What is CVE-2023-42450?

CVE-2023-42450 is a medium-severity vulnerability in Joinmastodon Mastodon, classified under HTTP Response Splitting. CVSS score: 5.4/10. Published 2023-09-19.

How severe is CVE-2023-42450?

Medium severity. CVSS v3 base score is 5.4 out of 10.

SSRF in Joinmastodon Mastodon

CVE-2023-42450

Mastodon is a free, open-source social network server based on ActivityPub. Starting in version 4.2.0-beta1 and prior to version 4.2.0-rc2, by crafting specific input, attackers can inject arbitrary data into HTTP requests issued by Mastodon. This can be used to perform confused deputy attacks if the server configuration includes `ALLOWED_PRIVATE_ADDRESSES` to allow access to local exploitable services. Version 4.2.0-rc2 has a patch for the issue.

Published 2023-09-19 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.005 (37.4th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 37th percentile — 37.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.004 · newest: 0.005 · change: +0.001

Affected products

Joinmastodon Mastodon — versions 4.2.0
Mastodon — versions >= 4.2.0-beta1, < 4.2.0-rc2

Weakness classification (CWE)

References

github.com/mastodon/mastodon/security/advisories/GHSA-hcqf-fw2r-52g4 (Vendor Advisory)
github.com/mastodon/mastodon/commit/94893cf24fc95b32cc7a756262acbe008c20a9d2 (Patch)

Frequently asked questions

What is CVE-2023-42450?: CVE-2023-42450 is a medium-severity vulnerability in Joinmastodon Mastodon, classified under HTTP Response Splitting. CVSS score: 5.4/10. Published 2023-09-19.
How severe is CVE-2023-42450?: Medium severity. CVSS v3 base score is 5.4 out of 10.