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.
EPSS Score:
4%
Comprehensive Technical Analysis of EUVD-2024-46737
1. Vulnerability Assessment and Severity Evaluation
Vulnerability Description:
The vulnerability involves the OpenSSL API function SSL_select_next_proto. When this function is called with an empty supported client protocols buffer, it may cause a crash or send up to 255 bytes of arbitrary private data from memory to the peer. This buffer overread can lead to unexpected application behavior, crashes, and loss of confidentiality.
Severity Evaluation:
- Base Score: 9.1 (CVSS:3.1)
- Vector: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
The high confidentiality impact (C:H) and high availability impact (A:H) contribute to the severity score. However, the vulnerability is assessed as Low severity due to the specific conditions required for exploitation and the limited use of NPN (Next Protocol Negotiation) compared to ALPN (Application Layer Protocol Negotiation).
2. Potential Attack Vectors and Exploitation Methods
Attack Vectors:
- Configuration Errors: Applications that accidentally configure a zero-length server list and pass it to
SSL_select_next_proto. - Programming Errors: Developers mistakenly calling
SSL_select_next_protowith a zero-length client list.
Exploitation Methods:
- Memory Leakage: An attacker could exploit this vulnerability to read up to 255 bytes of memory, potentially exposing sensitive information.
- Denial of Service (DoS): The crash caused by the buffer overread could lead to a DoS condition, disrupting the availability of the affected application.
3. Affected Systems and Software Versions
Affected Versions:
- OpenSSL 1.1.1 < 1.1.1za
- OpenSSL 3.1.0 < 3.1.7
- OpenSSL 3.2.0 < 3.2.3
- OpenSSL 1.0.2 < 1.0.2zk
- OpenSSL 3.0.0 < 3.0.15
- OpenSSL 3.3.0 < 3.3.2
Unaffected Versions:
- FIPS modules in OpenSSL 3.3, 3.2, 3.1, and 3.0.
4. Recommended Mitigation Strategies
Immediate Mitigation:
- Patching: Upgrade to the latest versions of OpenSSL that include the fix for this vulnerability.
- Configuration Review: Ensure that applications using OpenSSL are correctly configured to avoid zero-length protocol lists.
- Code Review: Conduct thorough code reviews to identify and correct any instances where
SSL_select_next_protois called with a zero-length client list.
Long-Term Mitigation:
- Security Training: Educate developers on secure coding practices and the proper use of OpenSSL functions.
- Regular Audits: Implement regular security audits and vulnerability assessments to identify and mitigate similar issues.
5. Impact on European Cybersecurity Landscape
Impact Analysis:
- Confidentiality: Potential exposure of sensitive data, which could be exploited by malicious actors.
- Availability: Possible disruption of services due to crashes, affecting business continuity.
- Compliance: Organizations may face regulatory penalties if sensitive data is compromised due to this vulnerability.
Mitigation in European Context:
- GDPR Compliance: Ensure that personal data is protected and that any breaches are reported promptly.
- Cybersecurity Directives: Adhere to EU cybersecurity directives and guidelines to maintain a robust security posture.
6. Technical Details for Security Professionals
Function Analysis:
- SSL_select_next_proto: This function is used for protocol negotiation in TLS applications supporting ALPN or NPN. It accepts protocol lists from the server and client and returns the first matching protocol.
- Vulnerability Condition: When called with a zero-length client list, the function fails to handle this condition correctly, leading to a buffer overread.
Exploitation Conditions:
- ALPN: The client list is guaranteed by libssl to never be zero in length, reducing the likelihood of exploitation.
- NPN: The client list can be zero in length, making applications using NPN more susceptible to this vulnerability.
References:
Conclusion: While the vulnerability has a high CVSS score, its practical impact is mitigated by the specific conditions required for exploitation and the limited use of NPN. Organizations should prioritize patching and reviewing configurations to ensure they are not vulnerable. Regular security audits and adherence to EU cybersecurity guidelines will further enhance the overall security posture.