Electrum: Malicious server can replace a just-broadcast tx and try to trick user into RBF-ing it

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

9.3 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-117601: 0.18% estimated exploitation activity in the next 30 days; percentile 7.17 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The main Electrum server the client connects to is trusted with mempool ("unconfirmed") transactions. This is documented design. [0] However now someone figured out a way to practically exploit it: - when the client broadcasts a new tx1, a malicious server can swallow it, - and update the client's history with a conflicting (invalid) tx2 that replaces the outputs with the attacker's output address. - Tx2 has invalid signatures, but this is not checked (due to light client design). - The wallet will display tx2 as an unconfirmed transaction - tx1 will not be displayed. - If the client then manually RBF fee-bumps tx2, the client will sign it with a higher fee, and hence pay the attacker.

Fixes Commit 90fe30822 contains the fix [1], included in version 4.8.3. The client now verifies the signatures of `wallet.is_mine` inputs for mempool txs (before adding them to its local history). Users…

Published by the source

28 Oct 2026, 21:23 UTC

Last modified by the source

28 Oct 2026, 21:45 UTC

Source last checked

29 Oct 2026, 06:10 UTC

1. Revision 2 · amended · 29 Oct 2026

   Amended from osv record GHSA-6jp4-8v48-cc5v

2. Revision 1 · created · 28 Oct 2026

   Published from ghsa record GHSA-6jp4-8v48-cc5v

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# Patch Next advisory brief: CVE-2026-117601

Advisory: Electrum: Malicious server can replace a just-broadcast tx and try to trick user into RBF-ing it
IDs: CVE-2026-117601, GHSA-6jp4-8v48-cc5v
Record: https://patchnext.com/advisories/cve-2026-117601 (revision 2, source checked 29 Oct 2026, 06:10 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: electrum 4.8.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI electrum: affected < 4.8.3; fixed in 4.8.3

## Suggested inspection (read-only)
- `python -m pip show electrum`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on electrum.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-6jp4-8v48-cc5v (OSV record GHSA-6jp4-8v48-cc5v lists affected ranges: PyPI electrum >=0 <4.8.3 (fixed))
- Verified: https://github.com/advisories/GHSA-6jp4-8v48-cc5v (GitHub-reviewed advisory GHSA-6jp4-8v48-cc5v lists affected ranges: PyPI electrum >=0 <4.8.3)

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