External risk intelligence

Curl Cleartext HSTS Bypass Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 9.1)

CVE-2023-23914

The vulnerability exists within the curl library, a command-line tool and data transfer library. While curl is ubiquitous in many environments, it is primarily used as a client-side utility or integrated into backend application logic rather than acting as a public-facing service or internet gateway itself. Direct exploitation of this specific HSTS handling flaw requires controlling the URLs being processed by the library.

Haxx Curl

7.77.0 to before 7.88.09.08.2.0 to before 8.2.129.0.0 to before 9.0.69.1.0

Halo Surface Signal: 2 out of 5 — less likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A vulnerability in the widely used curl data transfer tool could allow sensitive information to be transmitted in cleartext, potentially undermining security measures like HTTPS enforcement. This issue arises from a flaw in how curl handles HSTS (HTTP Strict Transport Security) when processing multiple URLs in sequence, which could lead to insecure connections. The main concern is confirming relevance and exposure.

Cleartext information transfer flaw in a common tool.
Undermines secure connection enforcement.
Confirm relevance and exposure.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by sending specially crafted URLs to a system using a vulnerable version of curl. If the system processes multiple URLs in a single command, the flawed handling of HTTP Strict Transport Security (HSTS) could be triggered. This might allow sensitive information to be transmitted in cleartext, potentially impacting the security of subsequent data transfers.

Requires a vulnerable curl version.
Triggered by serial URL processing.
Risk of cleartext information exposure.

Live Threat

Current exploitation, exposure, and threat context

When supported by the advisory, this vulnerability could allow an attacker to interfere with curl's ability to enforce HTTPS connections, potentially leading to sensitive information being transmitted insecurely between curl and a server. This occurs when the HSTS mechanism is bypassed due to a state management issue during serial URL transfers.

Server communication details could be exposed.
Insecure transmission occurs during serial transfers.
Compromised connections may leak sensitive data.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability in curl affects systems using specific versions of the library, potentially impacting NetApp products and Splunk Universal Forwarder. Responsibility for remediation likely falls to application owners, infrastructure teams, or vendor management, depending on how curl is integrated. The first practical step is to inventory all instances of the affected curl versions, determine their reachability and criticality, and then plan remediation based on the identified risk, coordinating with vendors as necessary.

Application or infrastructure teams should own remediation.
Verify curl usage and affected component reachability.
Plan updates during approved maintenance windows.

Supplementary metadata

CVSS vector

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Frequently asked questions

What is curl and why is it in my environment?

Curl is a command-line tool and software library used to transfer data across networks. It is deeply integrated into many operating systems and third-party applications, such as Splunk Universal Forwarder and various NetApp storage systems, to enable automated communication between services. Because it is a foundational piece of technology for moving data, it is frequently embedded as a component within larger software packages rather than installed as a standalone program.

What does CWE-319 mean for CVE-2023-23914?

CWE-319 identifies a weakness where sensitive information is transmitted in cleartext. In the context of CVE-2023-23914, this means the software fails to keep data private. Normally, HSTS forces a program to use encrypted HTTPS connections. This vulnerability causes the program to ignore those safety rules during certain sequences, allowing it to fall back to an insecure, unencrypted connection even when it should have protected the traffic.

How is this HSTS bypass triggered?

This flaw occurs specifically when curl is instructed to process multiple URLs in a single command line operation. The state management fails to carry over the necessary HSTS security information from the first URL to subsequent requests. It is important to note that this does not trigger when curl processes only a single URL at a time, as the state persistence error is unique to serial, multi-URL command sequences.

Do I need to worry about this if my system isn't public?

According to Halo Surface Signal, this vulnerability is categorized as having an 'Unlikely' risk profile for most environments. While the underlying flaw allows for cleartext transmission, curl typically acts as a client-side utility rather than a public-facing service. The primary risk exists where an attacker can influence the specific URLs being processed by the library. Assessing whether your internal applications pass untrusted, attacker-controlled URLs to curl is key.

When should I update my software to fix this?

The first step is to inventory your systems to locate versions of curl, Splunk Universal Forwarder, or NetApp products that fall within the vulnerable release ranges. Once you have identified these instances, prioritize patching based on whether the software processes external or user-provided URLs. Coordinate with your software vendors or internal application teams to schedule updates during your next maintenance window to resolve the underlying library flaw.

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.