CVE-2021-31608Network attack vector

URL-rewrite bypass via invalid scheme matching

Published Nov 17, 2022 · Updated Apr 30, 2025

Security-control bypass in Proofpoint Enterprise Protection before 18.8.0 allows remote attackers to deliver unrewritten links by email. The URL rewrite feature fails to match a URL with an invalid scheme that a browser automatically corrects into a valid scheme. Impact requires a recipient to click the delivered link; the bypass disables URL rewriting and click-time protection for that link but does not affect other filtering controls.

CVSS severity7.4
High
EPSS probability0.43%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Security-control bypass in Proofpoint Enterprise Protection before 18.8.0 allows remote attackers to deliver unrewritten links by email. The URL rewrite feature fails to match a URL with an invalid scheme that a browser automatically corrects into a valid scheme. Impact requires a recipient to click the delivered link; the bypass disables URL rewriting and click-time protection for that link but does not affect other filtering controls.

The record

CVE
CVE-2021-31608
Published
Nov 17, 2022
Updated
Apr 30, 2025
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-693, T1566.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Nov 17, 2022CVE publishedPublication date reported by the CVE source.
  2. Apr 30, 2025Record 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

4.3NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
7.4ProofpointCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:H/A:N
4.3CISA-ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:R/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.

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 exploitUnconfirmed

Weakness, pattern, technique

CWE-693Protection Mechanism Failure
T1566.002Spearphishing Link

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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