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mercredi 4 mars 2020

VU#782301: pppd vulnerable to buffer overflow due to a flaw in EAP packet processing

PPP is the protocol used for establishing internet links over dial-up modems,DSL connections,and many other types of point-to-point links including Virtual Private Networks(VPN)such as Point to Point Tunneling Protocol(PPTP). The pppd software can also authenticate a network connected peer and/or supply authentication information to the peer using multiple authentication protocols including EAP. Due to a flaw in the Extensible Authentication Protocol(EAP)packet processing in the Point-to-Point Protocol Daemon(pppd),an unauthenticated remote attacker may be able to cause a stack buffer overflow,which may allow arbitrary code execution on the target system. This vulnerability is due to an error in validating the size of the input before copying the supplied data into memory. As the validation of the data size is incorrect,arbitrary data can be copied into memory and cause memory corruption possibly leading to execution of unwanted code. The vulnerability is in the logic of the eap parsing code,specifically in the eap_request()and eap_response()functions in eap.c that are called by a network input handler. These functions take a pointer and length as input using the the first byte as a type. If the type is EAPT_MD5CHAP(4),it looks at an embedded 1-byte length field. The logic in this code is intended to makes sure that embedded length is smaller than the whole packet length. After this verification,it tries to copy provided data(hostname)that is located after the embedded length field into a local stack buffer. This bounds check is incorrect and allows for memory copy to happen with an arbitrary length of data. An additional logic flaw causes the eap_input()function to not check if EAP has been negotiated during the Line Control Protocol(LCP)phase. This allows an unauthenticated attacker to send an EAP packet even if ppp refused the authentication negotiation due to lack of support for EAP or due to mismatch of an agreed pre-shared passphrase in the LCP phase. The vulnerable pppd code in eap_input will still process the EAP packet and trigger the stack buffer overflow. This unverified data with an unknown size can be used to corrupt memory of the target system. The pppd often runs with high privileges(system or root)and works in conjunction with kernel drivers. This makes it possible for an attacker to potentially execute arbitrary code with system or root level privileges. The pppd software is also adopted into lwIP(lightweight IP)project to provide pppd capabilities for small devices. The default installer and packages of lwIP are not vulnerable to this buffer overflow. However if you have used the lwIP source code and configured specifically to enable EAP at compile time,your software is likely vulnerable to the buffer overflow. The recommended update is available from Git repoistory http://git.savannah.nongnu.org/cgit/lwip.git(Specifically patch referenced: http://git.savannah.nongnu.org/cgit/lwip.git/commit/?id=2ee3cbe69c6d2805e64 e7cac2a1c1706e49ffd86) This type of weakness is commonly associated in Common Wekaness Enumeration(CWE)with CWE-120 Buffer Copy without Checking Size of Input('Classic Buffer Overflow')

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Auteur: US Cert

Catégories: CertUSNombre de vues: 213

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