CVE-2020-20949
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedBleichenbacher's attack on PKCS #1 v1.5 padding for RSA in STM32 cryptographic firmware library software expansion for STM32Cube (UM1924). The vulnerability can allow one to use Bleichenbacher's oracle attack to decrypt an encrypted ciphertext by making successive queries to the server using the vulnerable library, resulting in remote information disclosure.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceBleichenbacher's attack vulnerability in STM32 cryptographic firmware library allows an attacker to decrypt RSA ciphertexts encrypted with PKCS #1 v1.5 padding by measuring timing differences in server responses. The oracle reveals whether RSA decryption intermediate values are within a specific range, enabling iterative ciphertext manipulation to gradually recover the plaintext.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE dataall versionsall versionsall versionsall versionsall versionsall versionsall versionsall versionsCVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- High
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- None
- Availability
- None
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify STM32Cube firmware in useReview project dependencies and firmware packages to confirm usage of STM32CubeF0, F1, F2, F3, F4, F7, G0, or G4 librariesAffected if Any of these STM32Cube firmware libraries are present in the project
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Confirm RSA PKCS#1 v1.5 decryption is implementedSearch codebase for RSA decryption function calls, specifically look for mbedtls_rsa_pkcs1_decrypt, HAL_CRYP_RSA_Decrypt, or similar RSA decryption APIs with PKCS#1 v1.5 padding modeAffected if Code performs RSA decryption using PKCS#1 v1.5 padding mode
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Check for constant-time RSA implementationInspect the cryptographic library configuration and source code for constant-time RSA decryption functions or CRYP_RSA_CALC_TIME constant-time build flagsAffected if No constant-time RSA decryption implementation is found in the firmware
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Verify OAEP padding is not used as alternativeConfirm whether OAEP padding mode (MBEDTLS_RSA_PKCS_V21, RSA_PKCS_OAEP) is configured instead of PKCS#1 v1.5 for RSA operationsAffected if RSA decryption continues to use PKCS#1 v1.5 padding without OAEP as an alternative
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Test for timing oracle in RSA responsesPerform repeated RSA decryption operations with varied ciphertexts and measure response timing using timing analysis tools or custom test harness to detect timing differences correlating with padding validationAffected if Timing measurements reveal measurable differences in response latency that correlate with padding oracle conditions
If the environment uses any STM32Cube firmware with RSA PKCS#1 v1.5 decryption and lacks constant-time implementation or OAEP padding, it is likely vulnerable to this Bleichenbacher attack.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataUpdate STM32 cryptographic firmware to a patched version that either implements constant-time RSA decryption or switches to OAEP padding. If update unavailable, implement timing-safe response padding to prevent oracle exploitation.
- Consultation8.0 h
- Implementation24.0 h
- Testing16.0 h
- Review / QA8.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2020-20949 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2020-20949 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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