5.3

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2023-29409

CVE-2023-29409: Extremely large RSA keys in certificate chains can cause a client/server to expend significant...

Extremely large RSA keys in certificate chains can cause a client/server to expend significant CPU time verifying signatures. With fix, the size of RSA keys transmitted during handshakes is restricted to <= 8192 bits. Based on a survey of publicly trusted RSA keys, there are currently only three certificates in circulation with keys larger than this, and all three appear to be test certificates that are not actively deployed. It is possible there are larger keys in use in private PKIs, but we target the web PKI, so causing breakage here in the interests of increasing the default safety of users of crypto/tls seems reasonable.

(GitHub Advisory)

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CVE-2023-29409

CVE-2023-29409:

5.3

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

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Root Cause Analysis

The vulnerability description indicates that extremely large RSA keys in certificate chains can cause excessive CPU usage. The fix involves restricting the size of RSA keys to <= 8192 bits. I analyzed the provided Go CL (Change List) information, which, although not directly fetchable via get_commit_infos, was accessible through its web interface. The CL (https://go-review.googlesource.com/c/go/+/515257) shows changes in src/crypto/tls/handshake_client.go and src/crypto/tls/handshake_server.go. Specifically, checks for RSA key sizes (if pub.N.BitLen() > maxRSAKeyBits) were added to the (*clientHandshakeState).doFullHandshake and (*serverHandshakeState).doFullHandshake methods. These methods are responsible for processing certificates during the TLS handshake. Before the patch, these methods lacked the key size validation, making them vulnerable to processing overly large RSA keys, which could lead to a denial of service due to high CPU load. The pkg.go.dev/vuln/GO-2023-1987 page also confirms that the vulnerability lies in crypto/tls and affects handshake-related operations. The identified functions are the precise locations where the vulnerable processing (or lack of validation) occurred and where the fix was applied.

Vulnerable functions

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WAF Rule

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Reasoning

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