How should we interpret CVSS severity scores?

CVSS estimates technical severity, but it does not automatically equal business risk. Prioritize using context like internet exposure, asset criticality, known exploitation, and whether compensating controls exist. A Medium CVSS on an exposed production system can be more urgent than a Critical on an isolated non-production host.

Vulnerability Database

Q: What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

Q: What's the difference between a vulnerability, an exploit, and a zero-day?

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. Risk increases sharply when exploitation becomes reliable or widespread.

Q: Why do vulnerabilities keep reappearing in our environment?

Recurring findings usually come from incomplete asset discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host.

Q: How do we prioritize remediation without burning out the team?

Use a repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress so remediation is steady, not reactive.

Q: How can SynScan help reduce vulnerability risk over time?

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.

352,427

Total vulnerabilities in the database

CVE-2026-45753 — symfony / html-sanitizer

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Description

symfony/html-sanitizer lets applications sanitise untrusted HTML. UrlAttributeSanitizer is the visitor responsible for validating URL-valued attributes and stripping dangerous schemes from them; it runs on every element regardless of configuration. Whether an attribute is kept is decided by the element/attribute allow-list; validating the scheme of a URL attribute is solely UrlAttributeSanitizer's responsibility.

UrlAttributeSanitizer::getSupportedAttributes() returned only ['src', 'href', 'lowsrc', 'background', 'ping']. The HTML URL-valued attributes action (<form>), formaction (<button>, <input type=image>), poster (<video>) and cite (<blockquote>, <q>, <del>, <ins>) were missing from that list, so DomVisitor never invoked scheme validation for them. As a result, when a configuration admits one of those attributes, a javascript: URI in it survived sanitisation.

Conditions for exploitation

allowSafeElements() is not affected: <form> and the formaction attribute are both flagged unsafe in W3CReference, and allowElement('form') resets the element's attribute list. Reaching the vulnerable attributes requires a deliberately permissive configuration, for example:

<form> + action: allowElement('form', '*'), allowElement('form', ['action', …]), allowElement('form')->allowAttribute('action', 'form'), or the allowStaticElements() preset (whose docblock already warns the output "may still contain other dangerous behaviors");
<button> / <input type=image> + formaction: allowElement(…, '*'), allowAttribute('formaction', …), or allowStaticElements();
<blockquote> / <q> / <del> / <ins> + cite, or <video> + poster: similarly via '*', allowAttribute(), or allowStaticElements().

For the action / formaction cases the victim must additionally submit the form or click the button.

Resolution

UrlAttributeSanitizer now also handles action, formaction, cite and poster. action / formaction / cite are validated against the link schemes (like <a href>, so javascript: is rejected and data: is dropped too); poster is validated against the media schemes (so data: images keep working). The behaviour of <a href> and <img src> is unchanged.

One behaviour change to be aware of: a relative action="/submit" on an allowed <form> is now dropped by default (the same as <a href> / <img src> today); ->allowRelativeLinks() re-enables it.

The patch for this issue is available here for branch 6.4.

Credits

Symfony would like to thank Himanshu Anand and Rémi Pelloux for reporting the issue and Nicolas Grekas for providing the fix.

Published: May 28, 2026
Updated: Jun 5, 2026
CVE: CVE-2026-45753
GHSA: GHSA-hhg7-c65m-h7ff
Severity: Low
Exploit:
CISA KEV:

No technical information available.

CWEs:

OWASP TOP 10:

A7 - Cross-Site Scripting (XSS)

Affected Software
References

Software	From	Fixed in
symfony / html-sanitizer	6.1.0	6.4.40
symfony / html-sanitizer	7.0.0	7.4.12
symfony / html-sanitizer	8.0.0	8.0.12
symfony / symfony	6.1.0	6.4.40
symfony / symfony	7.0.0	7.4.12
symfony / symfony	8.0.0	8.0.12

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Frequently Asked Questions

What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.

Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.

Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.