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CVE-2021-23980

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Cross-site scripting in Bleach

Moderate severity GitHub Reviewed PublishedFeb 1, 2021 in mozilla/bleach • UpdatedSep 13, 2024

Package

pipbleach (pip)

Affected versions

< 3.3.0

Patched versions

3.3.0

Description

Impact

Amutation XSS affects users callingbleach.clean with all of:

  • svg ormath in the allowed tags
  • p orbr in allowed tags
  • style,title,noscript,script,textarea,noframes,iframe, orxmp in allowed tags
  • the keyword argumentstrip_comments=False

Note: none of the above tags are in the default allowed tags andstrip_comments defaults toTrue.

Patches

Users are encouraged to upgrade to bleach v3.3.0 or greater.

Note: bleach v3.3.0 introduces a breaking change to escape HTML comments by default.

Workarounds

  • modifybleach.clean calls to at least one of:

    • not allow thestyle,title,noscript,script,textarea,noframes,iframe, orxmp tag
    • not allowsvg ormath tags
    • not allowp orbr tags
    • setstrip_comments=True

  • A strongContent-Security-Policy withoutunsafe-inline andunsafe-evalscript-srcs) will also help mitigate the risk.

References

Credits

  • Reported byYaniv Nizry from the CxSCA AppSec group at Checkmarx
  • Additional eject tags not mentioned in the original advisory and the CSP mitigation line being truncated in the revised advisory reported byMichał Bentkowski at Securitum

For more information

If you have any questions or comments about this advisory:

References

@g-kg-k published to mozilla/bleachFeb 1, 2021
ReviewedFeb 2, 2021
Published to the GitHub Advisory DatabaseFeb 2, 2021
Published by theNational Vulnerability DatabaseFeb 16, 2023
Last updatedSep 13, 2024

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack VectorNetwork
Attack ComplexityLow
Attack RequirementsNone
Privileges RequiredNone
User interactionNone
Vulnerable System Impact Metrics
ConfidentialityNone
IntegrityNone
AvailabilityNone
Subsequent System Impact Metrics
ConfidentialityLow
IntegrityLow
AvailabilityNone

CVSS v4 base metrics

Exploitability Metrics
Attack Vector:This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity:This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements:This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required:This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction:This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality:This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity:This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability:This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality:This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity:This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability:This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided byFIRST.
(62nd percentile)

Weaknesses

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.Learn more on MITRE.

CVE ID

CVE-2021-23980

GHSA ID

GHSA-vv2x-vrpj-qqpq

Source code

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