CriticalCVE-2026-117601

Electrum before 4.8.3 Missing Signature Verification of Mempool Transactions (CVE-2026-117601)

Published 27.10.2026Last modified 28.10.2026Sources: pwn.watch, OSV, Hersteller, CVE.org

Actively exploited
Source: VulnCheck KEV (opens in a new tab) How we classify

What is affected, what to do

The vulnerability is being actively exploited; treat it as a priority. Update Spesmilo Electrum to version 4.8.3.

Affected: Spesmilo Electrum lt *..4.8.3
Fixed in: Spesmilo Electrum 4.8.3

Not sure whether and how you are affected? Ask a question about this vulnerability

Metrics

9.3

Priority

XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

Source: Vendor (CNA), CVSS v4

7.5

CVSS v3

Severity High on the scale from 0 to 10.

Source: pwn.watch, OSV, Hersteller, CVE.org

0.2 %

EPSS score

Low — model estimates < 1% exploitation likelihood.

Source: FIRST EPSS, percentile 7.1

Show all metrics

Severity: Critical
Priority (0–325) The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first? Read in glossary: 76.50 Methodology
Exploit maturity An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack. Read in glossary: actively exploitedSource: VulnCheck KEV (opens in a new tab) How we classify
XIEM Effective CVSS XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command. Read in glossary: 9.3 Methodology
CVSS v3 CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions. Read in glossary: 7.5
CVSS v4 CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions. Read in glossary: 9.3
EPSS score The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“. Read in glossary: 0.2 %; Low — model estimates < 1% exploitation likelihood.
EPSS percentile The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking. Read in glossary: 7.1; Low — this CVE sits in the lower 10% of all CVEs scored today.
KEV status The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching. Read in glossary: —
Published: 27.10.2026
CWE CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect. Read in glossary: CWE-347

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2 NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management. Read in glossary: Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
CRA The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates. Read in glossary: Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
ISO/IEC 27001 ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security. Read in glossary: Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

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 should upgrade (and regularly do so in any case). As a workaround, without upgrading, when signing any transaction, for bumping the fee or not, they should verify the output addresses and amounts are as expected.

Clients that connect to servers they trust (e.g. run their own) are not affected.

Remarks

some servers are performing this attack in the wild
electrum received two user-reports of this. user1 noticed after broadcasting tx1 the wallet history contained a different tx. user2 bumped the fee of tx2 and lost money.
user1 reported tx1 was not propagated over the bitcoin p2p, so the server held it
both reported the attacker's output address in tx2 was grinded to match the last four chars of the legit address from tx1
based on user2's report, the server constructed a tx2 with zero fee (presumably in hope of the user bumping its fee)
even with the fix, we still only validate "is_mine" inputs (i.e. paying from addresses that the wallet owns). So e.g. "incoming" txs are not validated. The client lacks a full consensus engine / script interpreter, though even with that, to cover non-is_mine inputs, it would have to validate the unconfirmed parents recursively until reaching mined txs. Mined txs we already SPV.

Timeline

2026-10-11: user1 sends private disclosure, but the bump_fee vector is not realised
2026-10-18: user2 (Raju-K12) reports privately that they lost money after bump_fee
2026-10-23: version 4.8.3 git tagged, with the fix
2026-10-24: version 4.8.3 binaries released
2026-10-24: public disclosure

Credits

Electrum thanks user1 and Raju-K12 for finding and responsibly disclosing this issue to us.

Source: pwn.watch, OSV (Open Source Vulnerabilities), GitHub Security Advisories, cvelistv5, NVD (NIST)

Affected products

The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

Affected products
		Affected versions	Fixed in
spesmilo	electrum	`< 4.8.3`	`4.8.3`
pip	electrum	`< 4.8.3`	`4.8.3`
pypi	electrum	`0` `—`	— `4.8.3`

Vendors and products

Spesmilo Electrum

Weakness classes (CWE)

CWE-347Improper Verification of Cryptographic Signature
Base
The product does not verify, or incorrectly verifies, the cryptographic signature for data.
Entry at MITRE CWE-347

References & sources

github.comPatch CommitPatch, web
github.comGHSA-6jp4-8v48-cc5v: Malicious server can replace a just-broadcast tx and try to trick user into RBF-ing itpwn-watch, Vendor advisory, web
vulncheck.comVulnCheck Advisory: Electrum before 4.8.3 Missing Signature Verification of Mempool TransactionsThird-party advisory, web
pwn.watchpwn.watch: Electrum wallet: malicious servers swap just-sent transactions and trick users into fee-bumping a payment to the attackerpwn-watch
raw.githubusercontent.comElectrum release notes (4.8.3 security update)pwn-watch
news.bitcoin.comElectrum Wallet Attack Hid Bitcoin Theft Behind a Fee Increasepwn-watch
thedefiant.ioElectrum Patches Wallet Flaw Exploited Through Malicious Serverspwn-watch
NVD (NIST)vuln/detail/CVE-2026-117601advisory
github.comspesmilo/electrumpackage
binance.comen/square/post/381361716824742web

Changes to this CVE

Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

28.10.2026Record modifiedCISA ADP

CISA assessment (SSVC): exploitation: none known, automatable: yes

Show the original change

Individual changes of this event as published by NVD
Type	Action	Before	After
SSVC assessment	added	—	{"id":"CVE-2026-117601","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-10-28T15:56:45.794314Z"}

Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

27.10.2026ReceivedVulnCheck

CVSS V4.0: AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

CVSS V3.1: AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H

Weakness type: CWE-347

Description added or changed

New references:

Show 8 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Description	added	—	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.
CVSS	added	—	AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
CVSS	added	—	AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H
Weakness type (CWE)	added	—	CWE-347
Affected versions (publisher)	added	—	New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/117xxx/CVE-2026-117601.json">CVE-2026-117601</a>
Reference	added	—	https://github.com/spesmilo/electrum/commit/90fe30822561e515a9d2d6e5a352cc07780999eb
Reference	added	—	https://github.com/spesmilo/electrum/security/advisories/GHSA-6jp4-8v48-cc5v
Reference	added	—	https://www.vulncheck.com/advisories/electrum-before-4.8.3-missing-signature-verification-of-mempool-transactions

Source identifier: disclosure@vulncheck.com

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