CVE-2025-53539Public exploitNetwork attack vector

Service hang via polynomial regex backtracking

Published Jul 7, 2025 · Updated Jul 7, 2025

Regular expression denial of service in FastAPI Guard 3.0.0 and earlier allows remote attackers to hang services via crafted requests. SusPatternsManager scans request content with patterns containing unbounded quantifiers, causing polynomial backtracking and sustained CPU use on crafted input. Penetration detection is enabled by default, and one kilobyte-scale request can leave a single-threaded Uvicorn worker unable to process concurrent requests for hours.

CVSS severity0.0
None
EPSS probability0.42%
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 FastAPI Guard 3.0.0 and earlier allows remote attackers to hang services via crafted requests. SusPatternsManager scans request content with patterns containing unbounded quantifiers, causing polynomial backtracking and sustained CPU use on crafted input. Penetration detection is enabled by default, and one kilobyte-scale request can leave a single-threaded Uvicorn worker unable to process concurrent requests for hours.

The record

CVE
CVE-2025-53539
Published
Jul 7, 2025
Updated
Jul 7, 2025
Vendor
Renzo Franceschini
Product
FastAPI Guard
Classifications
CWE-1333, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 7, 2025CVE publishedPublication date reported by the CVE source.
  2. Jul 7, 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=< 3.0.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Renzo Franceschini · FastAPI Guardversion=< 3.0.1

Published CVSS scores

0.0GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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 exploitPublished

Weakness, pattern, technique

CWE-1333Inefficient Regular Expression Complexity
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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