VulnerabilityPublished 13 Nov 2026

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

Affects mcp-atlassianSource GCVE Database

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Authoritative record:CVE-2026-126192· Medium 6.3

Not known to be exploited · CVSS 6.3 Medium : upgrade to 0.23.1_1 when convenient.

Data as of 13 Nov 2026, 21:32 UTC

CVSS 4.0 base score

6.3Medium

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

Thin evidence

One scorer or none

CVSS scores by source
CNA (GitHub_M)	CVSS 4.0	6.3

Patch: Available – fixed in 0.23.1_1

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

Upgrade mcp-atlassian to 0.23.1_1.

Affected software

Product	Affected	Fixed in
Homebrewmcp-atlassian`pkg:brew/mcp-atlassian`Other threats	Affected >=0.11.9 <0.23.1_1	Fixed in 0.23.1_1

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

A patch is available in Soup Sieve version 0.23.1_1 that addresses this performance issue. Users should upgrade to this fixed version to mitigate the vulnerability. Additionally, developers should avoid using :has() selectors with general sibling combinators on elements with many children or untrusted HTML. Restricting inner selectors within :has() to combinators like > or + can reduce traversal and improve performance. If upgrading is not immediately possible, input filtering to deny selectors containing :has() or wrapping selector evaluation calls in timeouts can help prevent long-running CPU usage.

Description

CVE-2026-126192 describes a performance issue in Soup Sieve where using a general sibling combinator (~) inside the :has() CSS selector on untrusted HTML can cause quadratic time complexity. This leads to significant CPU consumption and potential denial of service when processing HTML with many sibling elements. The issue arises because the selector implementation performs nested scans that scale poorly with the number of siblings. The vulnerability affects versions >=0.11.9 and <0.23.1_1 of the product. A fix is available in version 0.23.1_1. Mitigations include careful use of :has() selectors, avoiding anchors with many children, and restricting inner selectors to combinators like > or +. Timeout wrappers or input filtering can also help prevent long-running evaluations.

Impact

The vulnerability impacts availability by allowing an attacker to cause high CPU usage and application stalls through crafted HTML inputs that trigger quadratic-time selector matching. This can lead to denial of service conditions in applications using vulnerable versions of Soup Sieve to parse untrusted HTML with selectors containing :has() and the general sibling combinator. The issue does not involve privilege escalation, data disclosure, or integrity loss but can degrade service responsiveness significantly.

Summary

The vulnerability in Soup Sieve involves the use of the general sibling combinator (~) inside the :has() pseudo-class selector, such as div:has(a ~ b). When this selector is applied to untrusted HTML containing many sibling elements, the matching process runs in O(n²) time due to nested scans for each candidate element. This can cause CPU usage to spike dramatically, stalling applications that evaluate such selectors over attacker-controlled markup. The adjacent sibling combinator (+) does not exhibit this issue as it only checks the immediate following sibling, resulting in linear time complexity. The issue is inherent to the design of :has() and the general sibling combinator, but optimizations and fixes have been introduced in Soup Sieve version 0.23.1_1. Users are advised to limit the use of :has() with general sibling combinators on large or untrusted inputs and to constrain selectors to improve performance.

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Affected software

Description

AI analysisMachine-generated, updated 13 Nov 2026, 21:32 UTC

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