CVE-2026-72897 (GCVE-0-2026-72897)
Vulnerability from cvelistv5 – Published: 2026-09-29 15:32 – Updated: 2026-09-29 17:16
VLAI
EPSS
VEX
Title
Out-of-Bounds Access After SSL_set_SSL_CTX() During a Handshake
Summary
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.
Severity
7.5 (High)
SSVC
Exploitation: none
Automatable: yes
Technical Impact: partial
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2026-09-29 17:16 UTC
CWE
- CWE-787 - Out-of-bounds Write
Assigner
References
5 references
Impacted products
Date Public
2026-09-29 14:21
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]
}
],
"credits": [
{
"lang": "en",
"type": "reporter",
"value": "Filipe Casal (Trail of Bits)"
},
{
"lang": "en",
"type": "reporter",
"value": "Brandon Luo"
},
{
"lang": "en",
"type": "reporter",
"value": "Luigino Camastra (Aisle Research)"
},
{
"lang": "en",
"type": "remediation developer",
"value": "Matt Caswell"
}
],
"datePublic": "2026-09-29T14:21:57.000Z",
"descriptions": [
{
"lang": "en",
"supportingMedia": [
{
"base64": false,
"type": "text/html",
"value": "Issue summary: A TLS server that calls SSL_set_SSL_CTX() to switch a\u003cbr\u003econnection to a different SSL_CTX part way through a handshake may access\u003cbr\u003ememory beyond the end of an internal array if the replacement context knows\u003cbr\u003eabout more provider signature algorithms than the context the connection was\u003cbr\u003ecreated from. Applications which never call SSL_set_SSL_CTX() are not\u003cbr\u003eaffected.\u003cbr\u003e\u003cbr\u003eImpact summary: A remote peer may be able to cause a small out-of-bounds\u003cbr\u003eread, and in some circumstances a fixed-value out-of-bounds write, on the\u003cbr\u003eserver heap. This may lead to a Denial of Service.\u003cbr\u003e\u003cbr\u003eCWE: CWE-787: Out-of-bounds Write\u003cbr\u003e\u003cbr\u003eDescription: A TLS connection records how many certificate slots it has\u003cbr\u003ewhen it is created, taken from the SSL_CTX that created it: the built-in\u003cbr\u003ecertificate types plus one slot for each provider TLS-SIGALG entry that\u003cbr\u003econtext was aware of. That count sizes an internal array of per-slot\u003cbr\u003ecertificate validity flags.\u003cbr\u003e\u003cbr\u003eAn application may replace a connection\u0027s SSL_CTX part way through the\u003cbr\u003ehandshake by calling SSL_set_SSL_CTX(), most commonly from a servername\u003cbr\u003ecallback in order to serve a different virtual host. Doing so did not\u003cbr\u003erefresh the recorded count. A provider signature algorithm\u0027s slot index is\u003cbr\u003eits position in the list of whichever context resolves it, so if the\u003cbr\u003ereplacement context is aware of more of them than the original, an\u003cbr\u003ealgorithm offered by the peer can resolve to an index beyond the end of the\u003cbr\u003earray. Processing the peer\u0027s signature algorithms then reads one four byte\u003cbr\u003eword past the end for each such algorithm and, where the word read is zero,\u003cbr\u003ewrites a fixed value over it. A peer offering many of them can corrupt heap\u003cbr\u003emetadata and abort the process.\u003cbr\u003e\u003cbr\u003eOnly provider signature algorithms which occupy one of the excess slots,\u003cbr\u003eand which the server also has configured, have this effect. Codepoints the\u003cbr\u003ereplacement context does not recognise are discarded without being resolved\u003cbr\u003eto a slot, and provider signature algorithms are usable only from TLS 1.3.\u003cbr\u003e\u003cbr\u003eThe two contexts must therefore be aware of different numbers of provider\u003cbr\u003esignature algorithms, which requires separate library contexts, a provider\u003cbr\u003eloaded between the two being created, or providers which differ in what\u003cbr\u003ethey advertise - in 4.0, for example, the default provider advertises SM2\u003cbr\u003ewhere the FIPS provider does not. A deployment meeting the condition is\u003cbr\u003ealso unable to negotiate the affected algorithms with legitimate clients,\u003cbr\u003esince the same stale count hides the corresponding certificates, so the\u003cbr\u003emisconfiguration is likely to be noticed. For that reason, and because the\u003cbr\u003econfiguration is not the default, this issue has been assessed as Low\u003cbr\u003eseverity.\u003cbr\u003e\u003cbr\u003eFIPS impact: no\u003cbr\u003eNo FIPS modules are affected by this issue as the affected code is outside\u003cbr\u003ethe OpenSSL FIPS module boundary."
}
],
"value": "Issue summary: A TLS server that calls SSL_set_SSL_CTX() to switch a\nconnection to a different SSL_CTX part way through a handshake may access\nmemory beyond the end of an internal array if the replacement context knows\nabout more provider signature algorithms than the context the connection was\ncreated from. Applications which never call SSL_set_SSL_CTX() are not\naffected.\n\nImpact summary: A remote peer may be able to cause a small out-of-bounds\nread, and in some circumstances a fixed-value out-of-bounds write, on the\nserver heap. This may lead to a Denial of Service.\n\nCWE: CWE-787: Out-of-bounds Write\n\nDescription: A TLS connection records how many certificate slots it has\nwhen it is created, taken from the SSL_CTX that created it: the built-in\ncertificate types plus one slot for each provider TLS-SIGALG entry that\ncontext was aware of. That count sizes an internal array of per-slot\ncertificate validity flags.\n\nAn application may replace a connection\u0027s SSL_CTX part way through the\nhandshake by calling SSL_set_SSL_CTX(), most commonly from a servername\ncallback in order to serve a different virtual host. Doing so did not\nrefresh the recorded count. A provider signature algorithm\u0027s slot index is\nits position in the list of whichever context resolves it, so if the\nreplacement context is aware of more of them than the original, an\nalgorithm offered by the peer can resolve to an index beyond the end of the\narray. Processing the peer\u0027s signature algorithms then reads one four byte\nword past the end for each such algorithm and, where the word read is zero,\nwrites a fixed value over it. A peer offering many of them can corrupt heap\nmetadata and abort the process.\n\nOnly provider signature algorithms which occupy one of the excess slots,\nand which the server also has configured, have this effect. Codepoints the\nreplacement context does not recognise are discarded without being resolved\nto a slot, and provider signature algorithms are usable only from TLS 1.3.\n\nThe two contexts must therefore be aware of different numbers of provider\nsignature algorithms, which requires separate library contexts, a provider\nloaded between the two being created, or providers which differ in what\nthey advertise - in 4.0, for example, the default provider advertises SM2\nwhere the FIPS provider does not. A deployment meeting the condition is\nalso unable to negotiate the affected algorithms with legitimate clients,\nsince the same stale count hides the corresponding certificates, so the\nmisconfiguration is likely to be noticed. For that reason, and because the\nconfiguration is not the default, this issue has been assessed as Low\nseverity.\n\nFIPS impact: no\nNo FIPS modules are affected by this issue as the affected code is outside\nthe OpenSSL FIPS module boundary."
}
],
"metrics": [
{
"format": "other",
"other": {
"content": {
"text": "Low"
},
"type": "https://openssl-library.org/policies/general/security-policy/"
}
}
],
"problemTypes": [
{
"descriptions": [
{
"cweId": "CWE-787",
"description": "CWE-787 Out-of-bounds Write",
"lang": "en",
"type": "CWE"
}
]
}
],
"providerMetadata": {
"dateUpdated": "2026-09-29T15:32:18.277Z",
"orgId": "3a12439a-ef3a-4c79-92e6-6081a721f1e5",
"shortName": "openssl"
},
"references": [
{
"name": "OpenSSL Advisory",
"tags": [
"vendor-advisory"
],
"url": "https://openssl-library.org/news/secadv/20260929.txt"
},
{
"name": "4.0.3 git commit",
"tags": [
"patch"
],
"url": "https://github.com/openssl/openssl/commit/e87ed26b298a74d8ba61a53e9c7bcd1acac6b814"
},
{
"name": "3.6.5 git commit",
"tags": [
"patch"
],
"url": "https://github.com/openssl/openssl/commit/9c54d209486f6b1ad79fe2179c40f13200fa4f61"
},
{
"name": "3.5.9 git commit",
"tags": [
"patch"
],
"url": "https://github.com/openssl/openssl/commit/4135f553c9d3ba4a09fe752f5d30af2a6a092b2e"
},
{
"name": "3.4.8 git commit",
"tags": [
"patch"
],
"url": "https://github.com/openssl/openssl/commit/00646e5085a0d12d29e0d2f9b9bc5f7111a50922"
}
],
"source": {
"discovery": "UNKNOWN"
},
"title": "Out-of-Bounds Access After SSL_set_SSL_CTX() During a Handshake",
"x_generator": {
"engine": "Vulnogram 0.2.0"
}
}
},
"cveMetadata": {
"assignerOrgId": "3a12439a-ef3a-4c79-92e6-6081a721f1e5",
"assignerShortName": "openssl",
"cveId": "CVE-2026-72897",
"datePublished": "2026-09-29T15:32:18.277Z",
"dateReserved": "2026-08-10T17:02:46.613Z",
"dateUpdated": "2026-09-29T17:16:38.842Z",
"state": "PUBLISHED"
},
"dataType": "CVE_RECORD",
"dataVersion": "5.2"
}
}
}
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Drag nodes to arrange them. Double-click a related vulnerability to open it, or a source to visit its link. The Physics button on the left tunes the layout and the simulation; the legend at the bottom left hides or shows nodes by kind and by sighting type.
Experimental. This forecast is provided for visualization only and may change without notice. Do not use it for operational decisions.
Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
Vulnerability timeline
Sightings and recorded dates, grouped by day · UTC
How durations are calculated
Following the VTEM framework (Black Hat USA 2015), time to patch starts when the vendor or a coordinator was informed, total time to patch at discovery, and disclosure to patch at public disclosure; vendor response time runs from the report to its acknowledgement. Time to exploit and time unpatched start at the earliest discovery or report, falling back to public disclosure. Unpatched time ends at the first recorded patch, or the calculation time when no patch is recorded. Exploitation to patch shows the interval between those two events; a published proof of concept is a public exploit, not exploitation.
Each result shows its source dates. A negative interval means the end event came first; when date-only inputs leave the order open, the result says so. Date-only inputs are approximate. The earliest evidence wins: KEV report dates and sightings are labelled as such. Records describing another vulnerability are listed but never calculated from, and a PoC alone does not establish exploitation. Missing patch evidence does not prove that no patch exists. Filters below change the event list only.
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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the
vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base
model, served locally by ML-Gateway.
They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
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Related by attack behaviour
Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.