VulnerabilityPublished 13 Aug 2026Updated 30 Oct 2026

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

Affects uvicornSource GCVE Database

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Not known to be exploited · CVSS 9.1 Critical : upgrade to 0.34.2_1 as a priority.

Data as of 30 Oct 2026, 06:59 UTC

CVSS 3.1 base score

9.1Critical

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

EPSS: 0.5% chance of exploitation in 30 days – higher than 48% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 0.34.2_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
Weaponized: 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 29 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.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), 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 uvicorn to 0.34.2_1.

Affected software

Product	Affected	Fixed in
Homebrewuvicorn`pkg:brew/uvicorn`Other threats	Affected >=0.23.2 <0.34.2_1	Fixed in 0.34.2_1

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

AI-suggested

A patch is available in h11 version 0.15.0, which corrects the chunked encoding parsing to strictly require '\r\n' as the trailing bytes. Uvicorn users should upgrade to versions including this fixed h11 release (>=0.34.2_1). Since exploitation requires both h11 and a buggy proxy, fixing either component mitigates the issue. If upgrading is not immediately possible, ensure that any reverse proxy or intermediary correctly implements chunked encoding parsing to avoid mismatches.

Description

A vulnerability in the h11 HTTP/1.1 library used by Uvicorn allows acceptance of malformed Chunked-Encoding bodies due to lenient parsing of trailing line terminators. This can cause request smuggling when combined with a buggy reverse proxy that parses chunked bodies differently. The issue is fixed in h11 version 0.15.0. Exploitation requires both h11 and a proxy to have incompatible parsing bugs.

Impact

This vulnerability enables HTTP request smuggling attacks when a reverse proxy with a buggy chunked encoding parser is used in front of a Uvicorn server using vulnerable h11 versions. Attackers can bypass proxy security controls, smuggle unauthorized requests, or cause session key leakage between users. The CVSS score of 9.1 reflects critical impact with high confidentiality and integrity consequences but no availability impact.

Summary

The vulnerability arises from h11's acceptance of any two trailing bytes after chunk content in HTTP/1.1 chunked-encoding bodies instead of strictly requiring '\r\n'. This leniency is harmless alone but can lead to request smuggling when paired with a proxy that incorrectly parses chunked bodies by reading until a '\r\n' rather than exactly two bytes. This mismatch allows an attacker to craft requests that are interpreted differently by the proxy and the backend server, potentially bypassing security controls or causing session key leakage. The flaw affects h11 versions up to 0.14.0 and is fixed in 0.15.0. Uvicorn versions >=0.23.2 and <0.34.2_1 are affected due to their use of vulnerable h11 versions.

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Affected software

Description

AI analysisMachine-generated, updated 30 Oct 2026, 06:14 UTC

Technical details

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