CVE-2023-22465: Http4s has fatal error parsing User-Agent and Server headers

Severity: medium (CVSS 5.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.9% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2023-01-04 · updated 2026-06-17
Weakness: CWE-20

Description

Http4s is a Scala interface for HTTP services. Starting with version 0.1.0 and prior to versions 0.21.34, 0.22.15, 0.23.17, and 1.0.0-M38, the `User-Agent` and `Server` header parsers are susceptible to a fatal error on certain inputs. In http4s, modeled headers are lazily parsed, so this only applies to services that explicitly request these typed headers. Fixes are released in 0.21.34, 0.22.15, 0.23.17, and 1.0.0-M38. As a workaround, use the weakly typed header interface.

Affected products

http4s: from 0.1.0 before 0.21.34; from 0.22.0 before 0.22.15; from 0.23.0 before 0.23.17; 1.0.0-milestone1, 1.0.0-milestone10, 1.0.0-milestone11, 1.0.0-milestone12, 1.0.0-milestone13, 1.0.0-milestone14 (+31) — fixed in 0.21.34, 0.22.15, 0.23.17

How to fix

Upgrade to a fixed version: http4s 0.21.34, 0.22.15, 0.23.17. Check the vendor advisory for mitigations.

References

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