CVE-2021-3801Public exploitNetwork attack vector

Service interruption via markup comment backtracking

Published Sep 15, 2021 · Updated Aug 3, 2024

Regular expression denial of service in Prism before 1.25.0 allows context-dependent attackers to exhaust CPU via crafted HTML comments. The markup grammar's comment token uses a lazy wildcard that retries scanning from successive nested comment openers, creating excessive backtracking on malformed input. Exploitation requires an application to pass attacker-controlled markup to Prism; CPU exhaustion can block highlighting and interrupt the hosting application.

CVSS severity6.5
Medium
EPSS probability1.04%
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 Prism before 1.25.0 allows context-dependent attackers to exhaust CPU via crafted HTML comments. The markup grammar's comment token uses a lazy wildcard that retries scanning from successive nested comment openers, creating excessive backtracking on malformed input. Exploitation requires an application to pass attacker-controlled markup to Prism; CPU exhaustion can block highlighting and interrupt the hosting application.

The record

CVE
CVE-2021-3801
Published
Sep 15, 2021
Updated
Aug 3, 2024
Vendor
PrismJS
Product
Prism
Classifications
CWE-1333, T1499.004
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 <=1.24.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

PrismJS · Prismversion=unspecified <=1.24.1

Published CVSS scores

7.5huntr.devCVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:N/A:P

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-1333Inefficient Regular Expression Complexity
T1499.004Application or System Exploitation

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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