Technical details and steps to reproduce can be found on the referenced GitHub pages below.

BIGNUM reciprocal lacks proper check for 0 divisor in
BN_RECP_CTX_set


 [Additional Information]
 This is a similar vulnerability to the root cause of CVE-2015-1794.

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 [VulnerabilityType Other]
 CWE-20 Improper input validation

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 [Vendor of Product]
 OpenSSL

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 [Affected Product Code Base]
All versions to date

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 [Affected Component]
 BIGNUM reciprocal implementation (exported function), crypto/bn/bn_recp.c:BN_RECP_CTX_set

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 [Attack Type]
 Remote

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 [Impact Denial of Service]
 true

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 [Attack Vectors]
 The root cause of this vulnerability is that the initial setting up function BN_RECP_CTX_set​ did not verify the divisor against 0 and returns success. Without a return value indicating illegal divisor, the developer may dismiss the divide-by-zero error and falsely set up some relavant logic.
 Unlike CVE-2015-1794<https://github.com/advisories/GHSA-x29w-fcg6-7f9m>, BIGNUM reciprocal is only used in BN_mod_exp_recp where the divisor is checked after invoking BN_RECP_CTX_set. Thus the defect does not make a critical vulnerability in OpenSSL itself. However reciprocal-related functions are exported, and with a simple code search I found a few projects using them directly, like rnpgp<https://github.com/rnpgp/rnp/blob/ed195f81b71a40eac2b0a61b0778b1cf717d6191/src/lib/crypto/elgamal_ossl.cpp#L83>. So it is a good idea to regard it as a vulnerability so downstream developers may get a chance to check if their usage is impacted.

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 [Reference]
Technical report and PoC:
https://github.com/openssl/openssl/issues/21111
[https://opengraph.githubassets.com/fab142bb94193345a564dac931a1be0735b574a65feca97f2bc032ee8ecafe8b/openssl/openssl/issues/21111]<https://github.com/openssl/openssl/issues/21111>
BIGNUM reciprocal lacks proper check for 0 divisor in `BN_RECP_CTX_set` · Issue #21111 · openssl/openssl<https://github.com/openssl/openssl/issues/21111>
Recently while reviewing historical vulnerabilities and their recurring bugs, I noticed CVE-2015-1794 in OpenSSL, which leads to DoS due to a zero p value. The deeper cause is lack of check for 0 d...
github.com



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 [Has vendor confirmed or acknowledged the vulnerability?]
 true


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 [Discoverer]
 fullwaywang from Tencent