KRACK, short for key reinstallation attack, is a set of weaknesses in the WPA2 Wi-Fi handshake disclosed in 2017. A nearby attacker can trick a device into reusing an encryption key, which lets some traffic be decrypted. Patched devices resist it, so WPA2 remains usable when clients and access points are updated.
How does it work?
When a device joins a WPA2 network, the two sides run a four step handshake to agree on a session key. The third message can be retransmitted if the access point does not see a reply. The flaw is that a client may reinstall the key each time it receives that message. Reinstalling resets the packet counter and replay counter, so the same keystream is used again for different data. Reused keystream is what makes decryption possible.
The researcher who found it, Mathy Vanhoef of KU Leuven, states that the weaknesses are in the standard, not in one product, so correct implementations are likely affected. His site lists ten CVE identifiers, CVE-2017-13077 through CVE-2017-13088 with gaps, covering the pairwise key, group key and integrity group key in the handshake, plus related handshakes for fast roaming, peer links and sleep mode responses.
What can an attacker actually do?
The attacker must be within radio range of both the victim and the network. According to the researcher's description:
- They can decrypt packets the client sends, and in some setups packets sent to the client.
- With certain ciphers (WPA TKIP or GCMP) they can also forge and inject packets.
- They can hijack connections over unencrypted HTTP, for instance injecting content.
- They cannot recover the Wi-Fi password, and they do not obtain the session key.
Some Android 6.0 and Linux wpa_supplicant implementations could be tricked into installing an all zero key, which made interception especially easy.
Does HTTPS protect you?
Partly. The researchers say HTTPS is still valuable, but it can be bypassed in certain situations and they cite earlier bypasses in apps, VPNs and banking software. Traffic to plain HTTP sites is the most exposed.
Is WPA2 still safe?
Once patched, WPA2 is considerably safer, and the researchers recommend continuing to use it rather than switching to WEP. The patches are backward compatible, so a patched device talks to an unpatched one, but protection requires that both ends are fixed, or that the access point supports a mitigation protecting its clients.
CISA's guidance on securing wireless networks goes further and says that WPA3 is currently the strongest encryption, recommending WPA3 capable equipment where possible.
Constructed example: the forgotten access point
A small office updates every laptop and phone but never touches the wireless access point installed years ago. Its firmware predates the 2017 fixes and it has fast roaming switched on, so some handshake paths stay open even though the clients are patched. An attacker in the car park does not need the Wi-Fi password to try them. Following the researchers' advice, the office patches the access point, or disables fast roaming and repeater mode if no update exists. The lesson is that one side being fixed is not enough, so inventory the access points and repeaters, not only the laptops.
The setup is invented for illustration.
What does this mean today?
KRACK is old, and vendors released patches soon after the 2017 disclosure. The remaining risk is long lived hardware that never received them: cheap routers, range extenders, embedded devices and old phones. That is why asset inventory matters, because you cannot patch a device you do not know is on the network.
How to check or apply this
- List every access point, repeater and wireless client you manage.
- Check each vendor's advisories for the 2017 fixes and install current firmware.
- Replace devices that no longer receive updates.
- Where an access point supports 802.11r fast roaming or repeater mode and no patch exists, follow the researchers' suggestion to disable those features.
- Move to WPA3 where both ends support it.
- Keep HTTPS on for all services and avoid plain HTTP.
- Do not rely on changing the Wi-Fi password as a fix.
Quick check
Can KRACK reveal my Wi-Fi password?
No. The attack decrypts or alters traffic but does not recover the password or the session key.
Do I need to patch the router and every device?
Patch both clients and access points where you can. Patching only one side leaves the other exposed, although some access point fixes also protect connected clients.
Sources
Last reviewed: 18 October 2026.
