{
  "id": "electrum-fee-bump",
  "url": "https://ohshit.app/incidents/electrum-fee-bump/",
  "title": "Electrum fixes an exploited malicious-server trick that turns a fee bump into an attacker payment",
  "level": 4,
  "level_name": "septic-tank-blew",
  "date": "2026-10-24",
  "date_is_approximate": false,
  "updated": "2026-11-07",
  "tags": [
    "Electrum",
    "Bitcoin",
    "exploited"
  ],
  "ecosystems": [
    "electrum"
  ],
  "summary": "A malicious Electrum server can replace a pending payment with an invalid transaction. If the user fee-bumps it, the wallet signs a payment to the attacker. Maintainers report one user lost funds; upgrade to 4.8.3.",
  "splash": "Electrum before 4.8.3 connected to a malicious main server; loss requires the user to sign the substituted transaction",
  "wipe": "Install Electrum 4.8.3 from electrum.org; verify signatures and every full destination address and amount before signing",
  "in_the_wild": "yes",
  "source_exception": null,
  "cves": [
    {
      "id": "CVE-2026-117601",
      "name": "Malicious-server substitution of an unconfirmed transaction",
      "description": "A server hides a broadcast and supplies a conflicting transaction with invalid signatures; a manual replace-by-fee operation can authorize the attacker payment."
    }
  ],
  "fixed": [
    {
      "product": "Electrum",
      "version": "4.8.3",
      "status": "Checks wallet-owned input signatures before accepting mempool transactions"
    }
  ],
  "checklist": [
    {
      "title": "Upgrade before signing another transaction",
      "hint": "Use electrum.org and its GPG verification instructions; confirm the running application is 4.8.3 or newer"
    },
    {
      "title": "Check complete outputs on every signing request",
      "hint": "Compare full destination addresses and amounts with the intended payment, including fee bumps. Reported attacker addresses copied the legitimate address's last four characters; suffix matching is insufficient"
    },
    {
      "title": "Use a server you actually trust",
      "hint": "The advisory excludes clients connected to trusted servers, such as their own. Switching to an arbitrary public server does not establish trust; upgrade anyway"
    },
    {
      "title": "Review suspicious pending transactions without signing them",
      "hint": "Compare wallet history and transaction IDs with an independent trusted view. Preserve conflicting transaction details and server information for investigation. A suspicious entry alone does not prove funds moved"
    }
  ],
  "sources": [
    {
      "name": "Electrum security advisory",
      "url": "https://github.com/spesmilo/electrum/security/advisories/GHSA-6jp4-8v48-cc5v",
      "says": "Active attacks, two reports, one reported loss, prerequisites, affected versions and disclosure timeline",
      "primary": true
    },
    {
      "name": "Electrum fix commit",
      "url": "https://github.com/spesmilo/electrum/commit/90fe30822561e515a9d2d6e5a352cc07780999eb",
      "says": "Validates signatures for wallet-owned inputs of mempool transactions",
      "primary": true
    },
    {
      "name": "Electrum 4.8.3 release notes",
      "url": "https://raw.githubusercontent.com/spesmilo/electrum/4.8.3/RELEASE-NOTES",
      "says": "October 23 source release is a security update",
      "primary": true
    },
    {
      "name": "Electrum official downloads",
      "url": "https://electrum.org/",
      "says": "Official 4.8.3 downloads and GPG verification instructions",
      "primary": true
    },
    {
      "name": "Security Alliance incident-response roundup",
      "url": "https://radar.securityalliance.org/seal-weekly-stats-28-oct-3-nov-2026/",
      "says": "Reports one 1.2 BTC loss tied to the fee-bump flaw. Overlap with maintainer reports is unknown.",
      "primary": true
    }
  ],
  "headline": null,
  "one_liner": null,
  "in_the_wild_note": "Maintainers confirm attacking servers and two reports, including one loss",
  "mitigation": null,
  "timeline": [
    {
      "date": "Oct 11, 2026",
      "title": "First private report identifies a substituted transaction"
    },
    {
      "date": "Oct 18, 2026",
      "title": "A second user privately reports losing funds after a fee bump"
    },
    {
      "date": "Oct 23, 2026",
      "title": "The fixed 4.8.3 source is tagged"
    },
    {
      "date": "Oct 24, 2026",
      "title": "Binaries and the public advisory are released"
    },
    {
      "date": "Oct 28, 2026",
      "title": "Review verifies the maintainer advisory now identifies CVE-2026-117601",
      "body": "This is the same flaw as GHSA-6jp4-8v48-cc5v, not an additional vulnerability or a new disclosure date."
    },
    {
      "date": "Nov 4, 2026",
      "title": "SEAL reports a 1.2 BTC fee-bump loss",
      "body": "The response roundup describes one victim. Its relationship to the maintainer reports is unknown."
    }
  ],
  "body_markdown": "The malicious server withholds a newly broadcast payment and inserts a conflicting transaction with attacker-controlled outputs into the wallet's pending history. Its old signatures are invalid for those changed outputs. If the user then signs a replacement to increase the fee, the replacement authorizes the attacker payment.\n\nThis needs a malicious main server and a further user signature. It is not automatic seed theft. The original payment may never have reached Bitcoin peers. Even a transaction previously checked on a hardware wallet must be checked again when signing a fee bump.\n\nVersion 4.8.3 rejects invalid signatures on inputs belonging to the wallet before accepting mempool history. It does not turn this light client into a full consensus validator: incoming unconfirmed payments remain outside that added check.\n\nThe project received two reports and says one reporter lost money. It gives no total loss or wider victim count. Active exploitation in a widely used wallet warrants L4; no broader loss estimate is inferred.\n\n**Update, October 28:** The maintainer advisory now identifies this same vulnerability as **CVE-2026-117601**, also tracked as **GHSA-6jp4-8v48-cc5v**. The October 24 disclosure date and Electrum 4.8.3 fix are unchanged; the advisory does not specify when the CVE was added.\n\nSEAL’s November 4 incident-response roundup reports one victim lost 1.2 BTC to the malicious-server fee-bump flaw. The roundup does not establish whether this is the loss already reported by the maintainers. Keep the reported amount separate from any wider loss estimate. The 4.8.3 fix and output-verification steps still apply."
}