CVE-2023-26103Public exploitNetwork attack vector

Service degradation via WebSocket header regex

Published Feb 25, 2023 · Updated Mar 11, 2025

Regular expression denial of service in Deno before 1.31.0 allows remote attackers to slow WebSocket servers via crafted headers. The upgradeWebSocket function splits Connection and Upgrade header values with whitespace-aware regular expressions that perform excessive matching on long crafted input. Unauthenticated network access to a server using this API is sufficient; the result is excessive CPU use and degraded service, not data access or modification.

CVSS severity5.3
Medium
EPSS probability1.23%
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 Deno before 1.31.0 allows remote attackers to slow WebSocket servers via crafted headers. The upgradeWebSocket function splits Connection and Upgrade header values with whitespace-aware regular expressions that perform excessive matching on long crafted input. Unauthenticated network access to a server using this API is sufficient; the result is excessive CPU use and degraded service, not data access or modification.

The record

CVE
CVE-2023-26103
Published
Feb 25, 2023
Updated
Mar 11, 2025
Vendor
Deno Land Inc.
Product
Deno
Classifications
CWE-1333, T1499.003
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 25, 2023CVE publishedPublication date reported by the CVE source.
  2. Mar 11, 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?

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

  1. Affected versionversion=0 <1.31.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Deno Land Inc. · Denoversion=0 <1.31.0

Published CVSS scores

5.3SnykCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
7.5NIST NVDCVSS: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.

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-1333Inefficient Regular Expression Complexity
T1499.003Application Exhaustion Flood

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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