VulnerabilityPublished 2 Apr 2026

CVE-2026-34876: Mbed TLS · CVE-2026-34876 is an out-of-bounds read

Source CVE Database V5

Not known to be exploited · CVSS 7.5 High · fixed in 3.6.6 : patch affected systems soon.

Data as of 26 Oct 2026, 14:05 UTC

CVSS 3.1 base score

7.5High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.3% chance of exploitation in 30 days – higher than 32% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 3.6.6

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
Integrity: None
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 2 Apr 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

AI-suggested

A fix is available in Mbed TLS version 3.6.6 and later. Users should upgrade to version 3.6.6 or newer to remediate this vulnerability. For Mbed TLS 4.x, although the internal issue exists, the vulnerable function is not exposed publicly, reducing risk. Patch status beyond version 3.6.6 is not explicitly provided; users should consult the vendor advisory for the latest remediation guidance.

Description

CVE-2026-34876 is an out-of-bounds read vulnerability in Mbed TLS 3.x before 3.6.6 affecting the multipart CCM API. The vulnerability arises from missing validation of the tag_len parameter in the mbedtls_ccm_finish() function, allowing attackers to read adjacent CCM context data. In Mbed TLS 4.x, the internal implementation has the same issue, but the vulnerable function is not exposed in the public API. Exploitation requires direct application-level use of the multipart CCM API with an oversized tag_len. This vulnerability has a high severity with a CVSS score of 7.5 and impacts confidentiality but not integrity or availability.

Impact

Successful exploitation of this vulnerability can lead to unauthorized disclosure of adjacent CCM context data, impacting confidentiality. There is no impact on integrity or availability. No known exploits are reported in the wild at this time.

Summary

Mbed TLS versions 3.x prior to 3.6.6 contain an out-of-bounds read vulnerability in the mbedtls_ccm_finish() function within the multipart CCM API. The root cause is the lack of validation of the tag_len parameter against the fixed 16-byte authentication buffer size, which allows an attacker to read adjacent memory areas. While Mbed TLS 4.x versions before the fix share the same internal issue, the vulnerable function is not accessible via the public API, reducing exposure. Exploitation requires that an application explicitly invoke the multipart CCM API with an oversized tag_len parameter.

CWE-125Out-of-bounds Read (opens in a new tab)
Base weaknessCWE Top 25 (2025): #8
The product reads data past the end, or before the beginning, of the intended buffer.
Implementation · Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
Architecture and Design · Language Selection
Use a language that provides appropriate memory abstractions.
OWASP Cheat Sheets
XML Security (opens in a new tab)
Mass Assignment (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.1 (opens in a new tab)
V5.1.4 (opens in a new tab)
V5.1.3 (opens in a new tab)
Attack patterns
Overread Buffers (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

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Exploitation evidence

What to do

Description

AI analysisMachine-generated, updated 10 Apr 2026, 00:04 UTC

WeaknessesCWE-125

Technical details

References

Community reviews

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