Description
GNU libidn2 before 2.2.0 fails to perform the roundtrip checks specified in RFC3490 Section 4.2 when converting A-labels to U-labels. This makes it possible in some circumstances for one domain to impersonate another. By creating a malicious domain that matches a target domain except for the inclusion of certain punycoded Unicode characters (that would be discarded when converted first to a Unicode label and then back to an ASCII label), arbitrary domains can be impersonated.
Fixed in Linux distributions
Distributions backport fixes, so the version that fixes this vulnerability differs per release — upgrade to at least the version shown. From the distributions' security trackers and advisories.
Debian
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian 14 | libidn2 | 2.2.0-1 | — |
| Debian 13 | libidn2 | 2.2.0-1 | — |
| Debian 12 | libidn2 | 2.2.0-1 | — |
Ubuntu
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Ubuntu 18.04 LTS | libidn2 | 2.0.4-1.1ubuntu0.2 | USN-4168-1 |
Show 2 more (older releases, extended support, other products)Fewer releases
| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Ubuntu Pro 16.04 LTS | libidn2-0 | No fix yet | — |
| Ubuntu Pro 14.04 LTS | libidn2-0 | No fix yet | — |
SUSE
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| SLES 15 SP1 | libidn2 | 2.2.0-3.3.1 | SUSE-SU-2019:3086-1 |
| SLES 15 | libidn2 | 2.2.0-3.3.1 | SUSE-SU-2019:3086-1 |
openSUSE
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| openSUSE Leap 15.1 | libidn2 | 2.2.0-lp151.3.3.1 | openSUSE-SU-2019:2611-1 |
| openSUSE Leap 15.0 | libidn2 | 2.2.0-lp150.2.3.1 | openSUSE-SU-2019:2613-1 |
Alpine
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Alpine v3.24 | libidn2 | 2.2.0-r0 | — |
| Alpine v3.23 | libidn2 | 2.2.0-r0 | — |
| Alpine v3.22 | libidn2 | 2.2.0-r0 | — |
| Alpine v3.21 | libidn2 | 2.2.0-r0 | — |
| Alpine v3.20 | libidn2 | 2.2.0-r0 | — |
Show 8 more (older releases, extended support, other products)Fewer releases
| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Alpine v3.19 | libidn2 | 2.2.0-r0 | — |
| Alpine v3.18 | libidn2 | 2.2.0-r0 | — |
| Alpine v3.17 | libidn2 | 2.2.0-r0 | — |
| Alpine v3.16 | libidn2 | 2.2.0-r0 | — |
| Alpine v3.15 | libidn2 | 2.2.0-r0 | — |
| Alpine v3.14 | libidn2 | 2.2.0-r0 | — |
| Alpine v3.13 | libidn2 | 2.2.0-r0 | — |
| Alpine v3.12 | libidn2 | 2.2.0-r0 | — |
Amazon Linux
| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Amazon Linux 2 | idn2 +2 | 2.3.0-1.amzn2 | ALAS2-2019-1373 |
Advisories 7
- SSA-398330 SSA-398330: Vulnerabilities in the additional GNU/Linux subsystem of the SIMATIC S7-1500 CPU 1518(F)-4 PN/DP MFP >= V3.1.0 and < V3.1.5 Siemens ProductCERT · Oct 13, 2026
- USN-4168-1 libidn2 vulnerabilities Ubuntu Security Notices (USN) · Apr 27, 2026
- openSUSE-SU-2019:2613-1 Security update for libidn2 SUSE / openSUSE Security Updates · Feb 4, 2026
- SUSE-SU-2019:3086-1 Security update for libidn2 SUSE / openSUSE Security Updates · Feb 4, 2026
- openSUSE-SU-2019:2611-1 Security update for libidn2 SUSE / openSUSE Security Updates · Feb 4, 2026
- ICSA-23-348-10 Siemens SIMATIC S7-1500 CISA ICS & IT advisories · Aug 12, 2025
- ALAS2-2019-1373 Moderate: libidn2 security update (Amazon Linux 2) Amazon Linux Security Center (ALAS) · Dec 18, 2019
References
- https://gitlab.com/libidn/libidn2/merge_requests/71
- https://gitlab.com/libidn/libidn2/commit/241e8f486134793cb0f4a5b0e5817a97883401f5
- https://gitlab.com/libidn/libidn2/commit/614117ef6e4c60e1950d742e3edf0a0ef8d389de
- USN-4168-1 vendor-advisory
- FEDORA-2019-f454c7a118 vendor-advisory
- FEDORA-2019-20e9736c97 vendor-advisory
- FEDORA-2019-28d3cd20c0 vendor-advisory
- FEDORA-2019-1ebb5c928e vendor-advisory
- openSUSE-SU-2019:2611 vendor-advisory
- openSUSE-SU-2019:2613 vendor-advisory
- FEDORA-2019-160303ebeb vendor-advisory
- GLSA-202003-63 vendor-advisory