.

Live Threat

Current exploitation, exposure, and threat context

Processing untrusted YAML documents containing specially crafted binary data could lead to an out-of-bounds read within the YAML::Syck library. This vulnerability may expose parts of the process's memory when decoding such data, potentially affecting application stability and the confidentiality of sensitive information.

Memory contents could be exposed.
Malicious YAML input is processed.
Application crashes or memory leakage.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability in YAML::Syck for Perl affects systems that parse untrusted YAML documents containing `!!binary` scalars. Application owners or platform teams responsible for the software utilizing this library should initiate an inventory of where YAML parsing occurs and assess the risk based on exposure to external data. Confirming ownership and identifying critical instances are the first steps before planning remediation, which may involve vendor coordination or temporary mitigations if immediate patching is not feasible.

Application owners should manage remediation.
Verify untrusted YAML parsing points.
Plan remediation based on risk.

Supplementary metadata

CVSS vector

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Frequently asked questions

What is YAML::Syck and why is it used in Perl?

YAML::Syck is a Perl library designed to parse and generate YAML data formats. Developers use it to help applications read configuration files or process data structures serialized in YAML. Because it is highly efficient, it is a common choice for applications that need to handle complex, human-readable data exchanges within the Perl ecosystem.

How does CVE-2026-57075 create a memory vulnerability?

This flaw is an out-of-bounds read, classified as CWE-125. The library's base64 decoder improperly handles specific binary data found in YAML files. When it encounters certain bytes, it interprets them as a negative index, causing the software to read memory locations outside the intended table boundaries. This can leak data from the application's memory or cause the process to crash.

Do I need to be logged in to trigger this bug?

No. The vulnerability is reachable via the default loading functions in the library. An attacker does not need special permissions or an active user session to trigger it. Simply providing an untrusted YAML document that contains a specific binary scalar is enough to reach the faulty decoding code. Documents lacking this specific binary tag do not trigger this memory read error.

Is my system at risk if it isn't internet-facing?

Halo Surface Signal indicates that risk depends on whether your application parses untrusted YAML. While internet-facing systems are often the primary concern, any application—internal or external—that accepts YAML input from untrusted sources is potentially vulnerable. You should prioritize instances where your software handles data originating from external users or external systems.

What should I do if I use YAML::Syck?

First, identify where your applications are using this library to parse YAML files. Prioritize systems that ingest data from untrusted or external sources. Once identified, evaluate if those data paths are necessary or can be restricted. Coordinate with your software maintenance teams to update to version 1.47 or later, which contains the fix, and monitor for updates from your specific vendor or platform maintainer.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.