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How the Mentioned CVEs Work
Symfony’s HtmlSanitizer component is designed to cleanse user-submitted HTML to prevent Cross-Site Scripting (XSS) attacks. It does this through a visitor pattern, where the `UrlAttributeSanitizer` is responsible for validating attributes that contain URLs. It strips out dangerous schemes like `javascript:` based on an allowlist.
Two separate, medium-severity vulnerabilities were discovered in how this sanitizer operates. First, in CVE-2026-45753, the `getSupportedAttributes()` method, which defines which attributes to validate, had an incomplete list. It initially included attributes like src, href, and `ping` but omitted other common URL-bearing attributes such as `action` (for <form>), formaction, cite, and poster. This oversight meant that if an integrator’s configuration explicitly allowed these elements or attributes (e.g., via `allowStaticElements()` or wildcard allowances), a malicious payload like `action=”javascript:alert(1)”` would pass through unsanitized, leading to stored XSS.
The second vulnerability, CVE-2026-48761, expanded the scope of the problem. The `UrlAttributeSanitizer` also failed to cleanse other URL-bearing attributes on elements that are not part of the standard “body” elements list. When integrators explicitly allowed elements like <object data="...">, <applet codebase="...">, <iframe longdesc="...">, and <img longdesc="...">, the sanitizer would not recognize their data, codebase, archive, or `longdesc` attributes as needing sanitization. Consequently, a payload such as `` would be rendered directly into the DOM without any URL scheme filtering.
A related issue was also found in how the sanitizer handles `` tags. The refresh mechanism embeds a URL within the `content` attribute’s string (e.g., content="0;url=..."), which the per-attribute `UrlAttributeSanitizer` could not detect or sanitize. An integrator who allowed `` tags in their configuration would find that a payload like `content=”0; url=javascript:alert(1)”` passed through unfiltered, creating a URL redirect to a `javascript:` URI.
These vulnerabilities affect Symfony versions where integrators have used a permissive configuration, such as the `allowStaticElements()` preset, which the documentation already warns is potentially dangerous. The default configuration, where these elements are not explicitly allowed, is unaffected. The issues were responsibly disclosed by Scott Arciszewski (Trail of Bits) and fixed by Nicolas Grekas in patches that extend the list of sanitized attributes and add a new `MetaRefreshAttributeSanitizer` to handle refresh URLs correctly.
DailyCVE Form:
Platform: Symfony (HtmlSanitizer)
Version: 6.1.0-6.4.40 / 7.0.0-7.4.13 / 8.0.0-8.0.13
Vulnerability : Stored Cross-Site Scripting (XSS)
Severity: Medium
date: 2026-05-27
Prediction: Already Patched (2026-05-27/28)
What Undercode Say:
Analytics show the issue was addressed in Symfony’s weekly security release. The following commands show how to check and upgrade your Symfony installation to the patched versions (6.4.41, 7.4.13, or 8.0.13).
Check Installed Version
composer show symfony/symfony
Update to Patched Version
composer update symfony/symfony:^6.4.41 or composer update symfony/symfony:^7.4.13 or composer update symfony/symfony:^8.0.13
Update HtmlSanitizer Component Only
composer update symfony/html-sanitizer
Exploit:
The exploit relies on an application’s permissive sanitizer configuration. An attacker submits crafted HTML that includes a malicious `javascript:` URI in a previously unvalidated attribute.
CVE-2026-45753 (Missing `action` / `formaction` / `cite` / poster):
<form action="javascript:alert('XSS via action attribute')" method="post">
<button type="submit">Click me</button>
</form>
<video poster="javascript:alert('XSS via poster attribute')"></video>
CVE-2026-48761 (Missing `data` / `codebase` / `archive` / `longdesc` / meta):
<object data="javascript:alert('XSS via object data attribute')"></object>
<applet codebase="javascript:alert('XSS via applet codebase attribute')"></applet>
<iframe longdesc="javascript:alert('XSS via iframe longdesc attribute')"></iframe>
<meta http-equiv="refresh" content="0; url=javascript:alert('XSS via meta refresh')">
When a user visits a page with this injected code, or interacts with the element, the JavaScript executes in their browser.
Protection:
Protection involves updating to a patched version and reviewing your sanitizer configuration.
1. Immediate Patch
Upgrade to a version containing the fix: 6.4.41, 7.4.13, `8.0.13` or newer, which include the updated `UrlAttributeSanitizer` and the new MetaRefreshAttributeSanitizer.
2. Configuration Review
Avoid overly permissive configurations, especially allowStaticElements(), which can inadvertently enable vulnerable elements and attributes. Review any use of `allowElement()` or `allowAttribute()` to ensure they are not re-introducing these vectors.
3. Defense-in-Depth with CSP
Implement a strict Content Security Policy (CSP) that disallows `unsafe-inline` scripts and restricts `script-src` directives to trusted domains.
Impact:
Successful exploitation allows an attacker to execute arbitrary JavaScript in the context of the victim’s browser. This can lead to:
Session Hijacking: Stealing session cookies or other authentication tokens to impersonate the victim.
Data Theft: Accessing sensitive information displayed on the page or making unauthorized API requests on behalf of the user.
Defacement: Modifying the content or appearance of the web page to display malicious or misleading information.
Phishing Attacks: Injecting fake login forms or other deceptive content to steal user credentials. The attack is triggered when a user interacts with the affected element (e.g., submitting a form, clicking a button) or simply loads the page containing the malicious HTML.
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Sources:
Reported By: github.com
Extra Source Hub:
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