VulnerabilityPublished 13 Nov 2026

CVE-2026-126192: Soup Sieve · General sibling combinator inside :has() matches untrusted HTML in quadratic time

Affects tootSource GCVE Database

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Authoritative record:CVE-2026-126192· Medium 6.3

Not known to be exploited · CVSS 6.3 Medium.

Data as of 13 Nov 2026, 21:18 UTC

CVSS 4.0 base score

6.3Medium

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

Thin evidence

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CVSS scores by source
CNA (GitHub_M)	CVSS 4.0	6.3

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: Present
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: None
Vulnerable system availability: Low
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

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

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, 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: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: Present: The successful attack depends on the presence of specific deployment and execution conditions of the vulnerable system that enable the attack. These include: a race condition must be won to successfully exploit the vulnerability (the successfulness of the attack is conditioned on execution conditions that are not under full control of the attacker, or the attack may need to be launched multiple times against a single target before being successful); the attacker must inject themselves into the logical network path between the target and the resource requested by the victim (e.g. vulnerabilities requiring an on-path attacker).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: None: There is no loss of integrity within the Vulnerable System.
Vulnerable system availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the Vulnerable System are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 12 Nov 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 13 Nov 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

Affected software

Product	Affected	Fixed in
Homebrewtoot`pkg:brew/toot`Other threats	Affected >=0.26.0	Fixed in–

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AI-suggested

This quadratic time behavior is expected given the design of :has() and the general sibling combinator and is not considered a bug. Users should carefully design selectors to avoid applying :has() with general sibling combinators on elements with many children. Prefer using more restrictive combinators such as > or + inside :has() to limit traversal. Avoid overly general anchor selectors like *. If the risk is unacceptable, user input containing :has selectors can be scanned and rejected. Additionally, wrapping selector evaluations in timeouts can prevent long-running CPU consumption.

Description

CVE-2026-126192 describes a performance-related vulnerability in Soup Sieve when using the CSS selector pattern with a general sibling combinator inside :has(), such as div:has(a ~ b). This pattern causes quadratic time complexity in the number of sibling elements when matching against untrusted HTML, potentially leading to significant CPU consumption and denial of service. The issue arises because the selector engine performs nested scans without short-circuiting when the right-hand sibling element is absent. This behavior is inherent to the design of :has() and the general sibling combinator, and while optimizations exist, linear performance cannot be guaranteed in all cases. The vulnerability impacts availability by allowing an attacker to stall processing with crafted input. Mitigations include careful selector design, limiting the number of children on the anchor element, using more restrictive combinators inside :has(), scanning user input for :has usage, or applying timeouts to selector evaluations.

Impact

The vulnerability impacts availability by allowing an attacker to cause excessive CPU consumption when Soup Sieve evaluates selectors containing a general sibling combinator inside :has() against attacker-controlled HTML. This can stall or significantly delay processing for minutes even on small inputs with a few thousand sibling elements. There is no indication of confidentiality or integrity impact. No known exploits in the wild have been reported. The issue arises from the quadratic time complexity of the selector evaluation algorithm in this specific scenario.

Summary

Soup Sieve's implementation of CSS selectors with a general sibling combinator (~) inside :has() exhibits O(n²) time complexity when matching against untrusted HTML with many sibling elements. Specifically, selectors like div:has(a ~ b) cause nested scans over siblings, resulting in quadratic CPU time when the right-hand sibling 'b' is absent. This can stall processing for minutes on relatively small HTML inputs with thousands of siblings. The adjacent sibling combinator (+) inside :has() does not have this issue and remains linear. The vulnerability affects availability by enabling CPU exhaustion through crafted input. The issue is not a bug but a known performance characteristic of :has() combined with general sibling combinators. Optimizations have been made, but the fundamental complexity remains. Users are advised to limit the use of :has() with general sibling combinators on elements with many children and to prefer more restrictive combinators to reduce traversal. Detection and denial of risky selectors or timeouts can also mitigate impact.

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Description

AI analysisMachine-generated, updated 13 Nov 2026, 21:18 UTC

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