CVE-2021-20103Public exploitNetwork attack vector

Stored XSS via null-byte extension bypass

Published Jun 29, 2021 · Updated Aug 3, 2024

Stored cross-site scripting in MachForm before 16 allows remote attackers to run JavaScript in users' browsers through uploaded attachments. The upload.php extension filter accepts a crafted HTML filename containing an appended null byte, stores the attachment, and serves it as active content. Exploitation requires a user to browse the uploaded attachment; the script then runs in MachForm's origin and can read cookies or perform actions as that user.

CVSS severity7.1
High
EPSS probability0.69%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stored cross-site scripting in MachForm before 16 allows remote attackers to run JavaScript in users' browsers through uploaded attachments. The upload.php extension filter accepts a crafted HTML filename containing an appended null byte, stores the attachment, and serves it as active content. Exploitation requires a user to browse the uploaded attachment; the script then runs in MachForm's origin and can read cookies or perform actions as that user.

The record

CVE
CVE-2021-20103
Published
Jun 29, 2021
Updated
Aug 3, 2024
Vendor
Laymance Technologies LLC
Product
MachForm
Classifications
CWE-434, CWE-79, T1189
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 29, 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?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=All versions prior to version 16

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Laymance Technologies LLC · MachFormversion=All versions prior to version 16

Published CVSS scores

7.1TenableCVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:L/A:N
4.3NVDAV:N/AC:M/Au:N/C:N/I:P/A:N
6.1NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/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.

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-434Unrestricted Upload of File with Dangerous Type
CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1189Drive-by Compromise

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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