CVE-2026-121439VulnerabilityPublished 3 Nov 2026

Wasmtime 46.0.0 before 48.0.7 and 49.0.0 before 49.0.4 contains a response splitting vulnerability in wasi-http p3

Affects wasmtimeSource GCVE Database

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Authoritative record:CVE-2026-121439· Medium 6.9

Not known to be exploited · CVSS 6.9 Medium : upgrade to 49.0.4 or 48.0.7 when convenient.

Data as of 3 Nov 2026, 16:28 UTC

CVSS 4.0 base score

6.9Medium

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

Thin evidence

One scorer or none

CVSS scores by source
CNA (VulnCheck)	CVSS 4.0	6.9
CNA (VulnCheck)	CVSS 3.1	5.3

Patch: Available – fixed in 49.0.4, 48.0.7

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: Low
Vulnerable system availability: None
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

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

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, 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: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact to the Vulnerable System.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 3 Nov 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 3 Nov 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.

ghsa-4v2q-64x5-xvrc

What to do

Upgrade github/bytecodealliance/wasmtime to 49.0.4 or 48.0.7.

Affected software

Product	Affected	Fixed in
crates.iogithub/bytecodealliance/wasmtime`pkg:cargo/github/bytecodealliance/wasmtime`Other threats	Affected >=46.0.0 <48.0.7 >=49.0.0 <49.0.4	Fixed in 48.0.7 49.0.4

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

AI-suggested

Fixed versions are 48.0.7 and later for the 46.x branch, and 49.0.4 and later for the 49.x branch. Users should upgrade to these versions to remediate the vulnerability. Patch status is not explicitly confirmed in the input, but the version ranges indicate fixes exist in these releases. No vendor advisory content is provided to contradict this. No other mitigation steps are specified.

Description

Wasmtime versions 46.0.0 before 48.0.7 and 49.0.0 before 49.0.4 contain a response splitting vulnerability in the wasi-http p3 component. This flaw allows malicious guests to set unvalidated content-length headers, enabling them to splice an existing body into a new response with an incorrect content-length. This can lead to response splitting or injection attacks.

Impact

An attacker controlling a malicious wasip3 guest can exploit this vulnerability to perform HTTP response splitting or injection attacks. This can lead to integrity issues such as injecting malicious content into HTTP responses, potentially affecting the behavior of clients consuming these responses. There is no direct confidentiality or availability impact reported.

Summary

The vulnerability in Wasmtime affects versions 46.0.0 prior to 48.0.7 and 49.0.0 prior to 49.0.4 in the wasi-http p3 module. It permits guests to manipulate content-length headers without proper validation, which can be exploited to perform HTTP response splitting or injection attacks by splicing existing response bodies into new responses with incorrect content-length values. The CVSS 3.1 base score is 5.3, indicating a medium severity issue with network attack vector, low attack complexity, no privileges required, no user interaction, and impact limited to integrity.

CWE-444Inconsistent Interpretation of HTTP Requests (HTTP Request Smuggling) (opens in a new tab)
Base weaknessOWASP Top 10 A06:2025: Insecure Design (opens in a new tab)
The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.
Implementation
Use a web server that employs a strict HTTP parsing procedure, such as Apache.
Implementation
Use only SSL communication.
Implementation
Terminate the client session after each request.
OWASP Cheat Sheets
REST Assessment (opens in a new tab)
REST Security (opens in a new tab)
Cross-Site Request Forgery Prevention (opens in a new tab)
Secure Design Principles (opens in a new tab)
OWASP ASVS 4.0.3
V13.2.5 (opens in a new tab)
Attack patterns
HTTP Response Smuggling (opens in a new tab)
HTTP Request Smuggling (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 3 Nov 2026, 16:28 UTC

WeaknessesCWE-444

Technical details

References

Community reviews

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