CVE-2024-5535 in OpenSSL
Summary
by MITRE • 06/27/2024
Issue summary: Calling the OpenSSL API function SSL_select_next_proto with an empty supported client protocols buffer may cause a crash or memory contents to be sent to the peer.
Impact summary: A buffer overread can have a range of potential consequences such as unexpected application beahviour 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 standardised 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 in 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 in length. 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, and 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.
The FIPS modules in 3.3, 3.2, 3.1 and 3.0 are not affected by this issue.
Due to the low severity of this issue we are not issuing new releases of OpenSSL at this time. The fix will be included in the next releases when they become available.
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Analysis
by VulDB Data Team • 06/22/2026
The vulnerability described in CVE-2024-5535 relates to a buffer overread condition within the OpenSSL library's SSL_select_next_proto API function. This flaw occurs when an application calls the function with an empty supported client protocols buffer, which can lead to either application crashes or the unintentional disclosure of up to 255 bytes of arbitrary private data from memory to the peer. The technical root cause stems from improper validation of input parameters within the SSL_select_next_proto implementation, specifically failing to detect when the client protocol list length is zero. According to CWE-129, this represents an insufficient input validation issue where the function does not properly handle edge cases involving empty buffers, leading to memory access violations that can compromise system confidentiality and integrity.
The operational impact of this vulnerability manifests primarily in scenarios where applications incorrectly configure or program their TLS protocol negotiation logic. The vulnerability is particularly relevant to applications using NPN (Next Protocol Negotiation) rather than ALPN (Application Layer Protocol Negotiation) since NPN permits opportunistic protocol selection even when no overlap exists between client and server lists. In ALPN implementations, the client protocol list is guaranteed to never be empty by libssl, making such applications less susceptible to this specific flaw. However, when applications do accidentally pass zero-length protocol lists to the SSL_select_next_proto function, they become vulnerable to memory disclosure attacks. This vulnerability aligns with ATT&CK technique T1552.001, which involves unsecured credentials, as the memory disclosure could potentially expose sensitive cryptographic material or session information.
The vulnerability requires specific conditions to be exploited, making it less likely to be actively targeted by attackers. Applications must be misconfigured to pass zero-length protocol lists to SSL_select_next_proto, and must also fail to properly handle the "no overlap" response condition that would normally result in handshake failure. The issue affects only applications that directly call this specific API function with malformed parameters, typically occurring due to programming errors or configuration mistakes rather than malicious intent. Additionally, the vulnerability is not present in OpenSSL FIPS modules for versions 3.3, 3.2, 3.1, and 3.0, indicating that specific security configurations provide protection against this particular memory access violation. The low severity classification reflects that this vulnerability requires specific application-level misconfigurations and is not typically under attacker control, though the potential for confidentiality loss makes it a legitimate concern for security-conscious deployments. Mitigation strategies should focus on proper input validation within applications and avoiding the use of deprecated NPN protocols in favor of standardized ALPN implementations.