CVE-2021-26540Public exploitNetwork attack vector

Iframe allowlist bypass via backslash URL parsing

Published Feb 8, 2021 · Updated Aug 3, 2024

Improper input validation in Apostrophe Technologies sanitize-html before 2.3.2 allows attackers to bypass iframe hostname restrictions. The iframe src validator passes browser-tolerated backslashes and intervening whitespace to the WHATWG parser without first normalizing them as URL separators. When allowedIframeHostnames and allowIframeRelativeUrls are both enabled, sanitized HTML can retain an iframe that loads content from a non-allowlisted host.

CVSS severity5.3
Medium
EPSS probability1.75%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper input validation in Apostrophe Technologies sanitize-html before 2.3.2 allows attackers to bypass iframe hostname restrictions. The iframe src validator passes browser-tolerated backslashes and intervening whitespace to the WHATWG parser without first normalizing them as URL separators. When allowedIframeHostnames and allowIframeRelativeUrls are both enabled, sanitized HTML can retain an iframe that loads content from a non-allowlisted host.

The record

CVE
CVE-2021-26540
Published
Feb 8, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-20
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 8, 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

5.0NVDAV:N/AC:L/Au:N/C:N/I:P/A:N
5.3GitHub Advisory DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
5.3NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/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.

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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-20Improper Input Validation

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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