CVE-2026-68750 in html_sanitize_ex
Summary
by MITRE • 08/06/2026
Inefficient Algorithmic Complexity vulnerability in the traversal engine in rrrene html_sanitize_ex allows an unauthenticated remote attacker to exhaust server CPU and memory via a flat run of sibling elements in sanitized HTML. The list clause of HtmlSanitizeEx.Traverser.traverse/2 recurses on the tail of a sibling list and then evaluates List.flatten([head] ++ tail) over the already flattened result, so every one of n siblings copies and re-walks the entire remaining tail. The flattening is only needed for the rare case where scrub returns several replacement nodes for one node, but the cost is paid across the whole tail at every step, making traversal quadratic in sibling count.
The traverser sits on every public entry point, so no particular scrubber or configuration is required and the payload needs only allowed tags. A 160 KB body of 20,000 sibling elements occupies a scheduler for roughly 1.7 seconds, and the cost grows faster than the body does.
This issue affects html_sanitize_ex: from 0.3.1 before 1.5.3.
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Analysis
by VulDB Data Team • 08/06/2026
The vulnerability in question represents a critical inefficient algorithmic complexity flaw within the html_sanitize_ex library's traversal engine, specifically affecting versions prior to 1.5.3. This weakness resides in the HtmlSanitizeEx.Traverser.traverse/2 function where the recursive processing of sibling elements creates an exponential performance degradation that can be exploited remotely without authentication. The core issue manifests when the system processes HTML containing numerous sibling elements, as the current implementation recursively traverses each element while simultaneously performing costly List.flatten operations on already processed data structures.
The technical flaw stems from a fundamental algorithmic design error where the traversal process repeatedly applies List.flatten([head] ++ tail) operations across the entire remaining sibling list at every recursive step. This means that for n sibling elements, each element triggers a complete reprocessing of all subsequent elements in the list, creating a quadratic time complexity pattern rather than the expected linear processing. The flatten operation is only necessary in rare cases where a scrubbing function returns multiple replacement nodes for a single input node, yet this expensive operation is executed universally across every traversal step regardless of whether the costly scenario occurs.
This vulnerability operates at the intersection of CWE-502 and CWE-400 within the Common Weakness Enumeration framework, specifically addressing insecure deserialization patterns and excessive resource consumption. The attack vector requires only that the target system accept HTML content with allowed tags, making it particularly dangerous as it can be exploited through any public entry point that utilizes the sanitization library. The operational impact is severe, with a relatively small payload of 160 KB containing 20,000 sibling elements causing the system scheduler to remain occupied for approximately 1.7 seconds, demonstrating how the computational cost scales disproportionately with input size.
The performance degradation follows a mathematical pattern where the processing time grows faster than the body size itself, indicating that even modest increases in element count can lead to dramatic increases in resource consumption. This characteristic makes the vulnerability particularly effective as a denial-of-service vector, allowing attackers to consume server resources efficiently without requiring complex payloads or specific configuration conditions. The attack surface spans all public entry points utilizing html_sanitize_ex, meaning organizations cannot simply disable specific sanitization rules or configurations to mitigate this risk, as the flaw exists at the core traversal mechanism level.
The implications extend beyond simple resource exhaustion, as this vulnerability can be leveraged for cascading system failures and service disruption attacks. According to ATT&CK framework category T1499, this represents a resource exhaustion technique that can lead to system instability and availability issues. Organizations should prioritize immediate patching of affected versions, implementing rate limiting measures on HTML processing endpoints, and monitoring for unusual processing patterns that might indicate exploitation attempts. The vulnerability demonstrates the critical importance of algorithmic complexity analysis in security design, as seemingly minor implementation details can create significant performance bottlenecks that translate directly into exploitable security weaknesses.