CVE-2026-117601: Electrum: Malicious server can replace a just-broadcast tx and try to trick user into RBF-ing it

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
• EPSS 0.2%, percentile 7: FIRST’s model estimates a 0.2% chance of exploitation activity in the next 30 days, higher than 7% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes electrum (PyPI) < 4.8.3, fixed in 4.8.3. Check your lockfile, including transitive dependencies.

What it is

Electrum before 4.8.3 fails to verify signatures of wallet-owned inputs in unconfirmed mempool transactions, allowing malicious servers to replace a just-broadcast transaction with a conflicting one. Attackers can swallow the user's transaction and display an invalid zero-fee copy paying an attacker address, tricking users into RBF fee-bumping and signing it.

From the GitHub advisory

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.ismine inputs for mempool txs (before adding them to its local history).

Weakness type: CWE-347.

Product: electrum (PyPI) · Assigned by: VulnCheck · Published: 2026-10-27

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: none
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation none, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-11-04): 0.18% probability of exploitation activity in the next 30 days; percentile 7.

Timeline

• 2026-10-27 — CVE record published by VulnCheck
• 2026-10-28 — CVE record last updated
• 2026-10-28 — GitHub-reviewed advisory GHSA-6jp4-8v48-cc5v published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

1. GitHub Security Advisory (GHSA-6jp4-8v48-cc5v) github.com
2. Patch Commit github.com

Vulnerability records

1. GitHub Advisory GHSA-6jp4-8v48-cc5v github.com
2. CVE.org record CVE-2026-117601 cve.org
3. NVD entry CVE-2026-117601 nvd.nist.gov

Other references from the CVE record and advisory

1. VulnCheck Advisory: Electrum before 4.8.3 Missing Signature Verification of Mempool Transactions vulncheck.com
2. binance.com binance.com

Critical PyPI advisories · Today’s Security Digest