VulnerabilityPublished 13 Aug 2026Updated 29 Oct 2026

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

Affects kaskadeSource GCVE Database

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Not known to be exploited · CVSS 9.1 Critical : upgrade to 4.0.4_1 as a priority.

Data as of 30 Oct 2026, 08:44 UTC

CVSS 3.1 base score

9.1Critical

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

Patch: Available – fixed in 4.0.4_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 kaskade to 4.0.4_1.

Affected software

Product	Affected	Fixed in
Homebrewkaskade`pkg:brew/kaskade`Other threats	Affected >=4.0.0 <4.0.4_1	Fixed in 4.0.4_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 apply the official fix. Alternatively, fix or replace any buggy reverse proxy that incorrectly parses chunked-encoding bodies. Since exploitation requires both components to be vulnerable, remediating either h11 or the proxy mitigates the risk.

Description

A vulnerability in h11 versions up to 0.14.0 allows malformed HTTP/1.1 chunked-encoding bodies to be accepted 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 0.15.0. Exploitation requires both h11 and a proxy to have incompatible parsing bugs.

Impact

The vulnerability enables HTTP request smuggling attacks under certain conditions, allowing attackers to bypass reverse proxy protections or cause session key leakage between users. This can lead to unauthorized access or privilege escalation on affected systems. The CVSS score of 9.1 indicates critical severity with high confidentiality and integrity impact but no availability impact.

Summary

h11 up to version 0.14.0 does not strictly validate the trailing CRLF bytes in chunked-encoding HTTP/1.1 message bodies, accepting any two bytes instead. This leniency is harmless alone but can lead to request smuggling vulnerabilities when paired with a proxy that incorrectly parses chunked bodies by reading until a CRLF rather than exactly two bytes. This discrepancy allows an attacker to craft requests that are interpreted differently by h11 and the proxy, potentially bypassing security controls or causing session key leakage. The vulnerability is fixed in h11 version 0.15.0.

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What to do

Affected software

Description

AI analysisMachine-generated, updated 30 Oct 2026, 08:44 UTC

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