CVE-2026-55953

Description

The Erlang/OTP ssl TLS 1.2 (and earlier) and DTLS client does not verify that the cipher suite selected by the server in ServerHello was among the suites offered by the client in ClientHello. The client-side tls_handshake:hello/5 handler validates the negotiated protocol version and the downgrade sentinel but hands the server-chosen suite directly to ssl_handshake:handle_server_hello_extensions/9, which installs it without a membership check. The TLS 1.3 client path performs this check (per RFC 8446), so it is not affected. An on-path attacker between the client and the intended server can respond with a ServerHello selecting an anonymous key exchange suite such as TLS_DH_anon_* or TLS_ECDH_anon_* that the client never offered. Anonymous suites do not require the server to present a certificate, so the entire verify_peer and cacerts configuration is bypassed: the attacker completes the handshake with its own ephemeral parameters, no certificate is validated, no hostname is checked, and ssl:connect returns {ok, Socket}. All subsequent application traffic is readable and modifiable by the attacker. This issue affects OTP from OTP 17.0 before OTP 27.3.4.15, from OTP 28.0 before OTP 28.5.0.4, and from OTP 29.0 before OTP 29.0.4, corresponding to ssl from 5.3.4 before 11.2.12.11, from 11.3 before 11.6.0.4, and from 11.7 before 11.7.4. Whether OTP before OTP 17.0, corresponding to ssl before 5.3.4, is affected is unknown.

Status

Show unmaintained releases

Package	Ubuntu Release	Status
erlang	26.04 LTS resolute	Fixed 1:27.3.4.6+dfsg-1ubuntu0.1
	24.04 LTS noble	Fixed 1:25.3.2.8+dfsg-1ubuntu4.7
	22.04 LTS jammy	Fixed 1:24.2.1+dfsg-1ubuntu0.7
	20.04 LTS focal	Fixed 1:22.2.7+dfsg-1ubuntu0.5+esm2
	18.04 LTS bionic	Fixed 1:20.2.2+dfsg-1ubuntu2+esm3
	16.04 LTS xenial	Fixed 1:18.3-dfsg-1ubuntu3.1+esm3
	14.04 LTS trusty	Fixed 1:16.b.3-dfsg-1ubuntu2.2+esm2

Get expanded security coverage with Ubuntu Pro

Reduce your average CVE exposure time from 98 days to 1 day with expanded CVE patching, ten-years security maintenance and optional support for the full stack of open-source applications. Free for personal use.

Get Ubuntu Pro 30-day free trial

Patch details

For informational purposes only. We recommend not to cherry-pick updates. How can I get the fixes?

Package	Patch details
erlang	Upstream: 064e236 Upstream: 0a82596 Upstream: e6ff938

Severity score breakdown

CVSS version:

Base score 9.1 · Critical

Base metrics

Parameter	Value
Attack vector	Network
Attack complexity	Low
Attack requirements	Present
Privileges required	None
User interaction	None
Vulnerable system - Confidentiality impact	High
Vulnerable system - Integrity impact	High
Vulnerable system - Availability impact	None
Subsequent system - Confidentiality impact	None
Subsequent system - Integrity impact	None
Subsequent system - Availability impact	None

Scores

Parameter	Value
Base score	9.1 · Critical
Base + Threat score	-
Base + Environmental score	-
Base + Threat + Environmental score	-

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

Base score 7.4 · High

Base metrics

Parameter	Value
Attack vector	Network
Attack complexity	High
Privileges required	None
User interaction	None
Scope	Unchanged
Confidentiality impact	High
Integrity impact	High
Availability impact	None

Scores

Parameter	Value
Base score	7.4 · High
Exploitability score	-
Impact score	-

Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

References

Related Ubuntu Security Notices (USN)

USN-8827-1
Erlang vulnerabilities
28 September 2026

Other references

https://www.cve.org/CVERecord?id=CVE-2026-55953