CVE-2026-121439
Wasmtime 46.0.0 before 48.0.7 and 49.0.0 before 49.0.4 HTTP Response Splitting via wasi-http
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 that allows guests to set unvalidated content-length headers. Malicious wasip3 guests can splice an existing body into a new response with an incorrect content-length to perform response splitting or injection attacks.
Attack profile
How easy it is to exploit
- Attack vector
- Network
- Complexity
- Low
- Privileges needed
- None
- User interaction
- None
What an attacker can affect
- Confidentiality
- None
- Integrity
- Low
- Availability
- None
Exploit likelihood
What each source reports
CVE List Score used
Wasmtime 46.0.0 before 48.0.7 and 49.0.0 before 49.0.4 HTTP Response Splitting via wasi-http
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 that allows guests to set unvalidated content-length headers. Malicious wasip3 guests can splice an existing body into a new response with an incorrect content-length to perform response splitting or injection attacks.
- Record
CVE-2026-121439- CVSS 4.0
- 6.9 Medium
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 - CVSS 3.1
- 5.3 Medium
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N - Weakness
- CWE-444
- Updated
- 2026-11-03
EUVDEUVD-2026-115861
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 that allows guests to set unvalidated content-length headers. Malicious wasip3 guests can splice an existing body into a new response with an incorrect content-length to perform response splitting or injection attacks.
- Record
EUVD-2026-115861- CVSS 4.0
- 6.9 Medium
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 - Updated
- 2026-11-03
GitHub AdvisoriesGHSA-4v2q-64x5-xvrc
Wasmtime 46.0.0 before 48.0.7 and 49.0.0 before 49.0.4 contains a response splitting...
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 that allows guests to set unvalidated content-length headers. Malicious wasip3 guests can splice an existing body into a new response with an incorrect content-length to perform response splitting or injection attacks.
- Record
GHSA-4v2q-64x5-xvrc- CVSS 4.0
- 6.9 Medium
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X - CVSS 3.1
- 5.3 Medium
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N - Weakness
- CWE-444
- Updated
- 2026-11-03
NVD
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 that allows guests to set unvalidated content-length headers. Malicious wasip3 guests can splice an existing body into a new response with an incorrect content-length to perform response splitting or injection attacks.
- Record
CVE-2026-121439- CVSS 4.0
- 6.9 Medium
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X - CVSS 3.1
- 5.3 Medium
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N - Weakness
- CWE-444
- Updated
- 2026-11-03
OSV
Wasmtime 46.0.0 before 48.0.7 and 49.0.0 before 49.0.4 HTTP Response Splitting via wasi-http
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 that allows guests to set unvalidated content-length headers. Malicious wasip3 guests can splice an existing body into a new response with an incorrect content-length to perform response splitting or injection attacks.
- Record
CVE-2026-121439- CVSS 4.0
- 6.9 Medium
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 - Weakness
- CWE-444
- Updated
- 2026-11-07
OSVRUSTSEC-2026-0375
`wasi-http` p3: Guests can set incorrect content-length for spliced response bodies
This is an entry in the RustSec database for the Wasmtime security advisory located at https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-9prg-84vf-g5fc For more information see the GitHub-hosted security advisory.
- Record
RUSTSEC-2026-0375- CVSS 3.1
- 5.3 Medium
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N - Updated
- 2026-11-04
Affected products
Affected packages
| Package | Affected | Fixed |
|---|---|---|
wasmtime-wasi-httpcrates.io |
46.0.0 up to 48.0.7, 49.0.0 up to 49.0.4 | 48.0.749.0.4 |
History
| Recorded | Changed | Sources |
|---|---|---|
| affected, aliases, facts, published, references | OSV | |
| published | EUVD, GitHub Advisories, NVD | |
| facts, published | EUVD | |
| aliases, published | EUVD | |
| first indexed | CVE List, GitHub Advisories | |
| first indexed | OSV |
Dates are when Every CVE recorded each change. Upstream dates may differ.
In short
CVE-2026-121439 is a medium vulnerability (CVSS 6.9) in Bytecodealliance Wasmtime, published October 19, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.19%, the 9th percentile. Fixed in wasmtime-wasi-http 48.0.7.
References
github.com/bytecodealliance/wasmtime/security/advisories/GHSA-9prg-84vf-g5fc
vulncheck.com/advisories/wasmtime-46.0.0-before-48.0.7-and-49.0.0-before-49.0.4-http-response-splitting-via-wasi-http
nvd.nist.gov/vuln/detail/CVE-2026-121439
crates.io/crates/wasmtime-wasi-http
rustsec.org/advisories/RUSTSEC-2026-0375.html