External risk intelligence

Citrix NetScaler Memory Overflow in Email OTP Authentication Leads to Denial of Service

CVE advisorySeverity: CRITICAL (CVSS 9.3)

CVE-2026-107415

The vulnerability exists in Citrix NetScaler ADC and Gateway, which are designed as internet-facing edge gateways and remote access appliances. Email OTP authentication is a common feature exposed on these public-facing portals to manage user access, making the vulnerable component typically accessible from the public internet by design.

Denial of Service

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

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A memory overflow vulnerability has been identified in the email OTP authentication feature of Citrix NetScaler ADC and Gateway products. This flaw could lead to unpredictable system behavior or service interruptions, impacting the availability of these critical access control platforms. The main concern at this time is confirming the relevance and potential exposure of this vulnerability within our environment.

Overflow error in email authentication.
Affects secure remote access to company resources.
Confirm relevance and exposure.

Attack Path

How an attacker could exploit the issue

An attacker could potentially exploit a memory overflow vulnerability within the Email OTP authentication feature of Citrix NetScaler ADC and Gateway. This feature is often exposed externally, allowing an attacker without prior authentication to trigger the vulnerability. Successful exploitation could lead to unpredictable system behavior or a complete denial of service, impacting the availability of network access and services.

No authentication required for attack.
Triggered via Email OTP authentication mechanism.
Leads to Denial of Service.

Live Threat

Current exploitation, exposure, and threat context

A memory overflow in the Email OTP authentication feature of Citrix NetScaler ADC and Gateway could lead to unpredictable service behavior or denial of service. This could impact the availability of the gateway for legitimate users when exposed externally.

System availability.
Unpredictable behavior or DoS.
Disruption of service access.

Operational Fix

Recommended remediation, mitigation, and detection steps

The presence of a memory overflow vulnerability in Citrix NetScaler ADC and Gateway's Email OTP authentication necessitates immediate attention from teams responsible for managing these critical internet-facing systems. The first practical step is to ascertain the extent of exposure, confirm business criticality, and identify the accountable owner to plan a risk-based remediation strategy.

Network and security teams should own the issue.
Verify public-facing exposure and business impact.
Plan remediation or implement temporary risk reduction.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is Citrix NetScaler ADC and Gateway?

These are networking appliances that serve as traffic controllers and secure entry points for corporate networks. Organizations use them to manage load balancing and provide remote access to internal applications and desktops for employees, acting as a gatekeeper between the public internet and private data centers.

How does this memory overflow impact Citrix NetScaler?

This vulnerability involves a memory management error that occurs when the system processes email-based one-time passwords. When the software attempts to handle more data than its allocated memory can support, it can crash or enter an unstable state, preventing users from authenticating and effectively shutting down access services.

Does any specific action trigger this vulnerability?

The flaw is triggered specifically through the email-based OTP authentication workflow. It does not require an attacker to have valid credentials or prior access to the system. Simply interacting with the email OTP feature in a way that exceeds memory capacity is sufficient to trigger the issue, whereas standard traffic not involving this specific authentication step remains unaffected.

Is my Citrix NetScaler instance at risk?

According to Halo Surface Signal, this vulnerability is particularly relevant because Citrix NetScaler ADC and Gateway are designed to face the public internet to facilitate remote access. If your appliance has the email OTP feature enabled and is accessible from the internet, it is exposed to potential triggers from external actors.

What should I do first to address CVE-2026-107415?

Begin by identifying which of your gateways have the email OTP authentication feature enabled and confirming their internet exposure. Once you have an inventory of these critical systems, coordinate with your network and security teams to prioritize them for risk-based remediation, such as applying updates or temporarily limiting access to the affected function.

References

support.citrix.com/support-home/kbsearch/article?articleNumber=CTX697305

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.