CVE-2021-3783Network attack vector

Reflected XSS via unsanitized upgrade parameters

Published Sep 15, 2021 · Updated Aug 3, 2024

Cross-site scripting in YOURLS 1.8.2 and earlier allows remote attackers to execute script in a victim's browser via crafted upgrade URLs. The admin/upgrade.php endpoint accepted oldver and oldsql GET parameters and passed them onward without sanitization, allowing attacker-controlled values to be reflected into generated pages. Exploitation requires a victim to request the crafted URL and interact with the resulting page; the demonstrated impact is script execution in that browser context.

CVSS severity6.1
Medium
EPSS probability0.70%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Cross-site scripting in YOURLS 1.8.2 and earlier allows remote attackers to execute script in a victim's browser via crafted upgrade URLs. The admin/upgrade.php endpoint accepted oldver and oldsql GET parameters and passed them onward without sanitization, allowing attacker-controlled values to be reflected into generated pages. Exploitation requires a victim to request the crafted URL and interact with the resulting page; the demonstrated impact is script execution in that browser context.

The record

CVE
CVE-2021-3783
Published
Sep 15, 2021
Updated
Aug 3, 2024
Vendor
YOURLS
Product
YOURLS
Classifications
CWE-79, T1189
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 15, 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=unspecified <=1.8.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

YOURLS · YOURLSversion=unspecified <=1.8.2

Published CVSS scores

6.6huntr.devCVSS:3.0/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H
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.

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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 exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1189Drive-by Compromise

Public exploit references

No public exploit references are available in this record.

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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