CVE-2026-34219

Description

The Rust libp2p Gossipsub implementation contains a remotely reachable panic in backoff expiry handling.
After a peer sends a crafted PRUNE control message with an attacker-controlled, near-maximum backoff value, the value is accepted and stored as an Instant near the representable upper bound. On a later heartbeat, the implementation performs unchecked Instant + Duration arithmetic (backoff_time + slack), which can overflow and panic with:
overflow when adding duration to instant
This issue is reachable from any Gossipsub peer over normal TCP + Noise + mplex/yamux connectivity and requires no further authentication beyond becoming a protocol peer.

Attack Scenario

An attacker that can establish a libp2p Gossipsub session with a target node can crash the target by sending crafted PRUNE control data:

1. Establish a standard libp2p session (TCP + Noise) and negotiate a stream multiplexer (mplex/yamux).
2. Open a Gossipsub stream and send an RPC containing ControlPrune with a very large backoff (chosen near boundary conditions, e.g. ~ i64::MAX - victim_uptime_seconds; example observed: 9223372036854674580 for ~28h uptime).
3. The value is parsed from protobuf and passed through Behaviour::handle_prune() into mesh/backoff update logic.
4. Initial storage path uses checked addition (Instant::now().checked_add(...)), so the malicious near-max value is retained.
5. On the next heartbeat (typically within ~43–74s), expiry logic computes backoff_time + slack using unchecked addition, which overflows and panics.

Differences from CVE-2026-33040

This advisory is related to CVE-2026-33040 but it is not the same defect. CVE-2026-33040 addressed overflow during backoff insertion by adding checked arithmetic when converting PRUNE backoff into an Instant. The issue in this advisory occurs at a different location and at a different time: a near-maximum backoff can still be stored successfully, and the crash happens later in the heartbeat path when slack is added to that stored Instant using unchecked arithmetic. This report covers a distinct secondary overflow path in heartbeat expiry handling that remained reachable after the original insertion-side hardening.

This vulnerability was originally reported by the Security team of the Ethereum Foundation.

Remote unauthenticated denial of service (critical).
Any application exposing an affected libp2p-gossipsub listener can be crashed by a network-reachable peer that sends crafted PRUNE backoff values. The crash is triggered during heartbeat processing (not immediately at PRUNE parse time), and can be repeated by reconnecting and replaying the message.

An arithmetic operation produces a value that exceeds the integer type's maximum, causing it to wrap to an unexpected small value. Typical impact: incorrect size calculations leading to heap overflows or logic errors.

CVE-2026-34219 has a CVSS score of 5.9 (High). The vector is network-reachable, no privileges required, and no user interaction. 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 (0.49.4); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-34219? CVE-2026-34219 is a high-severity integer overflow or wraparound vulnerability in libp2p-gossipsub (rust), affecting versions < 0.49.4. It is fixed in 0.49.4. An arithmetic operation produces a value that exceeds the integer type's maximum, causing it to wrap to an unexpected small value.
2. How severe is CVE-2026-34219? CVE-2026-34219 has a CVSS score of 5.9 (High). 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.
3. Which versions of libp2p-gossipsub are affected by CVE-2026-34219? libp2p-gossipsub (rust) versions < 0.49.4 is affected.
4. Is there a fix for CVE-2026-34219? Yes. CVE-2026-34219 is fixed in 0.49.4. Upgrade to this version or later.
5. Is CVE-2026-34219 exploitable, and should I be worried? Whether CVE-2026-34219 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
6. What actually determines whether CVE-2026-34219 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.
7. How do I fix CVE-2026-34219? Upgrade libp2p-gossipsub to 0.49.4 or later.