External risk intelligence

RRD::CGI::Image tzset() Vulnerability Allows Process Stalling

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2026-123776

The vulnerability resides in a CGI (Common Gateway Interface) module for Perl. CGI scripts are traditionally used to create dynamic, internet-facing web content, and web-based interfaces are frequently deployed as public-facing services. Because this component is designed to process external web query parameters directly, it is commonly encountered in internet-accessible web application environments.

Halo Surface Signal: 4 out of 5 — likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

This advisory concerns a vulnerability in a Perl CGI module used for image processing. The issue allows specially crafted web requests to potentially stall the affected CGI process by referencing a FIFO or blocking device. This could lead to service disruption for applications relying on this module.

Input handling error stalls CGI processes.
Affects internet-facing web applications.
Confirm relevance and exposure of this component.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this by sending specially crafted web requests to a vulnerable Perl CGI script. These requests would contain a malicious string in the "tz" query parameter, which the script passes directly to the `tzset()` function without proper validation. This function, when interpreting the input as a file path, could cause the CGI process to hang indefinitely by attempting to access a FIFO or device file.

Unauthenticated access to CGI script required.
Malicious "tz" parameter triggers `tzset()`.
Denial of service by process stalling.

Live Threat

Current exploitation, exposure, and threat context

A flaw in RRD::CGI::Image could allow an attacker to cause a denial-of-service by sending specially crafted query parameters. This could happen when the software processes requests containing a "tz" parameter, which is passed to the `tzset()` function without proper validation. If the value provided for "tz" begins with a forward slash or colon, `tzset()` may attempt to open it as a file path. If this path points to a FIFO or a blocking device, the process executing the CGI script could become stalled indefinitely.

CGI process stalling.
Malicious parameter in web request.
Service unavailability.

Operational Fix

Recommended remediation, mitigation, and detection steps

To address this vulnerability, application owners or the platform team responsible for the Perl CGI environment should initiate an inventory of affected systems. Confirming the reachability and business criticality of these systems is the crucial first step, followed by identifying the accountable owner for remediation planning.

Confirm affected CGI application ownership.
Verify reachability and business criticality.
Plan remediation based on risk assessment.

Supplementary metadata

CVSS vector

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Frequently asked questions

What is RRD::CGI::Image and how is it used?

RRD::CGI::Image is a Perl module designed for the Common Gateway Interface (CGI). Developers use it to generate dynamic, web-based image content. Because CGI is a standard interface for running scripts on web servers, this module is typically integrated into web applications to handle image processing tasks triggered by incoming browser requests.

What does CVE-2026-123776 mean for this software?

This vulnerability is classified as Improper Validation of Input (CWE-1287). It occurs because the module takes untrusted input from a URL query parameter and passes it directly to the system's time zone setting function. Because the input is not checked for safety, the function can be tricked into interacting with system files or device paths instead of setting a time zone.

How can an attacker trigger this process stall?

An attacker triggers the issue by sending a web request containing a 'tz' parameter that begins with a forward slash or colon. This input prompts the system to treat the string as a file path. If the value points to a FIFO pipe or a blocking hardware device, the process stalls while waiting for the resource to respond, effectively causing the CGI script to hang. Normal query parameters that do not start with these characters do not trigger this behavior.

Do I need to worry about this if my system is internal?

Halo Surface Signal indicates that this module is frequently used in internet-facing web services, which makes the risk profile higher for public-facing assets. However, any application that processes external web requests using this module, even if limited in scope, may be reachable by an attacker. You should evaluate whether your specific deployment is accessible to unauthorized users.

What are the first steps to address this vulnerability?

You should begin by creating an inventory of all applications within your environment that utilize the RRD::CGI::Image Perl module. Once you have identified these assets, assess their business criticality and determine if they are exposed to network traffic. This allows you to prioritize systems for remediation and identify the appropriate owners responsible for managing the security of those specific applications.

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.