CVE-2026-72897 Out-of-Bounds Access After SSL_set_SSL_CTX() During a Handshake
Remote attackers can cause denial of service via out-of-bounds write in OpenSSL when SSL_set_SSL_CTX is used during handshake.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-72897?
CVE-2026-72897 is a high-severity memory corruption vulnerability in Openssl Openssl, published on 29 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Weakness: CWE-787
Which versions are affected by CVE-2026-72897?
It affects 1 product.
Products include Openssl Openssl.
How do I fix CVE-2026-72897?
Apply the vendor's patch.
Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.
Is CVE-2026-72897 being exploited?
There is no known exploitation of CVE-2026-72897 in the wild and no public exploit code.
Other vulnerabilities in Openssl Openssl
Sources
Also known as ALPINE-CVE-2026-72897, AZL-105471, AZL-105498, AZL-105588, AZL-105633, AZL-105825 and 7 more.