CVE-2025-6497 is a medium-severity vulnerability identified in version 5.8.0 of tidy-html5, an open-source HTML parser and sanitizer maintained by HTACG. The flaw exists in the function prvTidyParseNamespace within the source file src/parser.c. Specifically, the vulnerability manifests as a reachable assertion failure triggered by crafted input during namespace parsing. This means that under certain manipulated conditions, the program encounters an assertion that fails, potentially causing the application to terminate unexpectedly or behave unpredictably. The vulnerability requires local access with low privileges (PR:L) and does not require user interaction or authentication. The attack complexity is low (AC:L), indicating that an attacker with local access can reliably trigger the assertion failure without sophisticated conditions. The CVSS 4.0 vector (AV:L/AC:L/PR:L/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:P) reflects that the attack surface is limited to local access, with partial impact on availability (VA:L) but no impact on confidentiality or integrity. The exploit has been publicly disclosed, increasing the risk of exploitation, although no known exploits in the wild have been reported to date. The vulnerability does not affect remote users directly and does not require elevated privileges beyond local access. The assertion failure could lead to denial of service by crashing the tidy-html5 process or any application embedding it, potentially disrupting HTML parsing workflows or services that rely on tidy-html5 for sanitization or formatting. Since tidy-html5 is often embedded in web development tools, content management systems, or automated HTML processing pipelines, this vulnerability could impact those environments if they use the affected version locally or in automated scripts.

For European organizations, the primary impact of CVE-2025-6497 is a potential denial of service (DoS) condition on systems that utilize tidy-html5 5.8.0 locally. This could disrupt internal development environments, automated content sanitization processes, or any service embedding tidy-html5 for HTML parsing. While the vulnerability does not allow remote code execution or data exfiltration, the availability impact could affect productivity and service reliability, especially in organizations with automated pipelines for web content processing. Sectors relying heavily on web content management, such as media, publishing, and e-commerce, may experience operational disruptions if affected systems are exploited. Additionally, organizations with strict uptime requirements or those using tidy-html5 in security-sensitive sanitization contexts might face increased risk of service interruptions. Given the local attack vector, insider threats or compromised internal systems pose the most likely exploitation scenario. The lack of known exploits in the wild reduces immediate risk but the public disclosure means attackers could develop exploits targeting vulnerable installations. European organizations should assess their use of tidy-html5 5.8.0 in local environments and embedded applications to understand exposure.

1. Upgrade tidy-html5: The most effective mitigation is to update tidy-html5 to a version later than 5.8.0 where this vulnerability is patched. Monitor HTACG project releases and apply updates promptly. 2. Restrict local access: Limit local user privileges and access to systems running tidy-html5 to trusted personnel only, reducing the risk of local exploitation. 3. Implement input validation: Where possible, validate or sanitize HTML input before it reaches tidy-html5 to prevent malformed namespace data that triggers the assertion. 4. Use sandboxing: Run tidy-html5 processes in isolated environments or containers to contain potential crashes and prevent impact on critical systems. 5. Monitor logs and crashes: Implement monitoring to detect abnormal termination or crashes of tidy-html5 processes, enabling rapid response to exploitation attempts. 6. Review embedding applications: Audit applications embedding tidy-html5 to ensure they handle failures gracefully and do not expose additional attack surfaces. 7. Incident response readiness: Prepare internal teams to respond to potential denial of service incidents related to this vulnerability, including rollback plans and patch deployment procedures.