Exposure of Sensitive Information to an Unauthorized Actor
CVE-2024-5535
Summary
Calling the OpenSSL API function "SSL_select_next_proto" with an empty supported client protocol buffer may cause a crash or memory content to be sent to the peer. Impact summary: A buffer overread can have a range of potential consequences such as unexpected application behavior or a crash. In particular, this issue could result in up to 255 bytes of arbitrary private data from memory being sent to the peer, leading to a loss of confidentiality. However, only applications that directly call the "SSL_select_next_proto" function with a 0-length list of supported client protocols are affected by this issue. This would normally never be a valid scenario and is typically not under attacker control but may occur by accident in the case of a configuration or programming error in the calling application. The OpenSSL API function "SSL_select_next_proto" is typically used by TLS applications that support ALPN (Application Layer Protocol Negotiation) or NPN (Next Protocol Negotiation). NPN is older, was never standardized and is deprecated in favour of ALPN. We believe that ALPN is significantly more widely deployed than NPN. The "SSL_select_next_proto" function accepts a list of protocols from the server and a list of protocols from the client and returns the first protocol that appears in the server list that also appears in the client list. In the case of no overlap between the two lists, it returns the first item to the client list. In either case, it will signal whether an overlap between the two lists was found. In the case where "SSL_select_next_proto" is called with a zero-length client list, it fails to notice this condition and returns the memory immediately following the client list pointer (and reports that there was no overlap in the lists). This function is typically called from a server-side application callback for ALPN or a client-side application callback for NPN. In the case of ALPN, the list of protocols supplied by the client is guaranteed by libssl to never be zero. The list of server protocols comes from the application and should never normally be expected to be of zero length. In this case, if the "SSL_select_next_proto" function has been called as expected (with the list supplied by the client passed in the client/client_len parameters), then the application will not be vulnerable to this issue. If the application has accidentally been configured with a zero-length server list, has accidentally passed that zero-length server list in the "client/client_len" parameters, and has additionally failed to correctly handle a "no overlap" response (which would normally result in a handshake failure in ALPN), then it will be vulnerable to this problem. In the case of NPN, the protocol permits the client to opportunistically select a protocol when there is no overlap. OpenSSL returns the first client protocol in the no overlap case in support of this. The list of client protocols comes from the application and should never normally be expected to be of zero length. However, if the "SSL_select_next_proto" function is accidentally called with a "client_len" of 0, then an invalid memory pointer will be returned instead. If the application uses this output as the opportunistic protocol, then the loss of confidentiality will occur. This issue has been assessed as Low severity because applications are most likely to be vulnerable if they are using NPN instead of ALPN - but NPN is not widely used. It also requires an application configuration or programming error. Finally, this issue would not typically be under attacker control, making active exploitation unlikely. This issue affects the openssl versions 1.0.2-beta1 through 1.0.2u, 1.1.1-pre1 through 1.1.1w, 3.0.0-alpha1 through 3.0.14, 3.1.0-alpha1 through 3.1.6, 3.2.0-alpha1 through 3.2.2, and 3.3.0-alpha1 through 3.3.1.
- LOW
- NETWORK
- NONE
- UNCHANGED
- NONE
- NONE
- HIGH
- NONE
CWE-200 - Information Exposure
An information exposure vulnerability is categorized as an information flow (IF) weakness, which can potentially allow unauthorized access to otherwise classified information in the application, such as confidential personal information (demographics, financials, health records, etc.), business secrets, and the application's internal environment.
References
Advisory Timeline
- Published