Summary
OWASP.AntiSamy mXSS when preserving comments
Workarounds
If you cannot upgrade to a fixed version of the library, the following mitigation can be applied until you can upgrade: Manually edit your AntiSamy policy file (e.g., antisamy.xml) by deleting the preserveComments directive or setting its value to false, if present. Also it would be useful to make AntiSamy remove the noscript tag by adding this in your tag definitions under the <tagrules> node (or deleting it entirely if present):
<tag name="noscript" action="remove"/>
As the previously mentioned policy settings are preconditions for the mXSS attack to work, changing them as recommended should be sufficient to protect you against this vulnerability when using a vulnerable version of this library. However, the existing bug would still be present in AntiSamy or its parser dependency (HtmlAgilityPack). The safety of this workaround relies on configurations that may change in the future and don't address the root cause of the vulnerability. As such, it is strongly recommended to upgrade to a fixed version of AntiSamy.
For more information
If you have any questions or comments about this advisory:
Email one of the project co-leaders, listed on the OWASP AntiSamy project page, under "Leaders".
Impact
There is a potential for a mutation XSS (mXSS) vulnerability in AntiSamy caused by flawed parsing of the HTML being sanitized. To be subject to this vulnerability the preserveComments directive must be enabled in your policy file and also allow for certain tags at the same time. As a result, certain crafty inputs can result in elements in comment tags being interpreted as executable when using AntiSamy's sanitized output.
Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.
CVE-2023-51652 has a CVSS score of 6.1 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.2.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Patched in OWASP AntiSamy .NET 1.2.0 and later. See important remediation details in the reference given below.
Frequently Asked Questions
- What is CVE-2023-51652? CVE-2023-51652 is a medium-severity cross-site scripting (XSS) vulnerability in OWASP.AntiSamy (nuget), affecting versions < 1.2.0. It is fixed in 1.2.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- How severe is CVE-2023-51652? CVE-2023-51652 has a CVSS score of 6.1 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of OWASP.AntiSamy are affected by CVE-2023-51652? OWASP.AntiSamy (nuget) versions < 1.2.0 is affected.
- Is there a fix for CVE-2023-51652? Yes. CVE-2023-51652 is fixed in 1.2.0. Upgrade to this version or later.
- Is CVE-2023-51652 exploitable, and should I be worried? Whether CVE-2023-51652 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2023-51652 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2023-51652? Upgrade
OWASP.AntiSamyto 1.2.0 or later.