CVE-2024-5535
CVE-2024-5535
Weakness (CWE)
CVSS Vector
v3.1- Attack Vector
- Network
- Attack Complexity
- Low
- Privileges Required
- None
- User Interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- None
- Availability
- High
Description
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.
Comprehensive Technical Analysis of CVE-2024-5535
1. Vulnerability Assessment and Severity Evaluation
Vulnerability Assessment:
CVE-2024-5535 is a buffer overread vulnerability in the OpenSSL API function SSL_select_next_proto. This issue arises when the function is called with an empty supported client protocols buffer, potentially leading to a crash or the disclosure of up to 255 bytes of arbitrary private data from memory to the peer. The vulnerability is specific to applications that directly call SSL_select_next_proto with a zero-length list of supported client protocols, which is typically an invalid scenario.
Severity Evaluation: The CVSS score for this vulnerability is 9.1, indicating a high severity. However, the OpenSSL team has assessed it as Low severity due to the following reasons:
- The issue primarily affects applications using NPN (Next Protocol Negotiation), which is deprecated and not widely used.
- The vulnerability requires an application configuration or programming error to be exploitable.
- Active exploitation is unlikely because the issue is not typically under attacker control.
2. Potential Attack Vectors and Exploitation Methods
Attack Vectors:
- Configuration Errors: An application might be misconfigured to pass a zero-length client protocols list to
SSL_select_next_proto. - Programming Errors: Developers might inadvertently call the function with an empty buffer due to coding mistakes.
Exploitation Methods:
- Memory Disclosure: An attacker could exploit this vulnerability to read up to 255 bytes of memory, potentially revealing sensitive information such as encryption keys, session tokens, or other confidential data.
- Denial of Service (DoS): The vulnerability could cause the application to crash, leading to a DoS condition.
3. Affected Systems and Software Versions
Affected Systems:
- Applications that use the OpenSSL API function
SSL_select_next_protofor ALPN (Application Layer Protocol Negotiation) or NPN. - Systems that have not yet applied the fix for this vulnerability.
Software Versions:
- The vulnerability affects OpenSSL versions prior to the next scheduled releases, which will include the fix.
- The FIPS modules in OpenSSL versions 3.3, 3.2, 3.1, and 3.0 are not affected.
4. Recommended Mitigation Strategies
Immediate Mitigation:
- Code Review: Conduct a thorough code review to ensure that
SSL_select_next_protois not called with a zero-length client protocols list. - Configuration Checks: Verify application configurations to ensure that valid protocol lists are provided.
Long-Term Mitigation:
- Update OpenSSL: Apply the upcoming OpenSSL releases that include the fix for this vulnerability.
- Monitoring: Implement monitoring to detect any unusual behavior or crashes that might indicate an attempt to exploit this vulnerability.
5. Impact on Cybersecurity Landscape
Impact:
- Confidentiality: The primary impact is the potential loss of confidentiality due to memory disclosure.
- Availability: The vulnerability could lead to application crashes, affecting service availability.
- Integrity: Although not directly impacting integrity, the disclosure of sensitive information could lead to further attacks that compromise data integrity.
Broader Implications:
- This vulnerability highlights the importance of thorough code reviews and proper configuration management in preventing security issues.
- It underscores the need for continuous monitoring and timely updates to mitigate potential risks.
6. Technical Details for Security Professionals
Function Behavior:
SSL_select_next_protois used to negotiate protocols between a client and server.- The function accepts two lists of protocols (client and server) and returns the first protocol that appears in both lists.
- If there is no overlap, it returns the first item in the client list and signals no overlap.
Vulnerability Details:
- When called with a zero-length client list, the function fails to handle this condition correctly.
- It returns the memory immediately following the client list pointer, leading to a buffer overread.
Exploitation Conditions:
- For ALPN, the client list is guaranteed by libssl to never be zero in length, reducing the likelihood of exploitation.
- For NPN, the client list comes from the application and should not be zero in length. If it is, the function returns an invalid memory pointer, leading to memory disclosure.
References:
Conclusion: CVE-2024-5535 is a critical vulnerability that requires immediate attention, especially for applications using NPN. While the likelihood of active exploitation is low, the potential impact on confidentiality and availability necessitates prompt mitigation strategies. Security professionals should focus on code reviews, configuration checks, and timely updates to address this issue effectively.