# CVE-2025-53539: ReDoS in fastapi-guard's penetration attempts detector

> Attackers can cause denial of service in FastAPI Guard by sending crafted inputs that trigger inefficient regex backtracking in penetration detection.

- **Severity:** Medium 6.9 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 7 Jul 2025
- **Updated:** 7 Jul 2025
- **Also known as:** GHSA-j47q-rc62-w448, PYSEC-2026-1359

## What is CVE-2025-53539?

CVE-2025-53539 is a medium-severity denial of service vulnerability in rennf93 fastapi-guard, published on 7 Jul 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

FastAPI Guard is a security library for FastAPI that provides middleware to control IPs, log requests, and detect penetration attempts. fastapi-guard's penetration attempts detection uses regex to scan incoming requests. However, some of the regex patterns used in detection are extremely inefficient and can cause polynomial complexity backtracks when handling specially crafted inputs. This vulnerability is fixed in 3.0.1.

Weakness: CWE-1333 (Inefficient Regular Expression Complexity)

## Which versions are affected by CVE-2025-53539?

3 open source packages ship an affected version, including fastapi-guard. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `fastapi-guard` | PyPI | All versions | 3.0.1 |
| `fastapi-guard` | PyPI | All versions | 3.0.1 |
| `fastapi-guard` | PyPI | All versions | 3.0.1 |

Products: Fastapi-Guard Fastapi Guard.

## How do I fix CVE-2025-53539?

Upgrade fastapi-guard to 3.0.1 or later.

## Is CVE-2025-53539 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2025-53539 exists.

## Other vulnerabilities in rennf93 fastapi-guard

- [CVE-2025-54365](https://insights.sbomify.com/CVE-2025-54365/): fastapi-guard patch contains bypassable RegEx (high 7.8)
- [CVE-2025-46814](https://insights.sbomify.com/CVE-2025-46814/): FastAPI Guard Remote Header Injection via X-Forwarded-For Manipulation (low 3.4)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-53539 and fix it if it is affected.

What it is: CVE-2025-53539 is a medium-severity denial of service vulnerability in rennf93 fastapi-guard, published on 7 Jul 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2025-53539 exists.

Affected packages and fixed versions:
- fastapi-guard (PyPI): affected all versions; fixed in 3.0.1
- fastapi-guard (PyPI): affected all versions; fixed in 3.0.1
- fastapi-guard (PyPI): affected all versions; fixed in 3.0.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-53539/ (Markdown: https://insights.sbomify.com/CVE-2025-53539/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-53539
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-53539
- OSV: https://osv.dev/vulnerability/CVE-2025-53539

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-53539/
Summaries are generated and may contain mistakes; the official description above is the record.