CVE-2021-32816Public exploitNetwork attack vector

Client hang via Autocrypt header regex backtracking

Published May 14, 2021 · Updated Aug 3, 2024

Regular expression denial of service in Proton Mail WebClient before 3.16.60 allows remote attackers to hang the client via a crafted email. Autocrypt public-key extraction applies three ambiguous, nested-quantifier regular expressions to attacker-controlled header text, causing exponential backtracking. A recipient must open or otherwise process the crafted message in the web client; the resulting availability impact is limited to a client-side hang.

CVSS severity7.5
High
EPSS probability1.01%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Regular expression denial of service in Proton Mail WebClient before 3.16.60 allows remote attackers to hang the client via a crafted email. Autocrypt public-key extraction applies three ambiguous, nested-quantifier regular expressions to attacker-controlled header text, causing exponential backtracking. A recipient must open or otherwise process the crafted message in the web client; the resulting availability impact is limited to a client-side hang.

The record

CVE
CVE-2021-32816
Published
May 14, 2021
Updated
Aug 3, 2024
Vendor
Proton AG
Product
Proton Mail WebClient
Classifications
CWE-400, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 14, 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=< 3.16.60

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Proton AG · Proton Mail WebClientversion=< 3.16.60

Published CVSS scores

6.5GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
5.0NVDAV:N/AC:L/Au:N/C:N/I:N/A:P
7.5NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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-400Uncontrolled Resource Consumption
T1499Endpoint Denial of Service

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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