FastAPI Guard Remote Header Injection via X-Forwarded-For Manipulation

published 2025-05-06 18 mo ago modified 2026-07-13 3 mo ago also EUVD-2025-13598 , GHSA-77q8-qmj7-x7pp , PYSEC-2025-242

cvss: 3.4; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.3%; EPSS 0.3% chance of exploitation in 30 days, percentile 27%
public exploits: 0; none known

FastAPI Guard is a security library for FastAPI that provides middleware to control IPs, log requests, and detect penetration attempts. An HTTP header injection vulnerability has been identified in versions prior to 2.0.0. By manipulating the X-Forwarded-For header, an attacker can potentially inject arbitrary IP addresses into the request. This vulnerability can allow attackers to bypass IP-based access controls, mislead logging systems, and impersonate trusted clients. It is especially impactful when the application relies on the X-Forwarded-For header for IP-based authorization or authentication. Users should upgrade to FastAPI Guard version 2.0.0 to receive a fix.

Patch / fix

fixed in 2 links 1

pypi fastapi-guard 2.0.0
fastapi-guard fastapi_guard 2.0.0

github.com/rennf93/fastapi-guard@0b003fd commit

CISA SSVC

Exploitation: Proof of concept is public
Automatable: no
Technical impact: partial
Assessed: 18 mo ago (2025-05-06)

Exploit prediction

0.34% chance of exploitation in the next 30 days, higher than 27% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

github.com https://github.com/rennf93/fastapi-guard/security/advisories/GHSA-77q8-qmj7-x7pp referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by	version	score	vector
CNA	3.1	3.4	CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:N/A:N
NVD	3.1	7.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
OSV	3.1	7.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Affected packages

ecosystem	package	affected	fixed in
pypi	fastapi-guard	<2.0.0	2.0.0

Affected products

rennf93 fastapi-guard

Classification

Weakness

CWE-74 Improper Neutralization of Special Elements in Output Used by a Downstream Component

References

github.com /rennf93/fastapi-guard/commit/0b003fda4c678c1b514e95f319aee88113e9bf4b
patch
github.com /rennf93/fastapi-guard/security/advisories/GHSA-77q8-qmj7-x7pp
exploit, vendor-advisory, evidence

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.

Summary