CVE-2021-41281

HIGHNVD 7.57.5—
EchelonGraph scoreMEDIUM confidence

This high-severity CVE scores 7.5 under NVD CVSS v3. EPSS exploit probability: 1.6% (74th percentile of EPSS-scored CVEs). GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
7.5EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 1.6%CVSS: 7.5Exploit: None knownExposed services: Not assessed

A fix is available — apply it.

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.

CVSS v3
7.5
EG Score
7.5HIGHmedium confidence
EG Risk
39
EG Risk 39/100CISA SSVC

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity75% × 45%
Exploitation2% × 40%
Automatability30% × 15%
CISA SSVC: Track at low or medium mission impact; Track or Attend at high (mission-essential systems).
Action: A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.
EPSS PROB
1.6%
EPSS %ILE
74th
KEV
Not listed

CISA SSVCTrack at low or medium mission impact; Track or Attend at high (mission-essential systems).

A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.

Exploitation none (no KEV listing, exploit record or EPSS ≥ 50%) · Automatable unknown (not published for this CVE) · Technical impact partial (CVSS below 9.0). Mission impact is CISA's Mission & Well-being decision point, and only you can judge it: high means the affected system is essential to your organisation's mission, or its compromise could cause irreversible harm to people. CISA's decision table

Published

November 23, 2021

Last Modified

November 21, 2024

References (10)

Affected Packages

(2 across 2 ecosystems)
Debian:14(1)
PackageVulnerable rangeFix by version rangeDependents
matrix-synapse0.19.2+dfsg-3 ... 1.9.1-1~bpo10+1 (199 versions)
  • every version up to 1.47.1-1: fixed in 1.47.1-1
—
PyPI(1)
PackageVulnerable rangeFix by version rangeDependents
matrix-synapse0.33.5 ... 1.9.1 (218 versions)
  • every version up to 1.47.1: fixed in 1.47.1
—

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 8× in last 7d / 32× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 185 total refreshes for this CVE.

  1. 2026-10-09 20:25 UTCOSV refresh
  2. 2026-10-09 20:04 UTCEPSS rescore
  3. 2026-10-08 16:40 UTCEPSS rescore
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  11. 2026-09-30 15:41 UTCEG score recompute
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  23. 2026-09-20 04:05 UTCOSV refresh
  24. 2026-09-19 15:38 UTCEPSS rescore
  25. 2026-09-18 19:23 UTCEPSS rescore
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  1. 2026-09-17 19:23 UTCEPSS rescore
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  49. 2026-07-30 18:51 UTCOSV refresh
  50. 2026-07-30 16:25 UTCEPSS rescore
  51. 2026-07-30 01:28 UTCEPSS rescore
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  58. 2026-07-23 02:04 UTCEG score recompute
  59. 2026-07-22 22:50 UTCEG score recompute
  60. 2026-07-22 14:06 UTCEPSS rescore
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  75. 2026-07-13 06:11 UTCEPSS rescore

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Frequently asked(5)

What is CVE-2021-41281?
CVE-2021-41281 is a high vulnerability published on November 23, 2021. 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…
When was CVE-2021-41281 disclosed?
CVE-2021-41281 was first published on November 23, 2021, with the most recent update on November 21, 2024. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-41281 actively exploited?
CVE-2021-41281 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 1.6% probability of exploitation in the next 30 days (74th percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2021-41281?
CVE-2021-41281 has a CVSS v3 base score of 7.5 (NVD).
How do I remediate CVE-2021-41281?
A fix for CVE-2021-41281 is available: update to the fixed version the vendor names in its advisory.

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