CVE-2026-72897

Issue summary: A TLS server that calls SSL_set_SSL_CTX() to switch a

connection to a different SSL_CTX part way through a handshake may access

memory beyond the end of an internal array if the replacement context knows

about more provider signature algorithms than the context the connection was

created from. Applications which never call SSL_set_SSL_CTX() are not

affected.

Impact summary: A remote peer may be able to cause a small out-of-bounds

read, and in some circumstances a fixed-value out-of-bounds write, on the

server heap. This may lead to a Denial of Service.

CWE: CWE-787: Out-of-bounds Write

Description: A TLS connection records how many certificate slots it has

when it is created, taken from the SSL_CTX that created it: the built-in

certificate types plus one slot for each provider TLS-SIGALG entry that

context was aware of. That count sizes an internal array of per-slot

certificate validity flags.

An application may replace a connection's SSL_CTX part way through the

handshake by calling SSL_set_SSL_CTX(), most commonly from a servername

callback in order to serve a different virtual host. Doing so did not

refresh the recorded count. A provider signature algorithm's slot index is

its position in the list of whichever context resolves it, so if the

replacement context is aware of more of them than the original, an

algorithm offered by the peer can resolve to an index beyond the end of the

array. Processing the peer's signature algorithms then reads one four byte

word past the end for each such algorithm and, where the word read is zero,

writes a fixed value over it. A peer offering many of them can corrupt heap

metadata and abort the process.

Only provider signature algorithms which occupy one of the excess slots,

and which the server also has configured, have this effect. Codepoints the

replacement context does not recognise are discarded without being resolved

to a slot, and provider signature algorithms are usable only from TLS 1.3.

The two contexts must therefore be aware of different numbers of provider

signature algorithms, which requires separate library contexts, a provider

loaded between the two being created, or providers which differ in what

they advertise - in 4.0, for example, the default provider advertises SM2

where the FIPS provider does not. A deployment meeting the condition is

also unable to negotiate the affected algorithms with legitimate clients,

since the same stale count hides the corresponding certificates, so the

misconfiguration is likely to be noticed. For that reason, and because the

configuration is not the default, this issue has been assessed as Low

severity.

FIPS impact: no

No FIPS modules are affected by this issue as the affected code is outside

the OpenSSL FIPS module boundary.