Electrum CVE-2026-117601

Exploitation requires: (1) a vulnerable Electrum client before 4.8.3 connected to an attacker-controlled/malicious Electrum server - clients that connect only to a trusted server they run themselves are explicitly NOT affected; (2) the server swallowing the user's just-broadcast transaction (holding it off the Bitcoin p2p network) and injecting a conflicting, invalidly-signed transaction paying an attacker address grinded to match the last four characters of the legitimate address, constructed with zero fee; and (3) the user then manually RBF fee-bumping that forged transaction and signing it. The decisive limiting factor is this required user action (manual fee bump plus signing) combined with the attacker needing to occupy the client's server position. The vendor's stated workaround without upgrading is to verify output addresses and amounts match expectations before signing any transaction, including fee bumps.

The vendor-supplied CVSS 4.0 base score is 9.3 (Critical) with AV:N/AC:L/AT:N/PR:N/UI:N and full VC/VI/VA:H, and the vuln.today decision label is act_now, which signals urgency. However, the signals must be read critically. The GHSA advisory states active in-the-wild exploitation ('some servers are performing this attack in the wild,' two user reports with one confirmed loss of funds), yet this CVE is NOT present in the CISA KEV data provided, and no EPSS score was supplied - so 'confirmed actively exploited (CISA KEV)' cannot be asserted; exploitation is vendor-reported only. The vendor vector marks UI:N, but the described mechanism requires the user to manually RBF fee-bump the forged transaction and sign it, which is active user interaction - this is a material disagreement between the published vector and the description, and it lowers real-world risk relative to the 9.3 headline. Exploitation also requires the victim's light client to be connected to an attacker-controlled Electrum server; clients that use their own trusted server are explicitly not affected. The real impact is theft of funds (integrity), not confidentiality, despite VC:H in the vendor vector. Net: a genuine, vendor-confirmed-exploited funds-theft flaw warranting prompt patching, but the practical bar (attacker server position plus user signing action) makes it more opportunistic/targeted than a fully automatable remote compromise.

A user running Electrum (pre-4.8.3) connects to a malicious public Electrum server and broadcasts a payment (tx1). The server swallows tx1, withholds it from the Bitcoin p2p network, and shows the wallet a conflicting invalid transaction (tx2) that redirects the outputs to an attacker address grinded to match the last four characters of the legitimate address, set at zero fee. Believing their payment is stuck, the user manually fee-bumps (RBF) and signs tx2, sending their funds to the attacker; no standalone public exploit code is referenced, but the vendor reports servers already doing this in the wild.