VulnerabilityPublished 13 Aug 2026Updated 27 Oct 2026

CVE-2025-43859: Fastapi · h11 accepts some malformed Chunked-Encoding bodies

Affects fastapiSource GCVE Database

Not known to be exploited · CVSS 9.1 Critical : upgrade to 0.115.12_1 as a priority.

Data as of 27 Oct 2026, 23:14 UTC

CVSS 3.1 base score

9.1Critical

How severe the flaw is on the 0–10 CVSS scale.

Patch: Available – fixed in 0.115.12_1

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
Integrity: High
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 13 Aug 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade fastapi to 0.115.12_1.

Affected software

Product	Affected	Fixed in
Homebrewfastapi`pkg:brew/fastapi`Other threats	Affected >=0.111.0 <0.115.12_1	Fixed in 0.115.12_1

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

Upgrade h11 to version 0.15.0 or later to resolve the parsing issue. Alternatively, fix or replace any buggy reverse proxy that mishandles chunked encoding. Since exploitation requires both components to be vulnerable, remediating either the server-side h11 or the proxy suffices to mitigate the risk.

Description

A vulnerability in h11 versions up to 0.14.0 used by FastAPI allows malformed HTTP/1.1 chunked-encoding bodies to be accepted due to improper validation of trailing line terminators. This leniency can lead to HTTP request smuggling when combined with a buggy reverse proxy that parses chunked bodies differently. The issue is fixed in h11 version 0.15.0.

Impact

This vulnerability enables HTTP request smuggling attacks when a FastAPI server using a vulnerable h11 version is fronted by a reverse proxy with a different, buggy chunked encoding parser. Attackers can bypass security controls or cause session hijacking by exploiting the differing interpretations of chunked bodies. The CVSS 3.1 score is 9.1 (critical), reflecting high confidentiality and integrity impact without availability impact.

Summary

The vulnerability arises from h11's incorrect parsing of HTTP/1.1 chunked-encoding message bodies, where it accepts any two bytes after the chunk content instead of strictly requiring '\r\n'. This discrepancy can cause desynchronization between h11 and a reverse proxy that expects proper chunk terminators, enabling request smuggling attacks. Such attacks may allow an attacker to bypass proxy protections or cause session key leakage by merging multiple requests into one. The flaw is fixed in h11 version 0.15.0.

References

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 27 Oct 2026, 23:14 UTC

Technical details

References

Community reviews

Related threats