External risk intelligence

Symfony X509Authenticator Allows Authentication via Malformed Distinguished Names.

CVE advisorySeverity: CRITICAL (CVSS 9.1)

CVE-2026-45063

The vulnerability exists in an authentication component of the Symfony PHP framework. Frameworks and authentication modules are commonly used to build internet-facing web applications and APIs, making this component a standard part of the exposed attack surface in many public-facing web deployments.

Sensiolabs Symfony

before 5.4.526.0.0 to before 6.4.407.0.0 to before 7.4.128.0.0 to before 8.0.12

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

A vulnerability in the Symfony PHP framework could allow an attacker to impersonate a legitimate user by manipulating certificate data. This issue impacts the framework's ability to correctly identify users, potentially leading to unauthorized access if exploited. The main concern at this time is to confirm if our systems utilize the affected versions of Symfony and the specific authentication component.

Impersonation risk via certificate data manipulation.
Confirms use of affected Symfony authentication.
Assess business relevance and exposure.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by presenting a specially crafted trusted certificate to an application using the vulnerable Symfony component. This certificate would contain an email address within another field, tricking the authenticator into recognizing the attacker as a legitimate user.

Requires attacker-controlled certificate.
Triggers by authenticating with a forged certificate.
Allows unauthorized user access.

Live Threat

Current exploitation, exposure, and threat context

When supported by the advisory, an attacker could authenticate as a legitimate user by providing a specially crafted certificate. This could potentially expose system data or user data if the affected component is used for authentication in an internet-facing application.

Affected user accounts.
Via specially crafted certificates.
Unauthorized access to user data.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability affects the Symfony PHP framework, specifically its X509Authenticator component. Application owners and platform teams are likely responsible for identifying and remediating this issue within their web applications. The first practical step involves determining where Symfony is deployed, assessing its exposure and criticality, identifying the accountable owner, and then planning remediation based on the identified risk.

Application owners should manage this issue.
Verify reachability and business criticality first.
Plan remediation based on exposure and risk.

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 Symfony and why is it used?

Symfony is a modular PHP framework developers use to build structured web and console applications. It provides pre-built, reusable components, such as the X509Authenticator, which handles secure identity verification. By using these components, developers can manage complex tasks like user authentication without writing custom code from scratch, making it a foundational piece of the software stack for many web services.

What is the vulnerability in CVE-2026-45063?

This CVE involves an improper authentication weakness, identified as CWE-290. The software uses a flawed regular expression to extract user identifiers from digital certificate data. Because the search pattern is unanchored, it can mistakenly match an email address hidden inside unrelated parts of a certificate's distinguished name, allowing an attacker to impersonate another user.

How does an attacker trigger this authentication flaw?

An attacker triggers the bug by presenting a specially crafted, trusted certificate to an application using the vulnerable authenticator. The flaw relies on the certificate containing an email address inside a secondary field. Simply accessing the application without a valid, trusted certificate does not trigger this vulnerability; the system must first accept the attacker's forged credentials during the authentication process.

Is my application at risk for CVE-2026-45063?

If your application uses the affected Symfony versions for certificate-based authentication, it is potentially at risk. According to Halo Surface Signal, this component is frequently used in internet-facing web applications and APIs, making it a common part of the public-facing attack surface. You should prioritize assessing any services that allow users to authenticate via X.509 client certificates.

What should I do first to address this Symfony issue?

Start by identifying all applications in your environment that utilize the Symfony framework and the X509Authenticator component. Once identified, evaluate whether these applications are internet-facing and perform sensitive operations. Your next step is to coordinate with the application owners to plan an upgrade to the patched versions—5.4.52, 6.4.40, 7.4.12, or 8.0.12—based on the specific risk profile of the deployment.

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.