Libslirp: libslirp: icmp echo reply length desynchronization leaks host ip to guest

Overview

A flaw was found in libslirp. When handling ICMP echo requests, the reply length is not updated to match the actual data received from the host. Under specific conditions where the normal ICMP socket is unavailable, this length mismatch causes the host's real IP address to be leaked to the guest in the echo reply, breaking the NAT isolation that user-mode networking is expected to provide.

Affected

• libslirp (all versions)
• container-tools:rhel8/libslirp (all versions)
• libslirp (all versions)
• libslirp (all versions)

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Workarounds

Ensure that the QEMU process user's GID is included in the system's ping_group_range (sysctl net.ipv4.ping_group_range), which allows the SOCK_DGRAM ICMP socket path and avoids the vulnerable raw socket fallback. Alternatively, use --net none or virtio-net with a TAP backend instead of user-mode (slirp) networking.