CVE-2020-5207
Request smuggling is possible in Ktor when both chunked TE and content length specified
In Ktor before 1.3.0, request smuggling is possible when running behind a proxy that doesn't handle Content-Length and Transfer-Encoding properly or doesn't handle \n as a headers separator.
Attack profile
How easy it is to exploit
- Attack vector
- Network
- Complexity
- Low
- Privileges needed
- Low
- User interaction
- Required
What an attacker can affect
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
Exploit likelihood
What each source reports
CVE List Score used
Request smuggling is possible in Ktor when both chunked TE and content length specified
In Ktor before 1.3.0, request smuggling is possible when running behind a proxy that doesn't handle Content-Length and Transfer-Encoding properly or doesn't handle \n as a headers separator.
- Record
CVE-2020-5207- CVSS 3.1
- 5.4 Medium
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N - Weakness
- CWE-444
- Updated
- 2026-11-02
EUVDEUVD-2020-0284
In Ktor before 1.3.0, request smuggling is possible when running behind a proxy that doesn't handle Content-Length and Transfer-Encoding properly or doesn't handle \n as a headers separator.
- Record
EUVD-2020-0284- CVSS 3.1
- 5.4 Medium
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N - Updated
- 2026-11-03
GitHub AdvisoriesGHSA-xrr9-rh8p-433v
Request smuggling is possible when both chunked TE and content length specified
Impact
Request smuggling is possible when running behind a proxy that doesn't handle Content-Length and Transfer-Encoding properly or doesn't handle alone \n as a headers separator.
Patches
https://github.com/ktorio/ktor/pull/1547
Workarounds
None except migrating to a better proxy.
References
https://portswigger.net/web-security/request-smuggling https://tools.ietf.org/html/rfc7230#section-9.5
- Record
GHSA-xrr9-rh8p-433v- CVSS 3.1
- 5.4 Medium
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N - Weakness
- CWE-444
- Updated
- 2026-11-02
NVD
In Ktor before 1.3.0, request smuggling is possible when running behind a proxy that doesn't handle Content-Length and Transfer-Encoding properly or doesn't handle \n as a headers separator.
- Record
CVE-2020-5207- CVSS 3.1
- 5.4 Medium
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N - CVSS 3.1
- 7.5 High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N - CVSS 2.0
- 5.0 Medium
AV:N/AC:L/Au:N/C:N/I:P/A:N - Weakness
- CWE-444
- Updated
- 2026-11-02
OSV
In Ktor before 1.3.0, request smuggling is possible when running behind a proxy that doesn't handle Content-Length and Transfer-Encoding properly or doesn't handle \n as a headers separator.
- Record
CVE-2020-5207- CVSS 3.1
- 7.5 High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N - Updated
- 2026-11-02
OSVGHSA-xrr9-rh8p-433v
Request smuggling is possible when both chunked TE and content length specified
Impact
Request smuggling is possible when running behind a proxy that doesn't handle Content-Length and Transfer-Encoding properly or doesn't handle alone \n as a headers separator.
Patches
https://github.com/ktorio/ktor/pull/1547
Workarounds
None except migrating to a better proxy.
References
https://portswigger.net/web-security/request-smuggling https://tools.ietf.org/html/rfc7230#section-9.5
- Record
GHSA-xrr9-rh8p-433v- CVSS 3.1
- 5.4 Medium
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N - Weakness
- CWE-444
- Updated
- 2026-11-02
Affected products
Affected packages
| Package | Affected | Fixed |
|---|---|---|
io.ktor:ktor-client-cioMaven |
before 1.3.0 | 1.3.0 |
io.ktor:ktor-server-cioMaven |
before 1.3.0 | 1.3.0 |
io.ktor:ktor-client-cioMaven |
before 1.3.0, plus listed versions 1.0.01.0.0-rc1.0.11.1.01.1.1 Show 16 moreShow less1.1.21.1.31.1.41.1.51.2.01.2.0-rc1.2.0-rc21.2.11.2.21.2.31.2.3-rc1.2.41.2.51.2.61.3.0-rc1.3.0-rc2 |
1.3.0 |
io.ktor:ktor-server-cioMaven |
before 1.3.0, plus listed versions 1.0.01.0.0-rc1.0.11.1.01.1.1 Show 15 moreShow less1.1.21.1.31.1.41.2.01.2.0-rc1.2.0-rc21.2.11.2.21.2.31.2.3-rc1.2.41.2.51.2.61.3.0-rc1.3.0-rc2 |
1.3.0 |
History
| Recorded | Changed | Sources |
|---|---|---|
| aliases | EUVD | |
| first indexed | CVE List, GitHub Advisories, NVD, OSV |
Dates are when Every CVE recorded each change. Upstream dates may differ.
In short
CVE-2020-5207 is a medium vulnerability (CVSS 5.4) in Jetbrains Ktor, published January 27, 2020. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.76%, the 55th percentile. Fixed in io.ktor:ktor-client-cio 1.3.0.