CVE-2026-68749 in html_sanitize_ex
Summary
by MITRE • 08/06/2026
Inefficient Regular Expression Complexity vulnerability in the CSS scrubber in rrrene html_sanitize_ex allows an unauthenticated remote attacker to exhaust server CPU via a long CSS declaration in sanitized HTML. The declaration regex in HtmlSanitizeEx.Scrubber.CSS.scrub/1 matches the property name with an unbounded greedy [-\w]+ followed by a mandatory :, so a long run of word characters not followed by a colon makes the engine give back one character at a time and retry the colon at every start offset. The work is quadratic in the length of the run, and no length cap is applied to the CSS handed to the scrubber. An 80 KB <style> body costs roughly 2.4 seconds of scheduler time, so a few concurrent requests saturate the BEAM scheduler pool and make the application unresponsive.
The impact is CPU exhaustion only. Nothing is read, modified or disclosed.
This issue affects html_sanitize_ex: from 0.3.1 before 1.5.3.
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Analysis
by VulDB Data Team • 08/07/2026
The vulnerability under discussion represents a classic example of inefficient regular expression complexity that can be exploited to cause significant performance degradation in web applications. This particular weakness exists within the CSS scrubber component of the html_sanitize_ex library, specifically in the HtmlSanitizeEx.Scrubber.CSS.scrub/1 function where regular expressions are employed to parse and sanitize CSS declarations. The core issue stems from an unbounded greedy quantifier pattern [-\w]+ that is followed by a mandatory colon character, creating a scenario where the regular expression engine must perform extensive backtracking operations when processing long sequences of word characters without subsequent colons.
The technical flaw manifests through the specific regex pattern used in the scrubber implementation which creates exponential backtracking behavior. When an attacker submits a carefully crafted CSS declaration containing a prolonged sequence of word characters followed by a colon, the regular expression engine must repeatedly attempt to match the pattern starting from each possible position in the input string. This results in quadratic time complexity where the computational effort grows dramatically with input length rather than maintaining linear performance characteristics expected in normal operation. The vulnerability is particularly insidious because it requires no authentication or privileged access, making it accessible to any remote attacker who can submit HTML content to the vulnerable application.
The operational impact of this vulnerability extends beyond simple resource exhaustion to potentially render entire applications unresponsive under attack conditions. The demonstrated example shows that an 80 kilobyte style block can consume approximately 2.4 seconds of scheduler time, which represents a substantial computational overhead that can easily overwhelm system resources when multiple concurrent requests are processed simultaneously. The BEAM scheduler pool, which governs the execution of processes in Erlang-based applications, becomes saturated with these CPU-intensive operations, leading to complete application unresponsiveness and denial of service conditions. This behavior aligns with attack patterns documented in the attack technique known as resource exhaustion, where attackers target computational resources rather than attempting to access or modify data directly.
Mitigation strategies for this vulnerability must address both the immediate regex implementation issue and broader architectural considerations for input validation and processing. The most direct fix involves implementing length constraints on CSS content fed to the scrubber function, preventing excessively long sequences from being processed through the vulnerable regular expression pattern. Additionally, regular expression optimization should be considered by replacing the greedy quantifier with a more efficient alternative that does not exhibit exponential backtracking behavior. This vulnerability also highlights the importance of following security best practices such as those outlined in the CWE database under category 134 which addresses improper use of regular expressions and their potential for denial of service attacks. Organizations should consider implementing input validation limits, rate limiting mechanisms, and thorough testing of regex patterns against known attack vectors to prevent similar issues from occurring in other components of their applications.
The affected version range of html_sanitize_ex from 0.3.1 through 1.5.2 demonstrates that this vulnerability has existed for an extended period, indicating that developers should carefully review their dependencies and ensure they are using patched versions of security-critical libraries. This type of vulnerability also serves as a reminder of the importance of automated testing including fuzzing and performance testing of regular expressions in security-sensitive code paths, particularly when processing untrusted user input through sanitization functions. The attack surface for this vulnerability is broad since any application using html_sanitize_ex for HTML sanitization and CSS processing is potentially vulnerable to this type of resource exhaustion attack.