CVE-2021-3163Public exploitNetwork attack vector

Stored script execution via preserved IMG event handler

Published Apr 12, 2021 · Updated Aug 3, 2024

Stored XSS in Slab Quill 4.8.0 allows remote attackers to execute JavaScript through a crafted IMG onloadstart attribute. The HTML editor preserves the event-handler attribute in submitted text, allowing it to run when the stored content is rendered in another user's browser. Execution requires a user to view attacker-controlled content and is limited to that page's browser context; the CVE is disputed as intended browser behavior.

CVSS severity6.1
Medium
EPSS probability1.31%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stored XSS in Slab Quill 4.8.0 allows remote attackers to execute JavaScript through a crafted IMG onloadstart attribute. The HTML editor preserves the event-handler attribute in submitted text, allowing it to run when the stored content is rendered in another user's browser. Execution requires a user to view attacker-controlled content and is limited to that page's browser context; the CVE is disputed as intended browser behavior.

The record

CVE
CVE-2021-3163
Published
Apr 12, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-79, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 12, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

6.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:P/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1203Exploitation for Client Execution

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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