VulnerabilityPublished 13 Aug 2026Updated 29 Oct 2026

CVE-2026-49477: Soup Sieve · Regular Expression Denial of Service (ReDoS) via Selector Parser

Affects mcp-atlassianSource GCVE Database

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Not known to be exploited · CVSS 7.5 High : upgrade to 0.21.1_8 soon.

Data as of 30 Oct 2026, 08:16 UTC

CVSS 3.1 base score

7.5High

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

Patch: Available – fixed in 0.21.1_8

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 13 Aug 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 29 Oct 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 0.21.1_8.

Affected software

Product	Affected	Fixed in
Homebrewmcp-atlassian`pkg:brew/mcp-atlassian`Other threats	Affected >=0.11.9 <0.21.1_8	Fixed in 0.21.1_8

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 soupsieve version 0.21.1_8 that fixes this vulnerability. Users should upgrade to version 0.21.1_8 or later to remediate the issue. Until patched, applications should avoid compiling or selecting CSS selectors from untrusted input or implement input validation to reject unterminated quoted attribute selectors. Because the attack causes thread blocking, consider applying timeouts or running selector compilation in isolated threads or processes to limit impact.

Description

A Regular Expression Denial of Service (ReDoS) vulnerability exists in soupsieve's CSS selector parser due to catastrophic backtracking in the regex pattern that parses attribute selectors with unterminated quoted values. This flaw allows an attacker to cause CPU exhaustion by submitting specially crafted CSS selectors with unterminated quotes, leading to exponential regex backtracking and thread blocking. The attack requires only about 300 bytes of input to trigger a hang exceeding 3 seconds. This affects applications using soupsieve versions from 0.11.9 up to but not including 0.21.1_8. A fix is available in version 0.21.1_8.

Impact

An attacker can cause a denial of service by exhausting CPU resources on servers running vulnerable versions of soupsieve when processing maliciously crafted CSS selectors with unterminated quoted attribute values. The attack requires no authentication and uses only printable ASCII characters, making it easy to exploit via typical input vectors such as URL parameters or form fields. The CPU time consumed grows exponentially with input length, causing thread blocking and potential service disruption. Memory usage remains negligible. The vulnerability has a CVSS v3.1 score of 7.5 (High) with no confidentiality or integrity impact but significant availability impact.

Summary

The soupsieve CSS selector engine for Beautiful Soup 4 contains a regex vulnerability in its attribute selector parser. The regex pattern for matching attribute selector values includes branches for double-quoted strings, single-quoted strings, and unquoted identifiers. When processing an attribute selector with an unterminated quoted value (e.g., an opening quote without a closing quote), the regex engine enters catastrophic backtracking due to overlapping character classes and lack of anchoring against unterminated strings. This causes exponential growth in backtracking steps proportional to input length, resulting in CPU exhaustion and thread blocking. The vulnerability can be triggered by a payload of approximately 300 bytes, causing the regex engine to hang for over 3 seconds. This impacts any server-side Python application using soupsieve's compile or Beautiful Soup's select methods with untrusted CSS selectors. The issue is fixed in soupsieve version 0.21.1_8.

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Affected software

Description

AI analysisMachine-generated, updated 30 Oct 2026, 08:16 UTC

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