Summary
Electrum: Malicious server can replace a just-broadcast tx and try to trick user into RBF-ing it
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.
Impact
CVE-2026-117601 has a CVSS score of 7.5 (Critical). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.8.3); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-117601? CVE-2026-117601 is a critical-severity security vulnerability in electrum (pip), affecting versions < 4.8.3. It is fixed in 4.8.3.
- How severe is CVE-2026-117601? CVE-2026-117601 has a CVSS score of 7.5 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of electrum are affected by CVE-2026-117601? electrum (pip) versions < 4.8.3 is affected.
- Is there a fix for CVE-2026-117601? Yes. CVE-2026-117601 is fixed in 4.8.3. Upgrade to this version or later.
- Is CVE-2026-117601 exploitable, and should I be worried? Whether CVE-2026-117601 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-117601 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-117601? Upgrade
electrumto 4.8.3 or later.