CVE-2021-41281

medium

Published 2021-11-23 · Modified 2026-02-26

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CVSS v3

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

CVSS v4 NEW

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not yet in upstream

VIR risk

5.5

Description

Synapse is a package for Matrix homeservers written in Python 3/Twisted. Prior to version 1.47.1, Synapse instances with the media repository enabled can be tricked into downloading a file from a remote server into an arbitrary directory. No authentication is required for the affected endpoint. The last 2 directories and file name of the path are chosen randomly by Synapse and cannot be controlled by an attacker, which limits the impact. Homeservers with the media repository disabled are unaffected. Homeservers with a federation whitelist are also unaffected, since Synapse will check the remote hostname, including the trailing `../`s, against the whitelist. Server administrators should upgrade to 1.47.1 or later. Server administrators using a reverse proxy could, at the expense of losing media functionality, may block the certain endpoints as a workaround. Alternatively, non-containerized deployments can be adapted to use the hardened systemd config.

Predictions

Exploit likelihood

30%

Patch ETA

—

Heuristic predictions, AS-IS, for prioritization only.

Mitigations

No mitigations published for this CVE yet.

The vendor-content worker queues fetches as references arrive (check back in a few minutes). Or — if you've already worked around this in production — publish your fix to the community-verified tier.

✚ Propose a mitigation on Community → Mitigations published via the community go through AI scoring + 2 human reviewers + 7-day silent objection window before landing here with source_tier=community-verified.

OS impact

Arch Fixed 1 release

Version	Status	Fixed in
—	Fixed	`1.47.1-1`

Debian Fixed 2 releases

Version	Status	Fixed in
sid	Fixed	`1.47.1-1`
forky	Fixed	`1.47.1-1`

Package impact

Ecosystem	Package	Vulnerable	Fixed
PyPI	matrix-synapse	`<1.47.1`	`1.47.1`
PyPI	matrix-synapse	`<91f2bd0907f1d05af67166846988e49644eb650c\|\|<1.47.1`	`91f2bd0907f1d05af67166846988e49644eb650c`

References

Community-verified mitigations for this CVE will appear above when contributors publish them.

Verify integrity in audit chain (admin only). AS-IS.