VulnerabilityPublished 13 Nov 2026

CVE-2026-126190: Soup Sieve · Polynomial-time denial of service in the descendant combinator

Affects bilixSource GCVE Database

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Authoritative record:CVE-2026-126190· High 7.5

Not known to be exploited · CVSS 7.5 High · fixed in 4.0 : patch bilix soon.

Data as of 13 Nov 2026, 21:02 UTC

CVSS 3.1 base score

7.5High

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 3.1	7.5

Patch: Available – fixed in 4.0

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

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

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, 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: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 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
Homebrewbilix`pkg:brew/bilix`Other threats	Affected >=0.18.3	Fixed in–

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

Description

## Summary Matching a selector that uses the descendant combinator (`a b`) takes time that grows polynomially with how deeply the document is nested, and each extra descendant combinator in the selector raises the degree by one. A selector with `k` descendant combinators takes `O(n^(k+1))` time on a chain of `n` nested elements. An attacker who controls the HTML or XML that an application passes to Soup Sieve (directly, or through Beautiful Soup's `select()`, `select_one()`, `css.select()`, etc.) can send a very small, deeply nested document that keeps a CPU busy for seconds to minutes against an ordinary selector such as `section div div`. ## Details To match `a b`, Soup Sieve checks each element against `b` and then walks up through its ancestors looking for one that matches `a`. When the selector has more descendant combinators, the rest of the selector is tried again from each ancestor on that walk, and so on for every combinator. Nothing was reused between walks, so the same ancestors were matched against the same part of the selector over and over. The worst case is when the leftmost part of the selector never matches, as in `section div div` against a document of nested `<div>` with no `<section>`: every walk goes all the way to the root and no search can stop early. ### Timings Example timings with `html.parser` on Soup Sieve 3.0.4 and 4.0, using a document of nested `<div>` (5 bytes each) and no matches: | Nesting depth | Input size | Selector | 3.0.4 | 4.0 | | --- | --- | --- | --- | --- | | 2,000 | 10 KB | `section div` | 1.6 s | 0.05 s | | 4,000 | 20 KB | `section div` | 5.1 s | 0.01 s | | 400 | 2 KB | `section div div` | 6.7 s | < 0.01 s | | 800 | 4 KB | `section div div` | 53.0 s | < 0.01 s | | 100 | 0.5 KB | `section div div div` | 2.5 s | < 0.01 s | | 200 | 1 KB | `section div div div` | 40.9 s | < 0.01 s | None of these selectors use unusual syntax. ## Proof of concept ```py import time import bs4 import soupsieve as sv soup = bs4.BeautifulSoup('<div>' * 200, 'html.parser') t = time.process_time() sv.select('section div div div', soup) print(time.process_time() - t) ``` On affected versions, doubling the nesting depth multiplies the time by about 2^(k+1), where `k` is the number of descendant combinators: about 4x for `section div`, 8x for `section div div`, and 16x for `section div div div`. ## Impact Applications are affected if they run selectors with descendant combinators against documents that untrusted users can influence, such as scrapers, HTML sanitizers or filters, and services that process uploaded or fetched markup. Descendant combinators are among the most common parts of CSS selectors, and a document of about 1 KB is enough to tie up a worker for close to a minute. Applications that only run selectors against trusted documents are not affected. ## Patches Fixed in 4.0 by [facelessuser/soupsieve#351](https://github.com/facelessuser/soupsieve/pull/351). Whether an element is, or has an ancestor that is, a match for part of a selector depends only on its parent, so Soup Sieve now caches that result per element and relation for every element visited on the walk. Each descendant combinator is then linear in the size of the document. Results are unchanged, and ordinary documents are no slower. Caching is enabled by default, so no changes are needed beyond upgrading. The `NOCACHE` flag is a temporary fallback for users who run into problems with caching, and it is planned for removal. It uses the old uncached matching, so selectors compiled with `NOCACHE` are still subject to polynomial time and shouldn't be used with descendant combinators on untrusted input. ## Workarounds If users can't upgrade: - Use as few descendant combinators as they can in selectors that run on untrusted documents. Where the structure of the document is known, the child combinator (`a > b`) only looks at the parent and is not affected. - Limit the size and nesting depth of untrusted documents before selecting on them. - Run selection on untrusted input with a time limit, for example in a worker process that can be stopped.

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